.

BTSC intSTEREO: Aural

Proof ofPerkirmance

Guide

BTSC STEREOAURAL PROOF of PERFORMANCE GUIDE

Includes Test Setup Procedures and

Tutorial with Test Charts and Logs

WRITTEN AND COMPILED BY

TFT, INC.

3090 OAKMEAD VILLAGE DRIVE

SANTA CLARA, CA 95051

(408) 727-7272

TWX 910-338-0584

FAX (408) 727-5942

©1986 TFT, Inc.All Rights Reserved

$25.00

First PrintingSeptember 1986

1

ACKNOWLEDGEMENTS

11-1, wishes to thank its Marketing and En- and critiques which made this book meaningfulgineering employees for their inputs, comments, to the TV broadcaster.

The Proof Forms included are printed with Publishing Corporation. These forms may bemodifications with permission from the Intertec reproduced at will.

3

SECTION 1

SECTION 2

TABLE OF CONTENTS

INTRODUCTION

FCC OST BULLETIN NO. 60 AND EIA / BTSCRECOMMENDED PRACTICES

2.1 FCC OST Bulletin No. 602.1.1 Spectrum and Modulation Level2.1.2 Main Channel THD2.1.3 Stereo Channel Separation2.1.4 Crosstalk2.1.5 Signal -to -Noise Ratio (Noise Floor)2.1.6 Other Recommendations2.1.7 SAP Channel2.1.8 Crosstalk

2.2 EIA / BTSC Recommended Practices

SECTION 3 COMPANDING, STEREO SEPARATION ANDMODULATION ACCURACY

3.1 Companding3.2 Stereo Separation and Modulation Accuracy

SECTION 4 CHECKING THE TEST EQUIPMENT AND PREPARINGDOCUMENTATION

4.1 Test Equipment4.2 Checking the Test Equipment for Response Variation

4.2.1 Checking the AC Voltmeter4.2.2 Checking the Audio Generator Distortion

Level4.2.3 Checking the Monitors

4.3 Test Documentation

SECTION 5

5.1

5.2

PRE -TESTING MAJOR SYSTEM COMPONENTS

Know the Characteristics and Interconnectionsin your Audio LinkFrom the Audio Console to the Aural Transmitter Input5.2.1 From the Audio Console to the STL Input5.2.2 From the STL Transmitter Input to the STL Receiver Output5.2.3 Pre -Testing the BTSC Stereo Generator

PAGE NO,1-1

2-1

2-1

2-1

2-22-32-32-42-42-52-52-6

3-1

3-1

3-2

4-1

4-1

4-24-2

4-24-34-3

5-1

5-15-35-35-45-4

5

5.3 Pre -Testing the Aural Transmitter 5-65.3.1 Sampling a Signal from the Antenna

Diplexer Output 5-65.3.2 Sampling a Signal from the Aural

Transmitter 5-65.3.3 Sampling a Signal from the IF

Stage of an IF Modulated Transmitter 5-7

SECTION 6 DATA SUMMARY SHEETS AND METHODS OFMEASUREMENT 6-1

6.1 Data Summary Sheets 6-16.1.1 Record of Test Equipment 6-26.1.2 Signal level and Line Impedance

at Major Test Points 6-36.1.3 Data Summary Sheet - Composite

Baseband Frequency Response 6-46.1.4 Data Summary Sheet - Main Channel 6-56.1.5 Data Summary Sheet for Stereo Channel 6-66.1.6 Data Summary Sheet for SAP 6-7

6.2 Composite Baseband Characteristics, Method ofMeasurement 6-8

6.3 Main Channel Characteristics, Method ofMeasurement 6-86.3.1 Total Harmonic Distortion 6-86.3.2 Signal -to -Noise Ratio 6-96.3.3 Crosstalk Into the Main Channel 6-9

6.4 Stereo Channel - Method of Measurement 6-96.4.1 Pilot Injection Level 6-96.4.2 Pilot to Interference Ratio 6-106.4.3 Stereo Operation With and Without

Companding 6-106.4.4 Signal -to -Noise Ratio in L and R Channel 6-106.4.5 L and R THD Measurement 6-116.4.6 Crosstalk Into Stereo Channel 6-116.4.7 Stereo Subcarrier Suppression 6-11

6.5 Second Audio Program (SAP) - Method ofMeasurement 6-126.5.1 SAP Noise Floor 6-126.5.2 SAP Channel THD 6-126.5.3 Crosstalk Into SAP Channel 6-126.5.4 SAP Subcarrier Frequency Measurement 6-13

6.6 Professional Channel 6-13

6

SECTION 1

INTRODUCTION

TV Stereo broadcasting is here to stayand the BTSC (BroadcastTelevision Systems Commit-tee) has outlined a standard for the U.S. market. It isa new challenge for TV broadcast engineers to converttheir stations from mono to stereo broadcasting. Thenecessary performance measurements are, quitepossibly, one of the most important tasks that a TVstation engineer has to face. The Proof is importantfor at least three reasons: First, a Proof gives you thesatisfaction that you are providing state-of-the-artstereo performance to the TV station's audience.Second, it gives a TV station the ability to demonstratethat it meets the FCC OST-60 technical guidelines, orat least better than the minimum guidelines. Third, ithighlights problem areas.

This publication was designed to pro-vide basic tutorial information on BTSC Stereo andthe FCC OST-60 Bulletin, as well as to fill the needfor a detailed guide to the TV aural transmitter andBTSC equipment performance measurements, basedon the recommended practices of the EIA TelevisionSystems Bulletin No. 5, MULTICHANNEL TELE-VISION SOUND, BTSC SYSTEM RECOMMEND-ED PRACTICES, dated July 1985. Every effort hasbeen made to answer questions that may arise and topresent an efficient test format, complete with easy touse forms and graphs. In addition to aiding theengineer with the preparation of the Proof, and in-depth analyses of each procedure and standard, thisbooklet provides the reader with a genuine under-

standing of what the FCC guidelines are, and howthese guidelines can be best met.

Ideally, every station would pass everyProof without a hitch, but in practice, this is notalways the case. Sometimes, it is more time con-suming to get a station to the point where it can passthe Proof than to actually make the measurements.For this reason, this guide also covers test proceduresfor sampling the BTSC signal at various points of theaural transmission system, and suggests methods ofisolating problems and correcting them so that theProof may be successfully completed.

While the use of this guide certainly hasthe effect of reducing the amount of time required tomake the equipment performance measurements, itsprimary intent is to assist the engineer to make a morethorough, accurate, and meaningful aural Proof ofPerformance.

Individual forms and graphs for record-ing test data are provided in this guide.

It is TFT's desire to update this publi-cation from time to time, and to supply this guide toTV broadcast engineers who would like to use it as astandard procedure to do a Proof. Your feedback,suggestions, or recommendations as to how this workcan be improved are greatly appreciated.

1-17

SECTION 2

FCC OST BULLETIN NO. 60 AND EIABTSC SYSTEM RECOMMENDED PRACTICES

2.1 FCC OST Bulletin No. 60

FCC OST Bulletin No. 60 containstechnical specifications for the BTSC Systemdeveloped by the Electronic Industries Association(EIA). These specifications have been publishedpursuant to the Report & Order in BC Docket 21323adopted March 29,1984, and are intended to be guide-lines for stations employing the BTSC system of multi-channel television sound (MTS) transmission andaudio processing.

The Commission, in BC Docket 21323,has adopted very general technical rules that will allowthe television aural baseband to be used for televisionstereophonic sound, second language programing,and any other broadcast or non -broadcast use.

The BTSC System uses a pilot auralsubcarrier at 15,734 Hz. The pilot allows receivers torecognize that transmissions are in stereo and toswitch into the stereophonic reception mode. Toensure compatibility and to prevent BTSC type re-ceivers from falsely detecting other stereo formats,FCC Rule 73.682(c)(3) restricts emissions at 15,734Hz by other TV broadcasters not using BTSC format.The purpose of this Bulletin is to give notice of theBTSC specifications referenced in Parts 73.681 and73.682(C)(3) of the Commission's Rules.

OST Bulletin No. 60 also definesspecial terminologies applied to the BTSC System,transmission standards, transmission system require-ments, modulation levels, baseband components, andencoding standards of the L -R channel.

2.1.1 Spectrum and Modulation Level

A summary of the BTSC compositesignal standard and modulation level of each baseband

AURALCARRIER

DEVIATION

kHz

MAINCHANNEL

L + R

fH

STEREO SUBCHANNELAM-DSB-SC

Encodd (L -R)

21H 31H

component is illustrated by the graph and table,Figures 2- 1A,B.

SECOND AUDIO PROGRAMFM CHANNEL

41H

1 PROFESSIONALCHANNEL

I III I

541 61H 6.51H

BASEBANO FREQUENCYFigure 2-1A 011=15.734 kHz)

2-1()

SIGNAL SPECIFICATIONS

Service orSignal

ModulatingSignal

MaximumModulatingFrequency

kHz

EncodingorphasisPreEm

SubcarrierFrequency

SubcarrierModulation

SubcarrierDeviation

kHz

Aural CarrierPeak

DeviationkHz

Monophonic L + R 15 75 µsec 25 t

Pilotf1-1 5

Stereophonic L - R 15 dbx Encoding 2 fH AM-DSB-SC 50 4.i

Second Audio

Program (SAP) 10 dbx Encoding 5 fH FM 10 15

Professional

Channel Voice or Data 1:5 150

0

µsec 61/2 FM

FSK3 3

t Total does not exceed 50 kHzTOTAL 73

Figure 2-1B: BTSC System Spectrum

2.1.2 Main Channel THD

Total harmonic distortion (THD) of the width shall not exceed the values in Figure 2-2 attransmitting system measured within a 30 kHz band- modulation levels of 25, 50, and 100%.

Audio Frequency AllowableTHD

MeasurementBandwidth

50 to 100 Hz 3.5% 30 kHz

100 to 7,500 Hz 2.5% 30 kHz

7,500 to 15,000 Hz 3.0% 30 kHz

Figure 2-2: Allowable THD versus Audio Frequency

102-2

2.1.3 Stereo Channel Separation

It is recommended that the transmissionsystem, excluding encoding, meet a 40 dB separationrequirement when 75 µsec pre -emphasis is substitutedfor sound encoding in the stereophonic sub -channel(L -R).

The minimum stereo separation withBTSC encoding at 10% modulation level and 75 µsecpre -emphasis network is shown in figure 2-3:

Figure 2-3: Minimum Stereo Requirement with Encoding of L -R

2.1.4 Crosstalk

OST-60 Bulletin also gives guidelinesof crosstalk between sub- channels as follows:

CHANNEL GUIDELINES REFERENCE

Stereo to Main -40 dB ±25 kHz Carr. Dev.

SAP/Pro to Main -60 dB ±25 kHz Carr. Dev.

Main to Stereo -40 dB ±50 kHz Carr. Dev.

SAP/Pro to Stereo -60 dB ±50 kHz Carr. Dev.

Figure 2-4: Crosstalk Guidelines Between Channels

2-311

2.1.5 Signal -to -Noise Ratio (Noise Floor)

And the signal-to-noise ratio in each ofthe service channels is as follows:

CHANNEL SNR100%

ReferenceMeasurement

Bandwidth De -Emphasis

Main -58 dB ±25 kHz 15 kHz Yes

Stereo -55 dB ±50 kHz 15 kHz Yes

Pilot (15,734 Hz) -30 dB ±5 kHz 1 kHz No

Stereo Subcarrier(31,468 Hz) -26 dB ±5 kHz 1 kHz No

Figure 2-5: Signal -to -Noise Ratio Guidelines

2.1.6 Other Recommendations

In addition to the recommendationslisted above, OST-60 recommends that the auraltransmitter operate satisfactorily with a frequencydeviation of ± 100 kHz, although the maximumallowable peak deviation is only ±73 kHz. It alsorecommends that the bascband frequency response ofthe transmitter be capable of transmitting frequenciesfrom 50 Hz to 120 kHz.

Other guidelines include that the pilot befrequency -locked to the horizontal scanning frequency

of the transmitted video signal. Although no recom-mendation is given to the phase relationship betweenthe pilot subcarrier and the horizontal scanning fre-quency of the transmitted video, it is generally agreedby the BTSC that the phase error should be less than±3 degrees in order to maintain good stereo per-formance.

The phase relationship between the pilotand the stereo subcarrier must also be kept within 3degrees (approximately ±0.53 micro -seconds).

122-4

2.1.7 SAP Channel

Electrical performance standards forSecond Audio Program (SAP) operation are alsogiven in OST-60. The SAP channel shall have a fre-quency response from 50 to 10,000 Hz. The SAP

subcarrier must be frequency -locked to the fifthharmonic of the horizontal line rate and it must be shutoff when the SAP subchannel is not in use. Theguidelines for THD in the SAP subchannel are asfollows:

Audio Frequency AllowableTHD

MeasurementBandwidth

50 to 100 Hz 3.5% 30 kHz

100 to 5,000 Hz 4.0% 20 kHz

5,000 to 10,000 Hz 3.0% 20 kHz

Figure 2-6: Allowable THD in SAP Subchannel

2.1.8 Crosstalk

Crosstalk into the SAP subchannel andsignal-to-noise ratio guidelines are as follows:

Channel Guidelines100%

ReferenceMeasurement

Bandwidth

Main Channel into SAP -50 dB ±10 kHz 50 Hz to 10 kHz

Stereo Channel into SAP -50 dB ±10 kHz 50 Hz to 10 kHz

SAP Channel SNR -50 dB ±10 kHz 50 Hz to10 kHz

Figure 2-7: Crosstalk Allowable into SAP Subchannel and Signal -to -Noise Ratioin SAP Subchannel

The signal quality of additional sub -carriers signal (e.g., Professional channel) on the au-ral carrier for non -program related purposes is notspecified in OST- 60 except that the arithmetic sum of

all subcarrieis, other than the stereophonic and secondaudio program, is limited to ± 3 kHz deviation of theaural carrier. Frequency response and THD are notspeci feed.

2-513

2.2 EIA / BTSC Recommended Practices

The Recommended Practices of theBTSC System for Multichannel Television Sound(MTS) have been prepared by the Ad Hoc WorkingGroup of the Multichannel Sound Subcommittee'sSteering Committee of the EIA Engineering Depart-ment's Broadcast Television Systems Committee.This document is intended to serve the industry in theform of recommendations for anyone wishing topractice Multichannel Television Sound (MTS) inaccordance with the BTSC system and the FCC Rulesgoverning its use in both the FCC Report and Order inDocket 21323 which authorizes MTS, and OSTBulletin No. 60.

The Recommended Practices is also ahandbook which gives comprehensive technicalcoverage of the transmission system requirements,monitoring and measuring, and discusses issues re-lating to TV receivers. Its Appendices and ReferenceSections consist of articles relating to BTSC stereoand recommended engineering practices.

This document can be purchased fromElectronic Industries Association, 2001 Eye StreetN.W., Washington, D.C., 20006.

14

SECTION 3

COMPANDING, STEREO SEPARATION AND MODULATIONACCURACY

3.1 Companding

The BTSC stereo system resembles theFM stereo broadcast system. The major differencesare the pilot and stereo subcarrier frequency, and theuse of companding in the L -R subchannel. BTSC com-panding is a noise reduction process used in both thestereophonic subchannel and the Second Audio Pro-gram (SAP) subchannel consisting of encoding (com-pression) before transmission and decoding (expan-

L - R

Matrix

Variable gainandPre -

emphasis

RMSdetector

L + R

sion) at reception. The specific BTSC encodingalgorithm is described in detail in OST Bulletin No.60. Tables for the amplitude and phase between theinput and output of the decoder can be found in theappendices of the EIA's Recommended Practices.BTSC decoding is complementary to the BTSC en-coding. A simplified system block diagram for theBTSC encoding and decoding is shown in Figure 3-1.

STEREOSUB -

CHANNEL

4- Bandpassfilter

75usPre -

emphasis

gainandDe -

emphasis

-+---Bandpassfilter

--0

MAINCHANNEL

RMSdetector

L - R

75usDe -

emphasis

Matrix

A

Figure 3.1: Block Diagram of the BTSC Companding System

Figure 3-1 shows that while the MainChannel (L+R) signal incorporates 75 µsec preem-phasis at the transmitter and 75 µsec de- emphasis atthe receiver (same as the monophonic standards), theL -R signal is "compressed" at the transmitter and

3-1

L + R

correspondingly "expanded" at the receiver. With aperfect transmitter path and perfect circuit com-ponents, the companding system shown in Figure 3-1is effectively transparent.

15

3.2 Stereo Separation and Modulation Accuracy

Stereo separation in the BTSC format isvery sensitive to gain and phase errors in the trans-mission path. This is because the L+R and L -Rsignals are treated differently. In particular, L -R iscompanded while L+R is simply pre -emphasized andde-emphasized. The L -R and L+R signals must arriveat the receiver's decoder matrix, which yields L andR, with very small errors in gain and phase across theentire audio band from 50 Hz to 15 kHz. Figure 3-2shows how stereophonic separation is affected bygain and phase errors in the L -R signal relative to theL+R signal at the input of the final matrix. Subjectivetests show that an average listener begins to perceive aloss in the spatial character of stereophonic musicmaterial when the separation drops below 18 dB, anda separation of 15 ± 3 dB is generally consideredadequate. Although the subjective effects ofseparation depend on the spectral distribution andother aspects of the audio material, it appears that agood engineering objective for the entire system is forseparation to exceed 20 dB in the mid -range,decreasing somewhat at frequencies above 8 kHz.Figure 3-2 shows that a separation of 20 dB requires again error smaller than 1 dB, and a phase error of lessthan 10 degrees. The BTSC standards require that thepotential separation of the radiated signal exceed 30dB in the mid -band from 100 Hz to 8kHz, but that itmay decrease at low frequencies to 26 dB at 50 Hz,and at high frequencies to 20 dB at 14 kHz. Thisrequires that the gain and phase errors

16 3-2

in the mid -band be smaller than 0.3 dB and 3.0 de-grees respectively.

The total modulation level accuracy inBTSC stereo is far more critical than in FM stereoradio broadcasting in order to produce acceptablestereo separation. Due to the fact that L+R and L -Rsignal paths in BTSC are critically different because ofthe companding in the L -R path, and the two paths arenot linearly related as in FM radio stereo systems, asmall change of modulation level in the BTSC systemwill affect the stereo separation because it alters theamplitude and phase relationship between the L+Rand L -R channel. If the total modulation level of theBTSC system is not maintained accurately, the dbx®decoder in the receiver will see an incorrect RMSlevel. The decoder, therefore, reproduces an L -R sig-nal having an altered amplitude and phase. An in-correct L -R signal is fed to the matrix, and conse-quently yields poor stereo separation. In other words,the decoding characteristics of the BTSC decoder aresensitive to the RMS level of its input signal. Sincethe RMS level to the input of the decoder is directlyproportional to the total modulation level therefore, thetotal modulation level in the BTSC transmitter must beaccurately monitored in order to maintain good stereoseparation.

® dbx is a registered trademark of dbx, Inc.

SEPARATION

dB

40

35

30

25

20

15

10

5

0

2°

30

4\4°

6\8°

10°

15° NO PHASE ERROR

20°25°30°Mk ----.

PHASE ERROR

GAIN ERROR

1 4 5 6 dB

Figure 3-2: Stereo separation as a function of gain and phase errors inthe L -R versus the L+R paths.

3- 3 17

SECTION 4

TEST EQUIPMENT AND DOCUMENTATION

4.1 Test Equipment

Frequently, half of the battle in doing aProof is in the preparation of the test equipment.Many hours of valuable time could be consumed at thetransmitter site if the equipment is improperly checkedout and out of calibration. Let's take a look at what

test equipment is principally required and discuss themethods of checking their operation. The better we pre-pare in advance, the less time it will take to do aProof. The following are recommended minimum testequipment and their features.

Type ofEquipment

Features

A. Audio Frequency Sine-wave Generator, two each

30 Hz to 120 kHz, 0.1% THDor better

B. Distortion Analyzer Having residual THD 0.1% orbetter

C. Digital AC Voltmeter ± 0.1 dB resolution and flatbetween 30 Hz and 120 kHz

Figure 4-1: Minimum Test Equipment and Features

Type ofEquipment

Features

A. Oscilloscope 10 MHz or higher

B. Variable Attenuator 600 ohm, 0.1 dB resolution

C. Digital Ratio Meter30 Hz to 120 kHz frequencyresponse, calibrated in dB

D. Digital Frequency Counter Up to 10 MHz response

Figure 4-2: Other Test Equipment and Features

Before proceeding, be sure that all testleads are equipped with the proper connectors formaking solid, well -shielded contact. Alligatorclip con-nections, sections of unshielded cable, and haphazard

4-1

grounds often cause needless grief and lost time.There is also an understandable psychological ad-vantage in starting out with a "tight ship".

19

4.2 Checking the Test Equipment for Response Variations

4.2.1 Checking the AC Voltmeter

Feed the output of the audio generatorinto the digital AC voltmeter, such as the TFT Model860 Multi -function Analyzer. The generator outputshould be held constant while the digital AC voltmeterindicator in dB is observed. Vary the frequency from30 Hz to 120,000 Hz and note any deviation from the400 Hz value which is more than 0.1 dB. If there isno change in the indication, then the frequencyresponse over the band that the Proof will cover is per-fect and no corrections will have to be made to thedata that are measured. If the digital voltmeter doesshow a deviation, record the error in 0.1 dB re-solution at 50 Hz, 100 Hz, 400 Hz, 1 kHz, 5 kHz, 10kHz, 14 kHz, 24 kHz, 31.5 kHz, 78.5 kHz, and 102kHz.

If the audio generator employed has abuilt-in AC voltmeter and is accurately calibrated overthe frequency range, you will find that it is quite easyto keep the output level perfectly constant while thefrequency is varied so that no calibration chart is re-quired. If the generator does not have a meter, thenobviously we are relying on the accuracy of the digitalAC voltmeter to check its output uniformity and itwould be a good idea to double-check it by using asecond meter and running the same spot checks. Ifyou must use a calibration chart with your generator,remember to subtract the generator errors from thetransmitter response deviations measured before en-tering them on the data sheet when you begin theProof.

4.2.2 Check the Audio Generator Distortion Level

A harmonic distortion analyzer readshum, white noise, and distortion as distortion.Consequently, we must remember that even if our gen-erator 'is producing a perfect waveform, its outputnoise level must be less than 0.1% or -60 dB for thedistortion analyzer to indicate less than 0.1%,assuming that the distortion analyzer is perfect. As apractical matter, most audio generators and harmonicdistortion analyzers exhibit a noise level of around -80dB or 0.01%. This is usually not a problem, as longas one is careful to avoid ground loops when makingthe connections to the equipment under test.

If the audio generator is fed directly intothe harmonic distortion analyzer, the total hum, noise,and distortion for the combination may be measured.For BTSC Proof measurements, if the reading is0.25% or less, the instruments may be consideredsatisfactory since this is 1/10 of the lowest recom-mendation for BTSC broadcast. Most distortion ana-lyzer and audio generator combinations yield a resi-dual hum, noise, and distortion level of about 0.1%.

CAUTION: The residual test equipmentdistortion MAY NOT be subtract-ed from the system distortion fig-ures when doing the Proof.

Subtracting the test equipment distortionis an invalid technique because distortions do notnecessarily add. As a matter of fact, the only timethey would add would be when all of the harmonicsare exactly in phase; a near impossibility when youconsider that this would have to be true for every mo-dulating frequency. Non-linearities can also canceleach other if their transfer characteristics arc comple-mentary. This accounts for the fact that a studio with1% distortion can be connected to a line amplifier with1% distortion and the line amplifier connected to atransmitter with 1% distortion. One might expect thesystem distortion to be 3%, but typically it would testat about 1-1/2 to 2-1/2% because of the fact that dis-tortion readings may not be added or subtracted.

It is a good rule of thumb that no por-tion of the system exceed 1/2 the distortion limit sinceat some modulating frequency the distortions couldadd. So to summarize, the fact that distortions usuallydon't add makes our broadcast systems better than thesum of their parts, but it also means that the test geardistortion cannot be subtracted.

The BTSC Committee recommends thatthe distortion measurements be made with a test band-width of 30 kHz (above the second harmonic of 14kHz, the highest frequency test input), a requirement

20 4-2

that is easily met since most harmonic distortionanalyzers will pass at least 40 kHz. Noise tests mustbe made with a 50 to 15,000 Hz bandwidth, also easyto meet since most audio voltmeters are flat to120,000 Hz.

A word of caution here: If the AC voltmeterhas a built-in 400 Hz high pass filter, it mustbe switched out. The required bandpassSTARTS AT 50 Hz. The high pass filter isgreat for getting hum out of the measure-ments, but not out of the transmitter! Thesefilters are installed in the test gear as adiagnostic aid to enable the user to determinehow much of a noise or distortion reading is

4.2.3 Checking the Monitors

Don't forget to check the operatingcondition of the modulation monitor as part of thisexercise. If the modulation monitor, such as the TFTModel 850 is equipped with a frequency synthesizedtype of FM modulation calibrator, adjustment of themeter calibration from the front panel pots is all that isnecessary. Otherwise, other type of modulationmonitors require additional test equipment to performa Bessel Null in order to assure the monitor's ac-curacy.

4.3 Test Documentation

A well -documented test procedure con-sisting of equipment list, setup block diagram, testpoints, and description will help speed up the Proofevery time you repeat it. It is well worthwhile tomake the initial investment so that it will pay off in thefuture.

hum and how much is white noise or distor-tion.

To check the residual noise level of theAC voltmeter or the distortion analyzer, short its inputand switch it to the most sensitive range. It shouldhave a noise level of more than 70 dB below themodulation monitor output level corresponding to100% modulation. Obviously, the noise in the testequipment may not be subtracted from the systemnoise measured. Since the BTSC residual noise limitis -58 dB, we should be careful to optimize the noiseperformance of the system and take care to keep thetest set-up noise free as this is not an easy test to pass,especially in stereo.

Don't forget the SAP and Professionalchannel monitor(s) if these subcarriers are part of thebroadcast operation. These monitors usually require asignal feed from the main modulation monitor. Cali-bration procedures can be located in the Owner'sManual.

Figure 4-3 gives an example of testpoints in a TV transmission facility, where the signalcan be sampled for monitoring and measuring theBTSC stereo performance.

4-321

STUDIO SITE

VIDEO

TFT 8500MicrowaveSubcarrierGenerator

(c ))AINlip

TRANSMITTER SITE

TV STL

F

STL -0 STL VIDEO VideoTx Rx Tx

BASEBAND

0

StereoGenerator

t tLeft Right

AUDIOCHANNEL

RF

TFT 850Modulation

Monitor

TFT 8501MicrowaveSubcarrier

Demodulator

0COMPOSITE

AUDIO

Diplexer

C) RF

C) IC)RFIF

IFMOD.

L.O.

AURAL TRANSMITTER

Figure 4-3: Test Points where Measurement can be Made

A written test procedure describing thecontrol settings of the test equipment and theequipment under test, the adjustment of all stationequipment relating to the test, and how the actualmeasurements are made, is an extremely usefuldocument for doing a Proof in the future. Havingthese records and documents also make it easier todelegate the job to subordinates.

Table 6.1.1 in Section 6 of this manualis an example of how the test equipment record can bekept for future reference. Having this informationavailable can speed up the process every time you dothe Proof. An equipment connecting diagram, similarto Figure 4-4, provides a permanent record of the testsetup as it illustrates the equipment interconnection.

22 4-4

AUDIO BTSCGENERATOR STEREO TRANSMITTER

OR 011 GENERATOR 0 UNDERSTEREO UNDER TEST

INPUT TEST

RF

ir

- - - - .; DEMOD

1

ir. COMPOSITE

rc0 0

00

TFTMODEL850/851

BTSC STEREOMONITOR

Figure 4-4: Example of Equipment Interconnections

4-5 23

SECTION 5

PRE -TESTING MAJOR SYSTEM COMPONENTS

5.1 Know the Characteristics and Inter -Connections in your Audio Link

Figures 5-1A and 5-1B show a trulybalanced, transformer -coupled circuit. Note that eachtransformer is center -tapped to ground. This circuitwill exhibit a degree of common mode rejection,which depends upon the accuracy of the transformercenter tapping or "balancing". The effect is identicalto that obtained with balanced push-pull amplifier cir-cuitry. Frequently, operational amplifiers are used toachieve the balanced input circuit. This type of inputcircuit usually exhibits better low frequency responsethan the transformer type input circuit.

Any hum or noise would have to enterthe top and bottom halves of the circuit, 180 degreesout of phase, to be passed to the output. Anyinterference affecting the top and bottom halves incommon, as a hum field would, causes its own can-cellation or rejection, hence, the term common moderejection. (This type of truly balanced circuitry makeslong distance wire transmission possible with sur-prising noise immunity, but is seldom found in studioequipment as the distance of transmission over theconnecting lines is usually a matter of feet rather thanmiles.)

(A)

(B)

T1

600 / 150 Q

It is very important to know how youraudio input is wired and unfortunately, the man-ufacturers usually call any transformer -coupled circuit"balanced". Figure 5-1C shows what would happenif the audio generator's unbalanced half groundedoutput were to be connected to a balanced input. Asyou can see, the impedance mis-match would be 2 to1. On the other hand, Figure 5-1C shows what hap-pens when a transformer with a balanced secondary isused to couple to a floating but unbalanced input con-figuration. It opens the door for "ground loops" dueto multiple ground connections. Adding a transformerto the audio generator's output is not a cure-all. Thereis no substitute for knowing the characteristics of theinput circuit and making an intelligent decision on theproper coupling technique. If connecting a match-ing transformer to your audio system is re-quired, be sure that it is included in your testequipment response checks so that any effectthat it might have is recorded. Load the secon-dary of the transformer with a resistor equal to thechannel input impedance, if it is an unbalancedsecondary. If the secondary is balanced, leave the

T2150 / 600

00 0

AUDIO GENERATOR

EFFECTIVE SHORTACROSS 1/2 PRIMARY

Figure 5-1A and 5-1 B: Direct connection across a truly balanced input can result in severemismatching and test conditions not simulating normal operation of the input circuit.

5-125

(C)

1

ANN, i Wr

Figure 5-1C: Multiple ground connections open the door for "ground loops"that could render the measurements invalid.

secondary center tap disconnected for coupling to theunbalanced voltmeter input so the transformer may bechecked.

If you have noise problems in your pre-test measurements, but these don't show up on theair, and are obviously in your test setup, try anisolation circuit between the modulation monitorinstrument output circuit and the harmonic distortionmeter/voltmeter input. We are assuming that the dis-tortion meter itself is well shielded; if changing theposition of the instrument changes the amount ofnoise, infiltration through the distortion meter housingitself should be suspected. The most common pro-blem, however, is the development of an RF potentialacross a ground loop. Remember that a quarter waveof FM and TV frequencies is only a couple of feetlong! Simple 60 Hz hum can invade the test input in

IEM 0

0 0

MODULATIONMONITOR

HI - Z OUTPUT

much the same way, of course.

If your modulation monitor does nothave a built-in distortion analyzer or digital AC volt-meter, you may want to consider the high impedanceisolation circuit shown in Figure 5-2 which usuallytakes care of either problem. Isolating the modulationmonitor ground from the distortion meter groundeliminates the ground loop, and the limited frequencyresponse of the transformer blocks the RF. The fre-quency response of the transformer must meet the30,000 Hz requirement. The transformer distortion isimportant too, because these measurements must bemade through it. Better quality matching transformerswill pass the instrument output voltage with less than0.1% distortion in most cases so it should not bedifficult to find a suitable unit.

221(Q

\ 00 0

Figure 5.2: A Ground Isolation Me thod

DISTORTIONMETER

265-2

5.2 From the Audio Console to the STL Input

For an engineer to be able to efficientlyprogress through the equipment performance measure-ments with a minimum of wasted time, the facilitymust of course be up to par. You must remember thatif you have completed part of the measurements, andthen find that an adjustment to the transmitter is re-quired, the tests that have been completed are usuallyinvalid. As a practical example, if we begin by mak-ing a complete frequency response and distortion mea-surement series only to find that a defect in the STLfrom the studio has rendered our noise level unusable,we must re -run the same series of tests after the audiopath problem has been serviced. Repeating the noisetest alone will not suffice because whatever repair or

adjustment was made to correct the noise could pos-sibly alter the frequency response or distortion per-formance.

Obviously, it doesn't take many of theseunexpected little setbacks to turn a seemingly simpleProof into an all -week affair. The best way to ensurethat this won't happen is to pre-test the major systemcomponents. There are many ways to quick -check afacility, but probably the best method is to determinewhich portions of the measurements will be the mostdifficult to pass and then prepare a pre-test procedurein order to be sure that the toughest requirements canbe met.

5.2.1 From the Audio Console to the Aural Transmitter Input

Use the techniques described in the be-ginning of this section. Feed an audio signal into theaudio console and terminate the output with the TFTModel 860 Analyzer or equivalent. Compare the mea-surements to the limits shown in Figure 5-3. Thesignal at the output of the audio console generallyfeeds the TV STL.

When the output of the audio console isterminated with the same impedance as the input of theTV STL Subcarrier Generator, the signal level mustbe adjusted to the same as the normal operating levelof the STL at 10% modulation on the subcarrier.Record this infomation on the data sheet provided inSection 6.

Pre -TestAudio

Frequency Limit

A. Check signal-to-noiseratio from audio consoleto the STL input

400 Hz Better than -66 dBbelow 100% modulationlevel

B. Spot check THD 50 Hz1000 Hz

14 kHz

0.5 %

C. Spot check audiofrequency response

50 Hz400 Hz14 kHz

Flat within ±0.1 dB

Figure 5-3: Pre -Test Limits from Audio Console to the STL Input

5-327

5.2.2 From the STL Transmitter Input to the STL Receiver Output

Similar methods may be used to checkthe STL system's audio channels. If dual subcarriersarc used to bring the R and L channels separately tothe transmitter site, each one of these channels shouldbe checked separately. The STL Owner's Manualgives detailed information as how to do a Proof of the

STL system, but the following data in Figure 5-4 canhelp you to determine if the STL has any major de-ficiency. Again, modulate the STL subcarrier genera-tor at the normal 100% level when doing thesechecks.

Pre -TestAudio

Frequency Limit

A. Check signal-to-noiseratio from the STLsubcarrier generator

400 Hz -65 dB below100% modulation

B. Spot check THD 50 Hz1000 Hz

14 kHzBelow 0.5%

C. Spot check frequencyresponse

50 Hz1000 Hz

14 kHz± 0.5 dB

Figure 5-4: Pre -Test Limits from STL Tx to STL Rx

5.2.3 Pre -Testing the BTSC Stereo Generator

There are a variety of stereo generatorsbeing used by TV broadcasters, some with built-inaudio processing and some without. It is important toreview the Owner's Manual to become familiar withthe controls and adjustments.

It is much easier to check out a stereogenerator, if the modulation monitor has a calibratedinput for the composite signal, as does the TFT Model

5-4

850. Using the Model 850 to measure an externalBTSC stereo composite signal requires that the ex-ternal stereo generator be matched to the monitor'sinput requirement. The best stereo separation isachieved when the output level of the encoder in thestereo generator matches the input level to the decoderin the monitor. The stereo generator must be capableof providing at least a 1 Volt RMS output level for±73 kHz deviation in order to operate the Model 850properly.

28

The level adjustment may be accom-plished by adjusting the output level of the stereo gene-rator or the input level to the monitor as follows:

a. Connect the BTSC stereo generator, a unitwhose encoder has been recently cali-brated, to the COMPOSITE INPUT on therear panel of the Model 850. Ensure thatthe INT/EXT switch on the monitor rearpanel is in the EXT position.

b. Apply a 1 kHz tone to the LEFT channelof the stereo generator.

c. Set the Model 850 to LEFT Channel(Keyboard Address 05). Set a referenceon the distortion analyzer using theLEVEL DB RATIO function.

d. Set the monitor to RIGHT channel (key-board Address 06) and adjust the follow-ing for best stereo separation:

1. Output of the stereo generator or,

2. The COMPOSITE INPUT LEVELADJ on the rear panal of the Model850.

e. When a composite input of 1 Vrms isachieved at the rear panel of the Model850, it is equivalent to ±73 kHz deviation.The system is now calibrated for makingmeasurements of the stereo generator.

f. Compare the data to the limits shown inFigure 5-5.

ModulationPre -Test LimitMonitor Readings

A. Check signal-to-noise 100 % 70 dBratio, use 400 Hz asreference

B. Spot check THD 100 % 0.5 %50 Hz1000 Hz14 kHz

C. Spot check stereo 10 - 30 % 36 dBseparation at 1000 Hz (Encoder in)

Figure 5-5: Pre -Test Limits for a BTSC Stereo Generator

5-529

5.3 Pre -Testing the Aural Transmitter

5.3.1 Sampling a Signal from Antenna Diplexer Output

Taking pre-test measurements from theoutput of the antenna diplexer can often reveal theperformance characteristics of the complete BTSCstereo system. Measurements made at this point re-flect the performance of the entire audio chain, the mo-dulator section, and the RF section.

The RF level available at the output ofthe diplexer or the RF feed line to the antenna isusually much greater than the signal from an outdoorantenna. It is necessary to know the approximatelevel of the RF voltage that appears at this test point,

since monitors such as the TFT Model 850 aredesigned to accommodate one to five volts of RF with-out overheating the input attenuator. If the voltage ismuch higher than 5V RMS, you should reduce it oradd a 10 to 20 dB attenuator pad in series with themonitor input.

After the test equipment is properly setup, connect the audio into the input of the aural trans-mitter. The limits for this pre-test should yield thefollowing minimum results:

Pre -Test AudioFrequency Limit

A. Check signal-to-noiseratio

400 Hz 60 dB below100% modulation

B. Spot check THD 50 Hz1000 Hz

14 kHzBelow 1.0%

C. Spot check frequencyresponse

50 Hz to 50 kHz50 kHz to 120 kHz

± 0.1 dB± 0.5 dB

Figure 5-6: Pre -Test Limits, Aural Transmitter

5.3.2 Sampling a Signal from the Aural Transmitter

If there is difficulty in obtaining thedesired performance in the pre-test step 5.3.1, goback one stage in the block diagram of the trans-mission link by making a pre-test measurement at theoutput of the aural transmitter. This process will ex-clude any degradation of performance due to the di -

plc xer.

The limits of this measurement ought toresult in data equal to or better than the numbersshown in Figure 5-6.

305-6

5.3.3 Sampling a Signal from the IF Stage of an IF ModulatedTransmitter

The TFT Model 850 monitor is equip-ped with a switch for selecting 41.25 MHz or 32.5MHz IF input at the rear panel for the purpose ofchecking the IF section of an IF modulated trans-mitter. If the aural transmitter tested per Paragraph5.3.2 does not result in meeting the pre-test limits, themodulating circuit, and/or the IF bandwidth of theaural transmitter may require service or adjustment,this will show up in this pre-test step.

If the station can pass the basic series oftests described in this section, then the chances arevery good that it will breeze through the completeProof, as the above requirements describe the all-outperformance demands placed on the system. It is wellworth the time that it takes to go through this elec-tronic assurance routine.

31

SECTION 6

DATA SUMMARY SHEETS AND METHOD OF MEASUREMENTS

6.1 DATA SUMMARY SHEETS

The purpose of the data summary sheetsis to provide an organized plan for recording the testresults as they are obtained. The value of this organi-zation will be appreciated later when we draw fromthis collection of figures for analysis of performance.

Tables 6.1.1 and 6.1.2 are for record -keeping of test equipment and characteristics of majortest points of the Transmission System as described inSection 4.3. The frequency response sections (Fig-ures 6.1.3 and 6.1.4) contain two columns: one forthe original audio generator output voltages and theother for the actual response deviation figures. It isimportant that any possibility for error in transferringthe test results from the meter face to the data sheet beeliminated, and that is the reason for recording theaudio generator output settings or exact level read by aAC digital voltmeter. While it is not difficult to figureout the response deviations mentally as the data is

measured, an error in addition or subtraction maynever be discovered and erroneous data would be re-corded. By recording the generator output, the engin-eer is free to concentrate on the tests at hand andworry about the math later. There is also a record ofthe original data that can be double-checked with thedeviation figures to ensure accuracy.

Filling in the distortion figure is straight-forward and can be done at the same time the responsedata are gathered. When recording distortion figuresof less than 1% or response deviations of less than 1dB, it is customary to place a 0 to the left of thedecimal point to preclude any ambiguity about whetherthe number is whole or fractional. In a group of num-bers, -.2 dB does not look too different from -2.0 dB,but -0.2 dB is at once recognizable as a differentnumeral.

6-1 33

6.1.1 Record of Test Equipment

Type Manufacturer Model No. Serial No. Date LastCalibrated

Audio Generator

Distortion Analyzer

Digital AC Voltmeter

Modulation Monitor

Stereo Generator

Frequency Counter

Oscilloscope

Recorded by:

Date:

6-234

6.1.2 Signal Level and Line Impedance at Major Test Points

Test Point Millivolts dBm Impedance

Input to Stereo Generator

Output from Stereo Generator

Input to SAP Generator

Output from SAP Generator

Input to the STL Subcarrier Generator

Output from the STL Subcarrier Demodulator

Recorded by:

Date:

6-335

6.1.3 Data Summary Sheet - Composite Baseband Frequency Response

5w

mN

0Lo+1

Freq(Hz)

Generatoroutput

AC meterreading

Correctedresponse

Freq

(kHz)Generator

outputAC meterreading

Correctedresponse

50 16.5

100 21.5

400 26.5

1k 31.5

5k 36.5

10k 41.5

14 k 47

waNIi

_Ne

LoCV

+1

63.7 k 94.4

71.2 k 98.7

78.7 k 103

86.2 k 111.5

93.7 k 120

Note: See Section 6.2 for instructions and limits

All Tests Performed By:

Date:

366-4

6.1.4 Data Summary Sheet - Main Channel

Lu

oNIY

1-0cJ

+1-o02eo0-

Freq(Hz)

Generatoroutput

AC voltmeterreading

Correctedresponse

50

100

400

1k

5k

10k

15k

>w0N

11-qN.-+1

0o2c)c)in

50

100

400

1k

5k

10k

15k

>''w0rJ

_Nc

Lr)N6+1

Oa2

LON

50

100

400

1k

5k

10k

15k

ggM

AM Signal -To -Noise Ratio - dB

FM Signal -To -Noise Ratio - dB

Crosstalk from L - R - dB

Crosstalk from SAP - dB

All Tests Performed by:

Date:

Note: See Section 6.3 for instructions and Section 2 (Fig. 2-2, 2-4 and 2-5) for limits.

6-537

6.1.5 Data Summary Sheet for Stereo ChannelPilot Injection (15,734 Hz) % Signal -To -Noise Ratio, Right Channel - dB

Pilot Interference Level dB Signal -To -Noise Ratio, Left Channel

Stereo Separation without Companding

Residual ResidualLeft Frequency Right

in Right (Hz) in Left(dB) (dB)

50

100

400

1k

5k

10k

14k

Total Harmonic Distortion

THDRight

Channel

Frequency(Hz)

THDLeft

Channel

50

100

400

1k

5k

10k

14k

Stereo Separation with Companding

Residual ResidualLeft Frequency Right

in Right (Hz) in Left(dB) (dB)

50

100

400

1k

5k

10k

14k

Crosstalk Into Stereo Sub -channel

Frequency(Hz)

Due Dueto Main to SAPChannel Channel

100

400

1k

5k

10k

Stereo Subcarrier (31,468 Hz) Suppression - dBAll Tests Performed By:

Date:

Note: See Section 6.4 for instructions and Section 4 for limits.

dB

386-6

6.1.6 Data Summary Sheet for SAP Channel

SAP Channel Signal -To -Noise - dB

Frequency Response and THD of SAP Channel

Freq(Hz)

50

100

400

1k

5k

10k

Generator AC meter Correctedoutput reading response

THD

Crosstalk due to Main Ch.(4.5 kHz, ±25 kHz Dev.)

dB

Due to Stereo Ch. and Main Ch. dB( 2 kHz L=R, 8.6 kHz L only)

SAP Subcarrier Frequency (78,670 Hz) Hz (±500Hz)

Note: See Section 6.5 for instructions and Section 4 for limits.

All Tests Performed By:

Date:

6-7 39

6.2 Composite Baseband Characteristics - Method of Measurement

Data Summary Sheet 6.1.3 is recom-mended for use to measure the baseband frequencyresponse of the aural transmitter. In order to achievestereo separation in excess of 40 dB in the radiatedsignal of a transmitter, it is required that the amplituderesponse should not vary by more than +/-1.0 dB andthe phase response should not vary by more than ±3.0 degrees, over a band of frequencies from 50 Hz to47 kHz and for an aural carrier deviation of 50 kHz.Since such tight tolerances of amplitude and phase aredifficult to measure, an alternate practice may be used.This alternate method is to measure stereo separationwithout the compander, (first table in Paragraph6.1.5). If the results are 40 dB or more over therange of frequencies and signal levels a kHzdeviation), the

amplitude and phase tolerance over the band of fre-quencies from 50 Hz to 47 kHz can be considered tocomply with the recommended practice.

For the frequency band from 47 kHz to120 kHz, the amplitude response should not vary bymore than ± 1.0 dB from a phase response. Thephase response should not vary by more than ± 10degrees, both conditions are for an aural carrierdeviation of + 25 kHz.

This measurement should be performedwith the aural modulation monitor in the split -soundmode.

6.3 Main Channel Characteristics - Method of Measurement

The main channel is the segment of thedetected aural composite baseband spectrum thatranges from 50 Hz to 15,000 Hz. The main channel

6.3.1 Total Harmonic Distortion

This measurement requires the use ofaural monitoring and measuring equipment in the split-

sound monitoring mode. The single tone audio inputto the transmitter should be supplied from a sourcehaving less than 0.055% total harmonic distortion.The measurement is typically performed with a totalharmonic distortion (THD) analyzer. The instrumentshould have a residual THD of its own of 0.1% orless. The amplitude of the audio signal should beadjusted to keep the aural carrier frequency deviationconstant at 6.25 kHz or 12.5 kHz or 25 kHz. The 75µsec deemphasis network is used. The aural carrier is

signal represents, within specified tolerances, the L+Rsignal that modulates the aural transmitter with anaudio frequency range of 30 Hz to 15 kHz.

modulated by the main channel only; the pilot and allsub -carriers are off. If the visual transmitter isequipped with a notch-diplexer, it can be turned eitheron or off. Verification that this has no influence onthe measurement results is recommended. Visualtransmitters without notch diplexers should be turnedoff. The monitor should be fed from a directionalcoupler in the transmission line that feeds both visualand aural (if not possible, aural) carriers to thetransmitting antenna. If an off -air feed is used, theoperator should be aware of the possibility thatmultipath effects may increase measured distortion.

6-840

6.3.2 Signal -to -Noise Ratio

The radiated aural signal may includeAM noise and FM noise. Additionally, the visualcarrier may have spurious visual phase modulation inthe frequency range of the main channel and the auralcarrier may be contaminated by spectral overflow.OST 60 (C) (a) (13) allows at most -58 dB FM noisein the main channel of 50 - 15,000 Hz (25 kHz peakdeviation as reference, 75 usec deemphasis). TheOST rule is to be interpreted to hold when the visualtransmitter is turned on. It is recommended that theType I (diagonal) pattern be used. Transmitters with-out notch diplexing should use a Type II pattern (multi -burst).

Set up a zero dB reference for 100%modulation (± 25 kHz deviation) by applying a 400Hz tone equally to the L and R channels of the stereogenerator. Measure the residual noise level on thedigital AC voltmeter after switching off the 400 Hzreference tone. Turn on and off the visual transmitterwith program material and record the difference innoise measurements. Add pattern to the visual carrierand check for degradation in SNR.

6.3.3 Crosstalk Into the Main Channel

Monitoring the output of the mainchannel output, set up a 0 dB reference (± 25 kHzdeviation) by applying a 400 Hz tone to the L and Rchannel, (L+R) of the stereo generator, using the split -sound monitoring mode. Remove the 400 Hz tone,and measure the residual noise in the main channel byapplying an audio signal L= -R to the stereo generatorwith the compressor bypassed. Adjust the level for ±50 kHz stereo subcarrier deviation of the aural

transmitter and sweep the audio generator from 50 Hzto 14 kHz; record the worst reading in dB and audiofrequency at which the reading is taken. Repeat thisprocess by modulating the SAP channel without thecompressor (encoder out) in the circuit. Adjust theSAP to a deviation of ± 10 kHz and an injection levelto the aural transmitter of ± 15 kHz. Measure theworst case residual noise in the main channel. Recordlevel in dB and frequency.

6.4 Stereo Sub -Channel - Method of Measurement

6.4.1 Pilot Injection Level

OST 60 specifies aural carriermodulation by the pilot subcarrier of 5 kHz deviationwith a tolerance of ± 0.5 kHz. For this measurement,a 15.734 kHz pilot frequency bandpass filter is usedto separate the pilot from other modulating signals.The operator should be aware that the radiated signal's

pilot carrier can be contaminated and interfered byhorizontal video components. The source of interfer-ence may be located by turning the visual carrier onand off and also by switching in different types ofvisual patterns.

6-9 41

6.4.2 Pilot to Interference Ratio

The ratio between the nominal pilot levelin the aural radiated signal and the RMS interferencein a 1 kHz bandwidth at a center frequency of 15,734Hz is 40 dB. OST 60 specifies this value as the cur-rent standard, but the EIA committee is proposing tolower this to 30 dB. The measurement shall beperformed with the monitor in the split -sound mode

and with thevisual carrier turned on. Also themeasurement is to be made in the split sound modeand in the intercarrier mode (if available). Adjust theaural carrier deviation by the pilot to 5 kHz. Measurethe pilot level on the monitor "pilot" position. Turnoff the pilot and take the reading on the digital ACvoltmeter.

6.4.3 Stereo Separation With and Without Companding

Recommended stereophonic separationwithout BTSC companding is 40 dB (OST 60) overthe band of frequencies from 50 to 14,000 Hz.Stereophonic separation, including BTSC compand-ing, at 10% and 75 µsec equivalent input modulation,shall meet or exceed the following requirements:

a. 30 dB separation from 100 Hz to 8 kHz;

b. Smoothly decreasing separation below 100 Hz to26 dB at 50 Hz;

c. Smoothly decreasing separation above 8 kHz, to20 dB at 14 kHz;

d. At other 75 µsec equivalent input modulationlevels between 1% and 100%, from 100 Hz to 8kHz, the separation shall equal or exceed 26 dB.

Note there is no BTSC recommendationregarding separation above 14 kHz when BTSCcompression is included. The reason for this is that atthe time of this writing, no information is available asto the long-term stability of skirt response of the verysharp cutoff (L+R) and (L -R) audio lowpass filters inthe BTSC stereo generator.

A sine wave signal variable in the bandof frequencies from 50 Hz to 14 kHz is supplied tothe left (or right) audio input port of the stereogenerator. The other audio input is grounded. Afteradjusting the aural carrier deviation, the reference levelis measured (without 75 µsec deemphasis whenmeasuring without companding) at the left (or right)audio output. Next, the residual level is measured inthe right (or left) audio output. A visual transmitterwithout notch-diplexer should be turned off.

6.4.4 Signal -to -Noise Ratio in L and R Channel

The aural carrier stereophonicsubchannel radiated signal may include thermal noise.Additionally, the visual carrier may be one of thepotential interference sources, some of which maycause noise and/or interference in some receivers.The potential visual interference sources are spuriousvisual phase modulation (example ICPM) and auraland visual phase noise in intercarrier-sound receiversas well as in split -sound receivers. The stereophonicsubchannel, when not (or only partially) energized,may include input noise amplified by the BTSCcompressor.

OST 60 specifies -55 dB or less FMnoise in the stereophonic channel radiated signal,

referenced to 50 kHz aural carrier deviation, andmeasured after demodulation without BTSC expand-ing but with deemphasis. The modulation monitor isfed with an RF signal from a directional coupler in thetransmission line feeding the antenna, using theforward wave. The function switch of the monitor isswitched to (L+R) + (L -R) position.

Apply a 400 Hz test tone to the input ofthe Stereo Generator and allow L= -R. Set the modu-lation level of the transmitter to 50 kHz deviation withthe BTSC decoder out of demodulation path. Thenmeasure the noise level after switching off the 400 Hzreference tone. The 75 µsec deemphasis networkshould be selected.

42 6-10

6.4.5 L and R Channel THD Measurement

OST 60 lists the following maximumdistortion percentages:

3.5% (RMS) (50 - 100 Hz)2.5% (RMS) (100 - 7,500 Hz)3.0% (RMS) (7,500 - 15,000 Hz)

The modulation percentages are 25, 50,100% of 75 usec equivalent input modulation, withharmonics to be included to 30 kHz. (BTSCcompanding is included.) The measurement should

be perfonned with the monitor in the split -soundmode.

6.4.6 Crosstalk Into Stereo Channel

Apply a sinewave of 50 Hz to 14,000Hz to the input of the Stereo Generator, allow L= -R.Adjust the modulation level of the aural transmitter tothe specified (12.5, 25, or 50 kHz). Measure thedistortion of the signal at the L (or R) audio output ofthe monitor with a distortion analyzer. Make certainthat the audio frequency accuracy should be sufficientto prevent measurements beyond 14 kHz.

Crosstalk into the stereophonic b.subchannel is the ratio of the reference level in thestereophonic subchannel to the level measured in thestereophonic subchannel when only the crosstalking c.channel(s) is (are) energized to its (their) nominalsubcarrier and/or aural carrier deviation. Crosstalkattenuation from the main channel into thestereophonic subchannel is required to exceed -40referenced to 50 kHz deviation and 60 dB fromanother subchannel. The measurement shall beperformed with the monitor in the split -sound mode.When measuring an internally diplexed transmitter,the visual carrier shall be turned off.

a. Apply a 400 Hz tone, L= -R, without BTSCcompanding. Adjust the level to 50 kHz stereosubcarrier deviation. Remove the pilot and allother subcarriers.

6.4.7 Stereo Subcarrier Suppression

OST 60 requires less than 250 Hz aural a.

carrier deviation by the level of the spurious 31,468kHz signal component in the radiated aural carrier.This level should be 46 dB below the reference level b.

of ± 50 kHz. The measurement shall be performedwith the monitor in the split -sound mode. When mea-suring an internally diplexed transmitter, the visual car-rier should be turned off.

Set the reference level (t 50 kHz deviation) usingthe L -R reading of the monitor.

Switch the modulation input to L=R and, ifpossible, switch off the stereophonic subchannelfrom the radiated signal. The digital AC volt-meter reading is the crosstalk from the main chan-nel (andunless the subchannel is off). To measure cross-talk from another subchannel, remove the L, Rinputs and energize the other subchannel to thenominal deviation and injection. The digital ACvoltmeter reading is the crosstalk. For each case,vary the modulating frequency in the talking chan-nel for constant deviation, to determine themaximum crosstalk in -dB.

Set up the reference level as under CrosstalkMeasurements.

Remove modulation, pilot, and all subcarriers.Read the level from the monitor's 2 x PILOTposition in dB.

6-11 43

6.5 Second Audio Program (SAP) - Method of Measurement

The SAP subchannel is the segment ofthe detected aural composite baseband spectrum thatranges from 63 to 94 kHz. The SAP subcarrier of anominal frequency of 78.670 kHz is frequency mo-dulated by the BTSC compressed SAP signal, limited

6.5.1 SAP Noise Floor

OST 60 requires the noise floor to be-50 dB or better below 100% modulation (± 10 kHzsubcarrier deviation and ±15 kHz injection level to theaural carrier). A SAP monitor used in conjunctionwith an AC digital ratio meter would be handy for this

6.5.2 SAP Channel THD

The SAP distortion is the total harmonicdistortion of the modulating signal of the SAP sub-carrier of the radiated aural carrier.

OST 60 lists the following maximumdistortion percentages:

3.5% (RMS) (50 - 100 Hz)4.0% (RMS) (100 - 5,000 Hz)3.0% (RMS) (5,000 - 10,000 Hz)

Measure at modulation percentages of25, 50, and 100% 75 µsec equivalent input mo-dulation. (100% corresponds to 10 kHz SAP sub-

carrier deviation.) The measurement shall be perform-ed with the BTSC monitor in the split -sound mode.

6.5.3 Crosstalk Into SAP Channel

The crosstalk into the SAP channel mustbe equal to or less than -50 dB according to OST 60.This measurement shall be performed with the auralmonitor and measurement equipment in the split-

sound mode. When measuring an internally diplexedtransmitter, the visual transmitter shall be turned off.

to an audio bandwidth of 10 kHz, to a peak deviationof ±10 kHz. The modulated SAP subcarrier mod-ulates the aural carrier to a peak deviation of ±15 kHz.A SAP monitor is required to perform this Proof.

measurement. First, set up a 100% modulation levelby reading the output via the SAP monitor. Let thisreading be the 0 dB reference at the ratio meter.Switch off the modulation and read the residual noiselevel in dB from the AC digital ratio meter.

When measuring an internally diplexed transmitter,the visual transmitter shall be turned off.

Adjust the SAP subcarrier deviation to75 µsec equivalent input modulation at the chosenmodulation percentage and at the chosen modulatingfrequency. To achieve this, replace the BTSC com-pression by 75 µsec preemphasis. Feed a sine waveof the chosen frequency into the BTSC SAP generatorand adjust the level until the desired deviation isreached as read on the SAP monitor. Next, switch theBTSC compression back into the circuit (replacing the75 µsec pre -emphasis). Measure the level at themonitor SAP output with a distortion analyzer andsubsequently measure the total harmonic distortion(THD).

a. The reference level in the SAP channel ismeasured as follows:

(1) Replace BTSC compression with 75 µsec pre-emphasis.

(2) Modulate the SAP subcarrier with a 400 Hztone and adjust the level to a deviation of 10

44 6-12

kHz as read on the SAP monitor. Verify thatthe SAP subcarrier injection is 15 kHz.

(3) Measure the SAP audio output with a digitalAC voltmeter via 75 µsec deemphasis (replac-ing BTSC expansion). Turn off the 400 Hzsignal.

b. To test crosstalk from the main channel into theSAP channel, modulate the aural carrier with amonophonic 4.5 kHz tone to a deviation of 25kHz. Measure the crosstalk on the digital AC volt-meter.

c. To test crosstalk from the composite stereo sub -channel into SAP, modulate the aural transmitteras follows:

d.(1) Apply a 2 kHz tone (L=R) to the main chan-

nel to produce an aural carrier deviation of12.5 kHz.

(2) Apply a Left -only 8.6 kHz tone at a level re-sulting in a total aural carrier deviaton by themain and (BTSC compressed) stereo subchan-nel of 25 kHz. (A second oscillator isnecessary.)

(3) Applying both the 2 kHz signal and the 8.6kHz signal should result in aural carrier devia-tion by the main channel only of 25 kHz andby the composite stereo signal (minus pilot)of 50 kHz.

(4) The pilot and the SAP subcarrier should beturned on. (The resulting modulation is des-cribed in detail in Appendix I of the BTSC Re-commended Practices.)

Measure the crosstalk at the SAP audio outputwith the digital AC voltmeter.

6.5.4 SAP Subcarrier Frequency Measurement

The SAP subcarrier frequency is nomi-nally equal to 5fH = 78.670 kHz (fH = horizontalscanning rate = 15,734 Hz). When frequency modu-lated, the average carrier frequency may show a

6.6 Professional Channel

The TV station operator has consider-able freedom in operating non- program related sub -channels; details are found in Section 2.2.8 of theRecommended Practices, where FCC and OST re-ferences are also listed. The professional channel sub -carrier frequency is centered at 102.271 kHz.

The only measurements relating to theProfessional Channel are to ensure the injection level

deviation from the nominal value. The SAPsubcarrier frequency may deviate from the nominalvalue of 5fH = 78.670 kHz by not more than +300 Hzin the absence of modulation.

is no more than + 3 kHz deviation of the aural carrierand the peak deviation to the Professional subcarrier isno more than ± 3 kHz.

The Professional Channel should notintroduce noise to the SAP and stereo channel. Thistest can be added on to the noise floor measurement inthe stereo and SAP noise floor test by turning theProfessional Channel subcarrier on and off.

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