RESEARCH DEP ARTTvlENT

SUBJECTIVE INVESTIGATION INTO ADJACENT C~l. INTERFERENCE -SITH 8,9 and 10kc/s CARRIER SEPARATION

Report No. G.039 Serial No. 1948/16

RESK~RCH REPORT N,-,. G. 039. -Serial Nu. 1948/16.- June 1948

Work ccrried out by -D. Mnurice.

Drawing Nos. 0039.1 to 0039.8.

J. Spencer, G. F. N:.:well.

lflritten by -G. F. Newell.

1. ~ummDry.

2. Introduotion.

SUBJECTIVE INVESTIG::..':::'ION IfITO i:.DJ~l.CENT cHdl'JN'4 UrTE@,El{ENCE 1:HTH, 8, 9 and 10 kcls C~lRRIER SEE1R~TION.

Thts report decls with experiments carried out to determine the permissible increase in the Rmplitude of en adjecentcsrrier (for El predetermined interference level) when;the carrier separation is increased frC'm 8 kC/s to 9 kC/s and then to 10 kC/so The experiments were made using two typic~l commercial broadce.st reoeivers and with various types of programme on both the wonted snd interfering cerriers. ki.nalysis of E; number of subject:!.ve observations indicates that for a given level of interference the amplitude of Fm 8djecent carrier cen be increased by between 4.5 end 6 db when the oarrier separation is increased from 8 kC/s to 9 kC/ s.

, Similarly 8 further increese of between 4.5 to 5.5 db is permissible if the carrier seperl1tion is mede 10 kC/so

In order to ascertein the effect on 8djacent crrrier interference of altering the carrier separet10n from 9 ke/s to 8 kcls or 10 kO/s it wes decided to measure, the retio of unwBntod to wanted carrier amplitudes which produced similar levels'of interference et aBeh of the three carrier seperat10n frequencies. For convenience it wes ettempted to synthesise the effect of two

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transmi tters hcving v3riable sep8rction and each modulBted with a different programme on e receiver tuned to either one of the cr.rriers.,. Two receivers were e~osen for these experiments. One JV8S (l good commercial receive~ of reesonably low cost and the other a more expensive type of receiver haAing foU.r bsndwidths. This letter receiver wns only tested with a medium oendwidth of approximetely +4.5'kc/s.

3. Description of Experiment.

3.1. General Method.

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The output terminals Of two standard signal generators Were connected in parallel through e common ,dummy antenna to the aerial terminals ot the reoeiver to 'be tested. The reoeiver was tuned to 1000 kO/so One of the signol generators was olso tuned to this frequency end used to supply the wanted programme. The other signel generator weB tuned to 8 kols lower then 1000 kc/so The generators were modulated with separste programmes arranged to peak to 1Oa% in order to simUlate broadcast transmitters. The wanted carrier was adjusted to produee 3 millivolts R.M. S. amplitude at the input of the dwnrny antenna. The unwanted earrier amplitude was adjusted until en observer sitting in front of the reoeiver decided that e predetermined level of interferenoe was produoed. The ratio of :the two earrier amplitudes wes noted and the unwanted cerrier tuned to 9 k~/s below 1000 kO/s. When the unwanted oarrier amplitude had been re-adjusted to produce the same level of interterenoe'es had previously been noted, the ratio of wanted to unwant~d oarrier amplitudes was again reoorded. The unwanted oarrier was tuned to 10 kc/s below 1000 kcls and the procedure repeated with this separation. This complete experiment weB repeated tor eaoh of the conditions 01' wanted and unwanted programmes outlined tn paregreph 3.2. Eaoh experiment was repeated by at least two observers and in some oases tour separate observers took part. The results of each experiment were then averaged to minimise theeftect of errors introduced by the observers unintentionally changing their oriterion 01' judgement 01' the interferenoe level. In order to test the effect of the wanted carrier amplitude on these results the experiments were repeated with en input trom the wanted carrier 01' 10 millivolts through the dummy antenna.

3.1.1. Level of .~udio Interterence Chosen.

The tests desoribed in the previous paragraph we~e all repeated with three levelsot audio interterence. These grades were called

1. !'P" or "Perceptible" interferenoe which corresponds with a 50 db programme to noise retio.

2. "S.D. "or "Slightly disturbing" interferenoe t 40 db.

3. "D" or "Disturbing" interferenoe, 30 db.

The grades differ from one another by approximately 10 db ~s shewn in the table above. The lowest level used, "perceptible", 1s 10 db obove the smallest noticeable level, "just perceptible", with oareful listening and during the absenoe of modulation.

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3.2. Types of Progrnmme Used.

The programmes were recorded for convenience ·of repetition. The recording WDS specially c8rried out by the Rcicording Section of Research

,~, Dept.' 2nd wes of the highest quelity obtainable with existing cellulose disc recording technique. The frequency chprecteristic wes substantially flet to 10 kc/s cnd the discs wero repl",ced when the,beckground noise level was considered likely to interfere with the rosults of the eXperiments. The tests were made with each of the following condi tioils

liJ~...NTED PROGR.: .. Mlv;E UNVUNTED PROGR.OOiiE

1 Piano solo News reading

2 {t .t Theatre Orchestra -

3 it f' Military BRnd 4 News Reeding, Theatre Orchestra

5 Sports Commentary It " .

3.3. Moduletion of CRrriers.

Fig 1 shows the arrengemcmt of equipment used in the tests. Both signal generators were moduleted by identical but independent programme channels, therefore only one will be described. The source of programme wes A gramophone turntable, whose pick-up supplied a pre-amplifier fitted with_a peak programme meter &~d control potentiometer. The output of th3 pre-amplifier was connected to a power amplifier producing an output capaile of modulating the appropriate signr::l generator up to 100%, using a B.B. C. peak programme meter. The, frequency ch3racteristics of the modulation were flat with ± 1 db between 30 c/s and 10 kC/so These ere shown in figs 4 and 5.

3.4. Description of Receivers used.

Receiver number one was 0 PYE 15:~ which is 8 three valve , I plus rectifier v

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The ove~all selectivity curve of this receiver is shown in Fig. 2 and the over-all fidelity curve is shown in Fig. 3. The second receiver was a Murphy ~4. 40C which is e more expensive reoeiver having 12 valves plus 2 rectifiers and a tuning indicator, and has automatic tuning control. In the condition in which the receiver was tested (i.e. medium wave-band and medium bandwidth with .:~.T.C. switched off) the circuit consists of a triods pentode mixer preceded by e band-pass coupled circuit. The .mixer is followed ~y a single I.F. amplifier stage feeding a diode detector. The overall selectivity curve is shown in Fig. 2 and the over-ell fidelity curve is shown in Fig. 3.

4. Discussion of Results,

Tables land 2 give the ratios of wanted to unwanted oarrier amplitudes for various conditions of test on the two receivers. Table 1 refers to the first receiver tested while Table 2 refers to the second. Each retio given in the tables is in fact the average of at least two· subjective tests, ihis number is not sufficient to smooth out individual voriation~ in assessing the interference level by subjective means. These individual variations are epparent on examination of Tebles 1 and 2. The effect of different types of programme is apparently masked by these individual variations. ~verage figures from Table 1 show that en unwanted carrier one quarter the amplitude of the wanted carrier will cause e "slightly disturbing" audio interference when the carrier separation is 8 kc/so If the separation is increased to 9 kC/s the unwanted amplitude may be increased to about one half that of the wanted carrier in order to produce the same grade of audio interference. With 10 kC/s separation the two eerriers become approximately equal for· the same gr~de of interference. Table 2 shows that for similar oond1tions using the second receiver the unwanted oarrier must be twice es big cs when reoeiver number one is used. Tables 10. end2e show the effect of ch~nging the oarrier separation from 9 kO/s to 8 kO/s and from 9 kO/s to 10 kO/s. These tables show that the ratio of unwanted carrier amplitudes for 9 to 8 kO/s or from 10 to 9 kO/s for 8 given grade of audio interference is apperently not affeoted IY the actual grade ohosen, nor iy the amplitude ot the wanted carrier within the renge covered iy these tests.

6. Conolusions.

The two receivers tested differed by 2:1 in the required· amplitude of adjacent carrier for 0 given grede of eudiointerferenoe. Equal emplitude carriers produce 8 "slightly disturbing" interferenoe with 10 ko/s separation using the first receiver and 9 kc/susing the second.

Tests on the first receiver indicate that the permissible inorease of an adJacentcerrier) for a constant level of interference, when the

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carrier separation is increased from e kols to 9 kc/s is 2:1 or 6 db. For a change of aarrier seporotion from 9 kc/s to 10 ko/s the figure is 1.8; 1 or 5.6 db. The results of the tests with the seoond receiver give for the same eonditions a figure of 1.75:1 or 5.db for an increase of carrier separation of from 8 kC/s to ~ kcls and a further 1.7 times or 4.5 db when increased from 9 ko/e to 10 kC/so These,figu:r:as are not appreciably ehenged by the type of programme on either carrier nor the actual grade of interference chosen for the tests, within the range from perceptible to disturbing interference. The selectivity of the rec€iver used will however definitely effect ,the figures quoted above.

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~ESEARCH DE~ BBC SUBJE.CTIVE INVESTIGATION INTO ADJACENT CAR2IER. INTE2F'ER.ENCE 8, 9 ~-+--=-l

~DS-/4-2-/P-i& 10 Kc/s CA~"IE.R. SEPARATION.

RE.PORT

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This dra w ing specification IS the property of the British Bro adcasting Corporation )nd may not be reprodua. or disclosed to a third party In any (orl11 withOut " writ ten pe rmiss ion of the Corporation

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Ratios of wanted carrier amplitude for the fl rst receiver tested unwanted cnrrier 8ffiplitude

wanted carrIer ampll tude unwonted carrier amplitude

RoM.So amplitude of wanted ca 'ler 3 mi111volts 10 mill1volts

Carrier separation in kc/a 8 kc/s 9 kcls 10 kcls 8 kcla 9 ko/s 10 kcls

Vtanted Unwanted Grado of ratio db ratio db rati< db ratio db ratio db iratl0 db pro,ramme pro~ralllJlle audio ,

interter- . ence

.i>1ano solo News reasl1l1g F 9.0 19 6.0 15.5 2.9 9.0 37.0 31.5 16.0 24.0 12.0 121. .. 5 tf .. " .. SD 5.3 14.5 2.7 8.5 1,,1 1.0 3.9 12.0 2.0 6.0 1.1 1.0 " .. 11 .. D 2 .. 0 60 0 0.81 -2.0 0.5 -6.0 1 .. 7 4.5 0 .. 9 '-1.0 0.6 -4.5

1'l8no solo Theatre Orch P 21.0 26.5 8.0 18.0 3.3 10.5 16.0 24.0 6.3 16 4.2 12.5 " .- .. " SI) 4.2 12.5 3.6 11.0 1.4 30 0 6.~ 16.5 1.5 3.5 0.07 -1.5 .. .. .. .. D 1.6 4.0 0.67 -""'3., 5 O. ?1 .1 ~ 5 1.4 3.0 0.85 -1.5 0.24 ~2.5 '.

l 'lano solo }.fili tary Band P 20.0 26 8.6 18 5 3.7 115 12.0 210 6 5 16.5 4.6 13.3 .. ft .. .. SI> 6.4 16 2.2 7.0 1.6 4.0 6.0 15.5 3.6 11.0 2.9 9.0 .. .. " .. D 2. 2 7.0 1.0 0 0.32 ·10.0 2.9 9.0 1.1 1.0 0.44 -7.0 News reading Theatre Orch o p 8.6 18.5 4.0 12 0 3, 9 12.0 20 0 26 0 10,,0 20.0 5.0 '14.0

n .. 'It " 3D 1.5 3.5 0.9 ,-1. 0 0.6 -4.5 5.0 14.0 3.3 10.5 2. 2 ?O .t ". .. .. D 1. 2 1.5 0.05 -1.5 0.3 10.5 1.9 5.5 1.2 2.0 0.77 - 2.5

Commentary Thl It re Orcll P 6.1 15.5 3. 2 1C.O 1.8 5.0 18,0 25.0 100 20.0 6,,7 16 0 5 .. .. .. SD 2.0 6.0 0.9 - 1.0 0.45 -7.0 30 0 9.5 1.5 3.5 1.0 0 I If " It D . o.? -3.0 0 • .32 ... 10.0 0.23 -14.0 1.3 2.5 0.?3 -3.0 0.4 -8.0 A'Yer~ge of 8 II programme p il.O 21.0 5.6 15.0 :3.3 10.5 18.0 25.0 8.7 19.0 5.? 15.0 conditions shown above 3D 3.1 10.0 1.7 5.0 0.9 -100 4.5 13.0 2.1 6.5 1.3 2.5 ;

D 1.5 3.5 0.7 . -3.0 O. 3~ -;.l.G 1 1.7 4.5 0.93 -.5 0.42 -7.5 : . ; I

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I TABLE 2

Iibtios f wantea carrIer amElltude for the second receiver tested. I o unwanted carrier amplitude

wanted carrier amEl1tude unwanted carrier amplitude

R.ll. S. amplitude of wanted carrier 3 millivolts 10 millivolts

Carrier separation in kc/s 8 kc/s 9 kC/s 10 kc/s 8 kc/s 9 kc/s 10 kc/s

Wanted Unwanted (trade of ratio db ratic db ratio db ratio db rat10 db ratio db programme programme audio

interfer-~nce_

Piano solo News reading P 5.6 15.0 3.1 10.0 1.7 4.5 5.0 14.0 2"S 9,,0 1.4 3,,0

" " It n SD 1.3 2.5 0.9 -1.0 0.5 -6.0 1.4 3.0 0.8 -2.0 0.4 -BoO .. .. .. .. D 0.3E -9.0 0.21 1-13.5 0.1:3 1-17.5 0.45 -7.0 0.31 -10.0 0.11 1-, 9.0 Pitmo solo Theatre Orch P 4. !5 13.0 3.3 10.5 3 0 3 7.0 5.7 15.0 4.0 12 2.7 8, 5 .. .. ..