A Second Generation Type-C One-Inch VTR

A Second Generation Type-C One-Inch VTR

By Hiroshi Tanimura, Yoshio Fujiwara, and Thomas E. Mehrens

This article deals with the design of a second generation Type-C videotaperecorder and the development of a new tape transport and movabletape-guiding system. Topics covered include the use of the NASTRANstructure analysis program to determine deck rigidity, and the developmentof new video and audio head technology, with a review of the compositesingle and polycrystal video head and the amorphous metal audio head.Also discussed are improvements in the technology surrounding DynamicTracking, and the application of this technology to provide for variable-speed play from three-times forward to one-time reverse; the applicationof microprocessors to video recording systems; and the specific applicationof a microprocessor to the servo sub-systems.

he sponsorship of standards by theT SMPTE and EBU in 1978 for theType-C format triggered a remarkableworldwide acceptance by professionaltelevision organizations of the I -in.helical scan VTR. During the veryshort period of four years, the numberof machines produced on the Type-Cformat is comparable to that of 2-in.quadruplex. Some of the reasons forthis rapid growth arc:

0 Excellent quality, high reliability,and ease of maintenance.

0 Easy operation, flexibility inediting applications, and portability.

0 Drastic reduction in operatingcosts and in capital costs.

Over the relatively brief four-yearhistory, a great deal of experience hasbeen gained in manufacturing. Thisexperience, combined with advances intechnology, allowed the developmentof a VTR utilizing new concepts. Thispaper describes certain aspects of sucha VTR. the SONY BVH-2000, whichwe feel deserves the title of "A SecondGeneration Videotape Recorder ." Thisrecorder makes use of major im-provements in the area of:

0 New tape transport and move-able guide system.

0 Improved video and audio per-formance.

0 Improved dynamic trackingperformance of slow motion (contin-uously variable speed from - 1 to + 3times normal speed).

0 Compact size and low weightwith total front access for case of op-eration and maintenance.

0 Built-in time-base corrector.0 Microcomputcr controlled servo

0 Full scale editing.Obviously, the length of this paper

will not allow for a complete discussionof all of these items in detail; thcrcforc.we will touch briefly on selectedareas.

Tape Transport and MovableGuide System

system.

A characteristic of videotape is the

ease with which i t can be physicallydistorted. The design of a tape trans-port is crucial to the reduction of bothstatic and dynamic loads placed on thetape. Any improvement in tape han-dling should result in increased tapelife and improved interchangeability.

The most significant improvementin tape handling is made by the re-duction of friction in the tape path.This can be accomplished by:

0 Minimizing the number of fixedguide posts. which in this design hasbeen reduced from 10 to 5.

0 Reducing the total wrappingangle around the fixed guide postsfrom 390" to 260'.

0 Using high-accuracy rollerguides wherever possible.

Ease of threading and operation isdesirable, but has been a problem forthe Type-C VTR due to the high angleof wrap around the scanning assembly.The introduction in the BVH-2000 ofan automatically operatcd movableentry and exit guide system (togetherwith the shield cover for the audio anderase heads), provides enough spacefor easy tape threading. A diagram of

IP ROLLER^ L v/s ERASE HEAD

Figure 1. New tape transport system.

SMPTE Journal. December 1983

PRESSURE WHEN PRESSURE IS APPLIED FROMBELOW OR ABOVE, MOVEMENT ISMADE IN THE DIAGONAL DIRECTIONINDICATED BY THE ARROW.

0 0O+@

PRESSURE

6PRESSURE

PRESSURE

U

I -I -i 5 5

PRESSURE

ALL DIRECTIONS ARE REGULATEDWHEN PRESSURE IS APPLIED FROMEITHER THE TOP OR BOTTOM.

PRESSURE

Figure 2. General concept of cross-roller guide system.

RIB

UNDER 5.0 - 10-2 (kgVrnrn2)

0 1.0 . 10-1 (kgVmm2)

x COMPRESSION STRESS

Figure 3. Distribution of maximum main stress (tensional) on the backsurface of the base plale (first stage).

SMPTE Journal, December 1983

the transport illustrating the basic tapepath is shown i n Fig. I .

tlistory has shown that the appli-cation of a movable guide bystem mustbe undertaken with great care, iisguide position errors can affect thestability of tape movement and track-ing accuracy. This new dcsign employsan extremely accurate cross-rollersystem which guarantees positioningof the guide to within 1 p over ex-tended use. Figure 2 illustrates thestructure of the cross-roller guidesystem. The rollers. having freedom ofmovement in the direction of rotation,arc arranged so that their axes are al-ternately perpendicular. This ar-rangement results in precise movementof the slant guide carrier with no needfor clearance between the rollers andthe guide carrier. When the guidecarrier end-stop is set, positional ac-curacy of the guide is guaranteed.

A reduction in size and weight of thetransport base plate without a loss ofstrength and rigidity requires carefulstructural analysis. During the dcsignof the new transport, the NASTRAN(NASA Structure Analysis) computerprogram was used. This programcomputes the stress concentration overa selected grid of elemcnts. Kepcatcdsimulation w h i I c v;i ry i ng Io;i d i ng, rc-straint, and degrees of freedom, cs-tablished an optimum structure basedon an aluminum die casting with cx-tensive rib reinforcement. Figure 3shows the distribution of stress on theinitial structure with the area of thecapstan motor mounting showing a

- RIB 0 1.0 10-1 (kgf/mmz)

o UNDER 5.0 . 10-2 (kgl/rnmz) x COMPRESSION STRESS

Rure 4. Distribution of maximum main stress (tensional) on the backsurface of the base plate (last construction).

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FILLING

CONTACTINGSURFACE

MAGNETIC GAP

r SINGLE-CRYSTAL

GLASS \

‘\\

FERRITE

~ POLY-CRYSTALFERRITE

I

Figure 5. Structure of single and polycryslal ferrite head.

FREQUENCY (HZ)

Figure 6. High-frequency characteristics: comparison of amorphous and permalloy head.

6

a& . I * ’ . ,\ FLEXIBLE

HEADTIP

--- -___- LZ-b,--- L1--.*

Figure 7. New Dynamic Tracking head.

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high concentration of tensional strcss.Thc result of using thc NASTRANprogram is sccn in Fig. 4, whcrc thestrcss has bccn distributed over a widerarea.

Video recording heads have, u n t i lnow, made usc of single crystal fcrritcs.Thcsc cxhibit excellent wear resistanceand have quite good magnetic pcrfor-mancc; howcvcr, bcing anisotropic,they exhibit noise which is the result ofthe contact prcssurc between hcad rindtape. This prcssurc deforms the crystal,and owing to its magnctostrictivcproperties, induces an elcctro-niotivcforce ( E M F ) ;icross the head winding.The resultant noise is called sliding orscrape noise. Polycrystallinc fcrritcmakes use of i1 rilndonlly orientedcollection of small particles. While therandom orientation of the pnrticlcsrcsults in no bulk magnetic anisotropyand reduced scrapc noisc, the rcsis-tancc to wcar of the polycrystallincstructure is inferior to the singlc crystalstructure.

Figure 5 shows the two mntcrialscombined in a singlc head. The singlcand polycrystal head has a higheroutput, ii lower scrapc noise, and ;Ihigher rcliiibility than the traditionalsinglc crystal ferrite head. Thc part ofthe head which contiicts the tape issingle crystal with high wcar rcsis-tancc, while the bulk rnagnctic prop-crtics are determined by the poly-crystiillinc material. This compositevidco hcad construction, combinedwith new processing tcchniqucs tocontrol deterioration of magneticcharacteristics duc to gaps a t theboundary, results in a I ~ I . 2 d B im-provcmcnt in scrapc noise and ;i betterspectral distribution of the rcmainingnoise.

The conventional audio hciid utilizespermalloy rather than ferrite material,since the magnetic characteristics arcbetter and thcrc is a lesser necd for amntcrial of high wear-resistance. Kc-cently i t has bcconie possiblc to makcpractical usc of amorphous metal,which has a better initial permeabilityand niagnctic f l u x density than perm-alloy; a lower variation in permeabilityand magnctostriction with tempera-turc; no crystal anisotropy; small cddycurrent loss; and iniprovcd wcar-rc-sistancc. Considering t hcse factors,amorphous mctal is an excellent ma-terial for audio hciids which have goodhigh-frequency response, signal-to-noisc ratios (SN R), and wcar charac-teristics. By making use of ferritemasking over the area not occupied bythc heads, resistance to wear is furthcr


cnhanccd. and ;I reduction in tape ad-hcsion is rcali/ed. In Fig. 6 , wc see therclativc high-frequency characteristics ofpermalloy and amorphous metal heads.

Dynamic Tracking SystemThe ability to obtain broadcast-

quality playback at other than normal

play spccds is bcing used in increas-ingly more sophisticated ways. Thishas cncouraged designers to improvesuch systcms in both quality and range.The recorder described here makes useof several major advances that havebccn made:

0 A new form of construction is

employed for the Dynamic Tracking(DT) head allowing a peak-to-peakdeviation of 750 p to be achieved anda self-resonance frequency of about1250 Hz.

0 A correlation search type of DTfollowing circuit has been developedguaranteeing perfect tracking even at

+B MAX ': 2"

-0 MAX '=. 2"

BOUNDARYI t

BIMORPH HEADPART HOLDER

300 .-DYNAMIC DEFLECTION

AT 80 Hi DRIVE360 "

400!i 2 a ; ; 1'0 1;' 1; 1'6 10 20 22LENGTH (mm)

(b)

Figure 8. DT head motion-tape contact. Figure 9. (a) Current twin DT head; (b) birnorph deflection curve.

Figure 10. Computer servo system.

SMPTE Journal. December 1983 1277

normal speed in reverse, where therequired deviation performance is aserious problem.

0 A digital trace control circuit hasbeen implemented giving stable high-speed responses to track jumping andtracing.

As a result, a speed range of - 1 to+3 times normal speed is now possible.Newly added features arc pro-grammed play and a variable-speedlearn function.

The Dynamic Tracking video headis mounted on a piezo-electric bimorphwhich moves the head transverselywhen voltage is applied. Early versionsof this head used a single bimorph,which caused the head to contact thetape at an angle when deflected. Byusing a dual bimorph construction asshown in Fig. 7, the head maintainsnormal contact to the tape. The char-acteristics are determined by LI, Lz,and L3. The deviation is proportional

to the square of L I , but with ii disad-vantageous reduction in resonant frc-qucncy. L7 increases the effectivebending length and deviation but thcrigidity of the joint maintains the res-onant frequency. The length of theflexible head holder (L3) is about 4mni. and does not significantly reducethe bending action while being able topcrforni the function of compensatingthe head contact angle. A carefulanalysis of the material, its dimensionsand form, is necessary for the flexiblehead holder to keep its stress below a nacceptable value. Figure 8 shows theresult.

I t is rather difficult to find the pre-cise relationship between the applicddrive voltage and resultant head dis-placement in such a conipositc struc-ture. In the initial design the cqua-tion:

UFigure 11. Servo software block diagram.

(kgcm

s

sc6

30 60 so 110

TAPE LENGTH (mitt)

Figure 12. Inertia (tape + reel + reel table).

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where:

b = End point displacementdll = Pic7.o-electric constantV = Field intensity in the bi-

hl + 2h7 = Thickness of ceramic and

I = F,ffcctivt: bimorph length

shows that the bending is proportionalto the square of the effective length.Each bimorph has its own bendingsensitivity, dependent upon manyfactors. Therefore. it is useful to havean empirical relationship.

6 = 0.0 I482 + 0.0 I4261

in or p h

shim

T he ex pcri men t a I equation:

+ 0.72531' - 0.00071-'

has been found to f i t the described DThead with good accuracy. Incidentally,this equation dcmonstrates the majorcontribution made by thc iz compo-nent.

The application of a trapezoidalshape to the bimorph results in a highresonant frequency being obtained.While an even higher resonant frc-quency may be obtained if the edgescontained a quadmtic curvature. about30 H z in 1250 Hz, the relativc ease ofmanufacture dictated the use of thetrapezoidal shape. The equation forapproximating the resonant frequencyis:

where:7

E / = [L , l (h l + h 2 ) ' - h 2 y3+ E J h ? ' ]

/ ) I = 2(.< Ihl + .V'h')

where E l , 152 arc Young's modulus forceramic and shim respectively; .T I . .szare the specific gravity of ceramic andshin1 respectively; dY(K) is a constant

TRACK C

HEAD SCAN

Figure 13. Operation of DT servo system.


TAPE POSITION(PITCH) 4- 2 JUMP

-I/- SPEEDRATIO

JUMP ZONE 10 de1efmln.4by following equ6tion

m : n a m r n - 1 : INTEGER

d : DETECTINQ POINT-

- 3 JUMP - 2 JUMP - 1 JUMP

Figure 14. The digital trace control circuit calculates the jump to another video track.

dependent on the trapezoid shapingbeing between about 3.5 for a rcctan-gular shape and 7.5 for ;i triangularshape. I t can be seen from this cqua-tion that nearly a 2:l increase in rcso-n n n t frequency cnn be obtained byadopting a trapezoidal shape. Also, anincrease in 1 reduces the resonant frc-qucncy in the siimc ratio as deviationis incrcascd.

Figure 0 shows ;I comparison of de-flection characteristics for the new DThead employed in the RVH-2000, theDThcad used in the BVII-I 100A,andthe older single biniorph DT head. Thecompensating effect of the flexiblehead holder in the double bimorph isclearly seen in the increased dellcctionof the latest version.

Microcomputer Controlled ServoSystem

The servo system in a videotape re-corder has traditionally been realizedthrough the use of ;i direct applicationof analog and digital circuitry. TheBVI 1-2000 uses a cornbination of an-;I log, d igi ta I , and ni icro-processortechniques to provide both high speeda nd d y n ;i ni ica I I y c h a ng i n g rcs ponscbased on varying conditions. Figure 10shows the block diagrani of the overallmachine servo system.

The system sends through the ccn-trill processing u n i t (CPU) all inputdata related to speed, phase, and tcn-sion; control of servo motors; and out-put data to the systeni control. Thebasic servo operation makes use ofanalog hardware where high-speedresponse (frequency response) is rc-quircd. This can be seen in the drumand capstan servo velocity loops.Where relatively stiltic values are re-quired, stable performance is assuredby mcms of digital processing tech-niques through the CPU. The mostsignificant feature of this CPU-basedservo system is its ability to respond tovarying conditions. These transients:ire controlled based on the systemprogramming being optimized for themost desirable tape speed and tensionrelationship.

System diagnostics and self checks


arc performed in both software and inhardware during operation to provide;i fail-safe condition in the case of anyniisoperation or hardware failure.

The CPU system consists of twoZ-XO microprocessors. Servo operationis divided between the CPUs, with onehandling hardware 1/0 control (thehard CPU). while the other (the hostCPU) provides servo system control,processes servo data, and provides forexternal monitoring of the system.Figure I I shows the relationship of theprocessors. The two processors operateindependently of each other and pcr-form their respective functions simul-taneously. As a result of the constantlychanging conditions in the system,large quantities of data must betransferred between the CPUs at ahigh speed. This transfer is through aconinion RAM area and is at a rate of4 times in each field. The maximumsanipling rate for the hard CPU is 480Hz, while the rate for the host CPU isI20 I lz. The operating system is con-tained in E-PROM. with the programoccupying less than 14.5K bytes for thehost CPU and 6K bytes for the hardCPU.

An exiin1ple of the system’s abilityto respond to varying conditions can beseen in the operation of the reel servo.Figure 12 illustrates the relationshipbetween inertia and tape length; how-ever, under some conditions the vari-ation of total inertia can be 10 to IStimes that shown. The best tape controlcan be obtained by calculilting theaccess torque control:

where:

J , = Tape inertia (calculated diam-

J, = Keel inertia (nicasured and cal-

J,,,, = Keel table and motor inertiaN, = Access command (from power

(Y , = Access sense (measured by rccl

During the design stage of the

eter)

culated)

control)

Fci )

BVH-2000, a high-performance di-agnostic and monitor system was dc-veloped, based on the requirenicntssurrounding program development anddebugging. The development of thissystem continues based on the analysisof discussions regarding requirementsof the production line. field service, andcustomers. Many of the diagnosticfunctions are presently resident i n thesysteni and are easily accessed fornormal maintenance, while otherspecialized test and diagnostic routinescontinue to be developed.

Dynamic Tracking ServoThe DT servo system is realized in

;I digital trace control and in a corre-lation search follower circuit. While adescription in detail would be toolengthy for this paper, the theory ofoperation can be seen by referring toFig. 13. The DT servo controls thehead scan in a manner similar to A orB or C when tape speed is other thannormal, or i f the recorded track varics.The circuitry required to perform thismay be considered as three separateitems: Ramp compensation, Positionshift, and Jump control. The followercircuitry is used to compensate forposition shift, and since this has beendescribed in detail in other papers, i twill not be discussed. The other twoi tem are controlled by the digital tracecontrol. Referring to Fig. 14. the dig-ital trace control circuit calculates themoment, the direction, and the dis-tance of the jump to another videotrack. This decision can be seen in thechart, where playback vertical phaseis represented in the vertical axis a sTape Position, and the playback syncfrequency represents Tape Speed inthe horizontal axis. The equationshown is used to determine the jumparea. Based on this, a speed range of- 1 to + 3 times, play can be assured.

ConclusionMany new and exciting dcvclop-

nients have occurred since the stan-dardization of the Type-C I-in. VTR.Some have been progressively intro-duced, but many have had to await theintroduction of a new model due to theneed for compatibility between sub-systems. Encouraged by the rapid ac-ceptance of the Type-C recorder, anextensive amount of time was investedto realize the numerous iniproveinentsi n performance achieved in theBVH-2000. The realization of theseadvances justify, it is felt, the title of“A Second Generation Type-C Onc-Inch VTR.”

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A Second Generation Type-C One-Inch VTR