A DIODE MATRIX VERTICAI INTERVAL VIDEO SWITCHER

Robert AhaTARC Electronics Inc.Westbury, New York

andF. Cecil Grace

Visual Electronics CorporationNew York, New York

The diode matrix vertical interval switchingsystem described is the direct result of a concen-trated effort by a number of people. The diodematrix circuitry was designed by engineers of theStorer Broadcasting Compary. Mr. H.A. Bondy,Chief Engineer, of WAGA-TV, Atlanta, Georgia,supervised the design, construction, and install-ation of a 15 input by 5 output studio and mastercontrol switcher utilizing this diode matrix whichhas been in use as the, main switching system atW.AGA-TV, since Fall, 1957. This project, contrib-uted to by all members of the WAGA-TV studioengineering staff, resulted in a switcher whichwas outstanding in its simplicity, reliability,serviceability, and low cost. Inexpensivegermanium diodes were used for the video signalswitching function. All vidVeo diodes and allrelays used for their control were made plug-in.

Visual Electronics Corporation recognizedthe advantages which such a switcher might bringto other operating TV stations if it were madeavailable to meet their reauirements. Licensearrangements were therefore made with StorerBroadcasting for the provision of all design dataon the diode video switcher for the incorporationof this principle in a new type vertical intervalswitcher to be manufactured for and marketed by'Jisual Electrcnics Corporation. The diode switch-ing matrix circuitry was tested exhaustively andthe design proved to be excellent. It had goodfremuency response, good switching action, andprovided a very simple compact method of videoswitching which was extremely fast-thus lendingitself to control switching during the verticalinterval.

The detailed design and production of thediode matrix vertical interval switcher hereindescribed were carried out by Tare ElectronicsInc., of Westbury, N.Y., under the supervisionof Mr. Robert S. Aha, Chief Engineer.

This switching system provides an overlapswitch of approximately 200 microseconds, sotimed that it falls within the mid-range of the1000 microseconds vertical sync interval. Thisis very difficult to accomplish in a relay typeswitcher due to the fact that relays whice havevery fast switching times do not have low cap-acitance and therefore do not lend themselvesto use in vileo switchers having extremely tightspecifications. In this switcher the videoswitching is accomplished by a diode switchingnetwork whose D.C. portion is controlled by theextremely fast standard plug-in Stevens-ArnoldMillisecond relay. Thus the high capacitance ofthis relay does not effect the video circuitry and

its extremely fast action and extremely long lifeare utilized to provide the vertical intervalswitching feature. Tests were run operating thisrelay over 100,000,000 operations with 300% loadcurrent with no deterioration of the unit. Notethat this relay is a completely sealed plug-incomponent resembling very much in size andappearance the standard vacuum tube.

With no worry about the capacitance of therelay and the associated wiring the diode matrixcan be set up to provide fast switching withextremely good frecruency response, differentialgain, differential phase, and excellent cross-talk isolation. Two other relays are used foreach point in the switching matrix-one providesadditional contacts for switching of audio cir-cuits, tally lights, etc., and the other providesfor the remote control operation of the switcherfrom one or several remotely located push-buttoncontrol panels. The control circuit which timesthe switching function so that it occurs duringthe vertical interval period consists of fivecommon type vacuum tubes. As these tubes provideonly on-off functions they can easily be replacedat a later date with transistors. Vacuum tubesrather than transistors are used in this currentmodel to provide greater reliability in theservicing of this unit in television broadcaststations operating throughout the country.

When the majority of station operating andmaintenance personnel become equipped with theknowl3edge and experience in handling transistors inthis type circuitry, the change to transistors maybe accomplished very simply by pulling out thevacuum tube module, changing the pawer supply,and plugging in the transistorized module, with-out changing any portion of the switcher itselfand without interfering with its high qualityperfomeance

Figure 1. shows the general block diagramof the switcher.

A. Input "TO Pad...provides a 75-ohm inputimpedance and a variable gaincontrol which feeds the correctsignal amplitude to the inputDistribution Amplifier.

B. Input Distribu-tion Amplifier..feedback type Distribution

Amplifier using two tubes toprovide a low output impedanceto drive the input bus.Excellent linearity and widefrequency response is achieved.

11

Fig. 1 - Block diagram of diode vertical interval switcher.

C. Input Bus.......low capacity low inductance

bus to distribute video to theresistance divider networks anddiode switching cimuits.

D. Divider..'0.... 0Precision resistors to divide

down the vtdeo signal so as toprovide isolation as well asa low impedance to the outputbus.

E. Diode SwitchingCircuit ....biased diodes which, when

unbiased by the video controlrelay, provide substantially ashort circuit from the dividernetwork to the output bus

F. Output Bus......low capacity, low inductance

bus similar to the input bus,which feeds the output CathodeFollower.

G. Output CathodeFollower . .....provides low capacity input

from the output bus which thendrives the output distributionamplifier.

H. Output Distribut-ion Amplifier...same type as input Distrib-

ution Amplifier with a 75-ohmoutput impedance.

I. Output 'T" pad..provides a constant impedancegain control.

Each matrix point is controlled by theextremely fast millisecond relay and itsassociated two service function relays. Thevertical interval timing circuit properly trig-gers the millisecond relay such that the 1Amillisecond video tranfer time occurs during themid-portion of the vertical sync interval.

The Switcher Control Circuit Contains:

A. Relays

1. Remote control - has a 15 millisecondoperate time and a 80 millisecond releasetime - activated by the push buttons andpresets the switching operation. It alsoresets the switcher for the next ope ration.

2. Control - has a 10 millisecond operate andrelease time - makes all tally circuits.

3. Video - has a 0.5 millisecond operate andrelease time - turns the diode circuitryon and off.

4. Trigger - has a 10 millisecond operate aadrelease time - presets the thyratron.

5. Automatic black level starting has a0.5 millisecond release and operate

12

I

I~~~~~~~~\

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I I~~~~~~~~~~~~~~~~~~~~~~~~~

ICONT-ROLRELAY

I~~~~~~I~~~~~~~~~~~~~ I I

REMOtTE

RELA

BUT TON

time - keeps the output video bus tiedto black level or no signal when theswitcher is first turned on.

B. Control panel.,..illuminated push buttons,each row on a separate frame.

C. Vertical IntervalControl...........consists of:

Vertical trigger amplifierThyraton controlD.C. inverterKey on tubeOne-shot multi-vibratorRelay hold

Figure 2. shows the rack mounting arrange-ment for a typical switcher.

Note that video feeds into the top unitwhich is the input Distribution Amplifiers then,through the matrix unit, out through the CathodeFollower and output Distribution Amplifier.

Fig. 2 - Rack and panel layout diode verticalinterval switcher.

Immediately below is the control relay chassisand immediately below that is the relay timingcontrol unit. The regulated positive and negativepower supplies and 24 volt line regulated D.C.supply are also supplied in the system. Theremotely located control panel is entirely D.C.operated, and consists of rows of self-illum-inated momentary contact push-buttons. Connect-ions to the control panel are also plug-in.

Operation of the Vertical Interval Switcher

To begin with, we must assume the follow-ing set of conditions: First, we have a neg-

ative vertical drive of between 3 and 5 volts.Second, a video signal is going to inputs 1and 2. Third, all power is off. when thepawer supplies are first turned on, V105A'conducts through the video relay contacts,through the automatic black control relayto B+, and the automatic black level controlrelay is energized. The relay, througn itscontacts, now applies the necessary voltage tothe black level diodes. This is the normllyblack, or no-signal, level condition.

Since all switching must be overlap, one

pair of diodes must be conducting at all times.Therefore the automatic black control sets up orpresets this condition. We now have the automaticblack control relay conducting through the platecircuit of V105A. The grid circuit of V105A isgrounded through the remote control relay contacts.

For the first operation, we push input #1button on the control panel. This in turn oper-ates the remote control relay #1 which grounds thethyratron cathode, and presets the thyratron. Italso opens up the contacts which grounded thegrids of the control tubes V105A and V105B and thuspresets these tubes. It also ties in parallelvideo relay #1 and control relay #1 by virtue ofthe two Form A contacts on the remote controlrelay #1 and applies both of them to the platecircuit of V103. It also starts the operatecycle of the trigger relay. This constitutespreset condition #1.

The trigger relay, which takes approximately10 milliseconds to operate after the remotecontrol #1 has completed its operating time,closes its contacts. When this trigger relayroperates, it removes the very large bias on thegrid of the thyratron, presetting it to a condit-ion let us call preset condition #2. When thishas occurred, we are now ready to fire. the thyra-tron. The thyratron fires by being triggeredfrom the leading edge of the negative verticaldrive signal. The negative vertical drive signalis differentiated in the grid of VlOlA, takingthe leading edge and clipping off the trailingedge. The leading edge pluse is amplified andinverted in VlOA and applied to the thyratrongrid V102. When the trigger, that is, the leadingedge of the vertical drive, comes through afterpreset condition #2, the thyratron fires. This,in turn, drops the junction of R108 and R109, whichoriginally was set to approximately 100 volts, toa very low potential. VlOlB, the D.C. inverter(which is normally won"), is now biased "offJthrough the divider networks of R108, R109, andR12, R113 and Rll). Since VlOlB now is cut offsthe plate circuit and R111 now approach B.. Thissets up the condition wherein a large positivevoltage is applied to the grids of V103. Thetube which normally is cut off, now conductsheavily.

The circuit R120 and C107 is primarily acathode peaking circuit which peaks the leadingedge of the keying pulse to provide a very fastoperate time of the video relays and the controlrelays.

13

INPUT

4SLt

.ACK

F|

rlBLACK

iNF

Z4V

v103

ViolB

ilCIA

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Fig.

3

When V103 is made to conduct, V104, a one-shot multi-vibrator operated by a positive pulseon the grid, is triggered and produces a 30millisecond negative pulse. The start of thispulse coincides with the leading edge of verticaldrive.

This pulse is now partially integratedthrough R128 and C108 and applied to the gridsV105A and V105B. As previously stated, V105A wasconnected to the automatic black control throughall the video relay contacts. V105B has novoltage in its plate circuit and therefore has noplate current. The negative pulse of 30 milli-seconds, applied to the grids V105A, cutting offits plate current, starts the release action ofthe automatic black control relay. Simultaneous-ly, V103 starts to conduct very heavily throughcontrol relay #1 and video relay #2 in parallel.Both relays then start their operate time cycle.Video relay #1 operates very quickly-that is,within a half millisecond. The control relay #1does not operate quite as fast. It operates inapproximately 10 milliseconds.

Now, there no longer is voltage on the coilof remote control relV #1, but its contacts arestill closed due to its slow release character-istics. It operates in approximately 15 milli-seconds and releases in 80 milliseconds. Sinceit releases in such a long time, the entirefunction of the vertical interval switchingoperation is accomplished before the start of therelease cycle.

We now have control relay #1 and video relgr#1 in their operated state. VlO, the one-shotmulti-vibrator, having completed its 30 milli-second pulse time, permits V105A and V105B toconduct as in their normal operating condition.Now video relay #1 is locked through its contactsto the plate circuit of V105A. We have the samething happening in control relay #1 through theplate circuit of V105B. But we also have bothrelays still in parallel and still tied togetherthrough the two Form A contacts on the remotecontrol relay #1, and still tied to the platecircuit of V103. At the instant that video relay#1 closes, which is approximately half a milli-second after the vertical drive pulse initiatesthe entire operation, the diodes on input #1conduct and we have the video going through thediodes to the output circuit. We have an overlapcondition due to the video relay #1 pulling inor operating before the automatic black controlrelay releases. Therefore we have vhat we callthe normal overlap condition of two sets of diodesconducting for a very short time, i.e., approx-imately two hundred microseconds.

We now come to the end of the release time ofof the remote control relay #1. This releases t1etwo form A contacts which tied together videorelay #1 and control relay #1 to the plate circultof V103. They are, however, held through their

holding contacts to the plate circuits of V1C5Aand V105B respectively.

The trigger relay is released through theopening of the contacts of remote control relay#1. This restores sufficient bias on the gridof the thyratron to make it inoperative. Wealso disconnect the cathode circuit of thethyratron from ground, which resets the thy-ratron. The reason we must open the plate-cathode circuit of'the thyratron is to resetit to its original non-conducting condition.The D.C. inverter following the thyratron isalso back to its normal condition by virtue ofthe thyratron being non-conducting and gridbias being removed, causing the plate current toflow. This completes the operation of going fromblack to the first selected video signal.

If the operator pushes a button to selectanother video signal, the same cycle is repeatedproviding overlap switching to this new signalduring the vertical interval.

In conclusion, this switcher provides:

Fast swaitch#CPrevious switchers have been in the order of

ten to 15 milliseconds switching time. The videotransfer time here is less than /1A millisecondand occurs only during the vertical interval.

Reliable switching

Sealed relays are used. This type of con-struction eliminated problems wlhich plague oldertype switchers caused by dust, dirt, moisture,etc., and long life is provided. The relays aswell as the switching diodes are arranged on aneat, accessible plug-in basis. Thus thesecomponents can be easily and cuickly replaced.

Simplicity

Due to the simple straight-forward arrange-ment of the video circuitry any reasonablycompetent station maintenance man can easilykeep thisswitcher operating within its verystringent specifications.

Low cost

Due to the fortunate choice of inexpensivereadily available diodes and relays, this switchercan provide the advantages of vertical intervalswitching and greater reliability at a lowercost than the majority of the relay switchingsystems which have been installed in the pastseveral years.

The speaker feels that all who have contrib-uted to this project should be Justly proud thatit has produced the best switcher available todayand one that can grow with the state of the art.

EIGHTH

ANNUAL

BROADCAST

Sfl2OSIUM

Sept

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