Philips tech. Rev: 36,233~241, 1976, No. 8 233

Digital circuits in the video' telephone

H. P. J. Boudewijns, E. C. Dijkmans, P. W. Millenaar, N.A. M. Verhoeckx and C.H. J. Vos

A previous article [1] gave a general description of the experimental video-telephonesystem that has been in use at Philips Research Laboratories in Eindhoven since 1972and is also connected to several other places in the Netherlands. An interesting featureof the signals used in this system is that they alternate from analog to digital; the videosignals are analog, whereas the synchronization, speech-transmission and signallingsignals are digital. This alternation in signal type is rather unusual, and this is why weare including the special article on the function of the digital circuits in the video tele-phone.

Features of a video-telephone system

In a video-telephone system three kinds of signalhave to be transmitted. First there is the video signal.This requires by far the largest bandwidth, which isabout 1MHz in the Dutch experimental system [1],

and in other systems. At each terminal a video signalis generated simultaneously for transmission to theother terminal. There is also the speech signal, occupy-ing at least telephony bandwidth, and thirdly there aresignals for communicating with the exchange (diallingsignal, calling signal and various tones); the whole ofthis last group is used for the 'signalling'.The last two kinds of signal are also found in or-

dinary telephone practice. It seems fairly obvious thatthey can be transmitted over a standard telephone cir-cuit, and in fact this is what is done in some video-telephone systems. The video circuit is then an additionto the existing telephone circuit. The separate exchangenecessary for switching the wideband circuit is con-trolled from the existing telephone exchange. The sub-scriber wishing to make a video-telephone call can forexample indicate this by dialling certain digits beforethe ordinary telephone number. This system has theadvantage that if the video circuit develops a fault, thespeech circuit and signalling are not affected. A powersupply failure at either end of the video line is suf-ficient to make the picture disappear, since each videotelephone is powered from its local mains. An ordinarytelephone, on the other hand, derives its current fromthe exchange, where there is always emergency equip-

H. P. J. Boudewijns, formerly with Philips Research Laboratories,is now with the Philips Commercial Estimating and PlanningDepartment, Eindhoven; E. C. Dijkmans, P. W.Millenaar andIr N. A. M. Verhoeckx are with Phillps ResearchLaboratories;Ir C. H. J. Vos, formerly with Philips Research Laboratories, isnow with the Philips Audio Division, Eindhoven. .', , ','

ment to ensure that it continues to function if the publicpower supply fails.The introduetion of a video-telephone system with a

structure of this kind grafted on to.'the public telephonesystem depends upon the existence ofmodern telephoneexchanges controlled by programmed processors. Thenumber of such exchanges is growingbut it will be manyyears before they are general throughout the system.Another aspect is the quality of the sound. The video

telephone requires a loudspeaking telephone circuit.The quality of the sound is considerably improved if itis given a bandwidth larger than the 3.5 kHz normallyused in telephony.A way of doing this has been devised for use in the

experimental system in the Netherlands. It consists incoding the sound in digital form and transmitting itduring the line-blanking intervals of the video signal.This has a second important advantage: the soundsignal is regenerated at the receiver and the level of thesound does not therefore depend on the line attenua-tion. In particular, this simplifies the voice-actuatedswitch in the loudspeaking telephone circuit.In the cross-country sections of the experimental

system a telephone line for signalling runs alongsidethe video pairs. At Philips Research Laboratories,however, where a special video-telephone exchangehad to be built, it was decided to incorporate the signal-ling in the video signal _indigital form, thus enablingthe extra telephone line to be eliminated. This meansthat the entire video-telephone circuit can be accom-modated in the four conductors of the outgoing andincoming video pairs.

[1) E. A. Aagaard, P. M. van den.Avoort and F. W. de Vrijer,An experimental video-telephone system, Philips tech. Rev.'36, 85-92, 1976 (No. 4).

234 H. P. J. BOUDEWIJNS et al. Philips tech. Rev. 36, No. 8

The use of digital signals creates a number of newpossibilities. With digital transmission it is of coursenecessary to use synchronous bit-clock oscillators atthe transmitting and receiving stations, and theseoscillators can obviously also be used for field and linesynchronization. It has been found possible to developa system for this that is extremely insensitive to noiseand interference. Moreover, the field-blanking periodis long enough tó permit the transmission of additionaldigital inforrnation, a facility that can be put to variousgood uses.When a subscriber dials a number the exchange has

to decode the digital dialling information it receives.Previous synchronization is essential here, and fairlycomplicated receivers are therefore required at theincoming side of the exchange. To limit expense eachsubscriber can be supplied with an 'off-hook detector'instead of a complete receiver, and receivers can beallocated to calling subscribers as required.This article presents a description of the digital aids

used in video-telephone sets in the experimental system.It will be concerned chiefly with two aspects: the syn-chronization of the receiver in one video-telephone setwith the transmitter in the other set, and digital speechtransmission with the voice switch. Finally, someextensions of the application of the system that arelargely due to the digital approach will be considered.

Block diagram of the video-telephone set

A general description of the use of digital signals inthe video-telephone set will now be given with the aidofthe block diagram infig. 1, where the circuits operat-ing with analog signals are shown in black and thedigital circuits in colour (red for outgoing, blue forincoming signals). It will be observed that only thevideo signal from the camera Cam retains its analogform, and this accounts for the analog components inthe outgoing line signal. All other signals are given adigital form.The time scale of all signals proceeding in a partic-

ular direction is determined by the oscillator G, whichgenerates the clock signal in the transmitter. The clocksignal is taken to a chain of frequency dividers DivCh;the results of these divisions are combined in logiccircuits TxLog to give the 21 control signals requiredfor the various operations in the transmitter.These include the picture scanning in the camera.

The clock rate (1.0016 MHz) has been chosen such thatthe line frequency (7825 Hz) and the picture rate(25 Hz) can be derived from it by division. Anotherfunction performed by the control signals is the samp-ling of the speech .signal ; this is done eight "time~perpicture line in the delta modulator DMod, and depend-

ing on the sign of the difference between successivesamples a '1' or a '0' is generated. The bit rate is62.6 kbit/s, which is adequate for a sound bandwidth ofabout 5 kHz; this value corresponds to the bandwidthused for radio broadcasts in the medium-wave band.The eight bits generated per picture line are accum-ulated in a speech buffer SpB and transmitted at anaccelerated rate in the blanking interval at the end ofthe picture line.

The information signalled by the subscriber when heuses pushbuttons PBut is coded digitally; it is thenstored in a signalling buffer SigB and transmitted tothe exchange in the field-blanking intervals.A third digital signal, of great importance for syn-

chronization between transmitter and receiver, is gen-erated in the synchronization-word generator Sy WGoThis is a 32-bit word that is transmitted once per field;the word in the odd field is the logical inverse of theword in the even field. The three digital signals arecombined in a digital multiplexer DMux with trains ofsquare-wave pulses at halfthe clock rate in the line- andfield-blanking intervals; these pulses are used for bit-clock synchronization. The output signal is againcombined with the analog video signals in an analog-digital multiplexer ADMux and the combined signalis then transmitted.

At the same time a signalof similar composition isreceived over the incoming line. The analog com-ponents of this signal modulate the beam current in thepicture tube. First, the digital components are used tosynchronize the receiver. The synchronization signal isproduced by preamplifying the entire signal, 'clamping'it to restore the d.c. voltage level, and then clipping itseverely. The video information is lost in this process.The resultant signal is applied to various stages, each ofwhich processes part of the information it contains.Two of the stages are for synchronization. The bit-

clock oscillator of the receiver reacts to the trains ofsquare-wave pulses in the incoming signal. Their phaseis compared with that of the locally generated clockpulses and from the comparison a d.c. voltage arisesthat adjusts the frequency and phase of the oscillator(phase-locked loop, block PLL in the figure). This givessynchronization at the highest frequency (1.0016MHz).There is also a synchronization-word detector Sy WDthat emits a pulse on reception of a synchronizationword. If this pulse exceeds a certain threshold in thesynchronization verification stage SyVer, it resets thecounters in the divider chain Divïlh. The synchroniza-tion words thus act as a time marker rec~rring at thefield frequency (50 Hz). (In fact the synchronizationwords for odd and even fields are different, thusuniquelyspecifying the picture frequency of 25Hz.) Allintervening frequencies, generated in the divider chain

Philips tech. Rev. 36, No. 8 VIDEO TELEPHONE, II

HUB~c==) ~ --,Cam

~~-----------------+--~Man

of the receiver, are then available in the correct phaseand the receiver logic RxLog combines them into therequired control signals.

These are used for decoding the speech signals. Theincoming eight-bit words are stored in a buffer SpB andtaken in the form of a continuous stream of bits to thedelta demodulator DDemod, at whose output theanalog sound signal becomes available. The delta de-modulator also provides a signal that indicates theamplitude of the incoming speech signal and is com-pared in the voice-actuated switch VSw with a corre-sponding signal from the delta modulator DMod.lfthesignal from DMod is the weaker of the two, the micro-phone channel is disconnected. This ensures that thereis no closed electro-acoustic circuit in which acousticfeedback due to the built-in amplification might lead tooscillation.

The receivers forming part of the exchange equip-ment are basically the same as those in the video-

235

Fig. 1. Block diagram of the video-telephone set. Black: analog signals.Red: digital signals from the trans-mitter. Blue: digital signals intendedfor the receiver.Cam cameraDMad delta modulatorVSw voice-actuated switchSpB speech bufferPBut push buttons for

subscriber diallingSigB signalling bufferDMux digital multiplexerADMux analog-digital

multiplexerSyWG synchronizatio n-word

generatorG bit-clock generatoroi-c« divider chainTxLag transmitter logicMan monitorDDemad delta demodulatorCl clamp circuitHLim voltage limiterPLL bit-clock oscillator,

adjusted by phasecomparison

SyWD synch roniza tion-wo rddetector

SyVer synchronizationverification

RxLag receiver logic

telephone set. However, no optical and acoustical out-put stages are included and they incorporate some ad-ditional units for processing the signalling information.

The block diagram of the video-telephone set infig. I has been simplified by the omission of two sub-assemblies: the 'self-view' switch, which enables thelocal su bscriber to monitor the picture being trans-mitted, and a test-signal switch, which generates acheckerboard pattern from the existing signals. Thispattern is also transrnitted if the subscriber presses aspecial button that prevents the transmission of hispicture.

Synchronization

Synchronization of the bit-clock. oscillator

As we noted earlier, the receiver is synchronized tothe incoming signal at both ends of the frequency spec-trum: at the clock rate (1.0016 MHz) and at the picture

..----------------------~~-~-~- --~

236 H. P. J. BOUDEWIJNS et al. Philips tech. Rev. 36, No. 8

frequency (25 Hz). The bit-clock oscillator is syn-chronized by using series of alternating 'zeros' and'ones', inserted into the signal at fixed positions by thetransmitter. These form a square wave at a frequencyof 500.8 kflz, Six cycles, i.e. twelve bits, are accom-modated in every line-blanking interval; in the field-blanking interval five complete lines are occupied byalternating 'O's and '1 's (fig. 2).

The receiver has two synchronization modes. One ofthese is the acquisition mode; the receiver is in thismode when the connection is first established, andreturns to it when noise or interference causes syn-chronization to be lost. In the acquisition mode theincoming signal is applied continuously to the bit-clockoscillator of the receiver. During this time the syn-chronization-verification circuit keeps a continuouswatch for the detection pulse that will occur when asynchronization word is received; to prevent errors ahigh threshold is applied.

The other mode is the maintenance mode, to whichthe receiver switches when synchronization has beenachieved. The bit-clock oscillator no longer has theentire signal applied to it but only the 500.8 kHzsquare-wave signals at the end of every line. The watchfor the detection pulse is confined to the bit interval inwhich the detection pulse should appear; the detectionthreshold is lowered.

The long series of 'O's and '1's during the field blank-ing is necessary for rapid acquisition of synchroniza-tion. This is because the received signal has to undergoa number of processing operations, as we saw earl ier.The video signal has a voltage swing that is ten timesthat ofthe digital signal 'fragments' and ifby chance thevideo signal contained a run that looked like a succes-sion of 'O's and '1 's, this would certainly destroy syn-chronisation. To eliminate the video signal as much aspossible, only a small voltage swing is permitted, as wesaid earlier; this is restricted to a few per cent of thatof the digital fragments (hard limiting). The trains ofdigital pulses have previously been raised to the correctd.c. level (clamping), which is equivalent to restoringthe d.c. component lost in the transmission.

The reconstitution of the d.c. level consists in deter-mining the mean voltage of the' digital-signal fragmentsand raising this to the nominal zero level; this also hasthe effectof restoring the black level of the video signal.The circuit determining the mean voltage is activatedby a pulse each time the 500.8-kHz square wave isreceived in the line-blanking interval. If, however, thereceiver has not yet been synchronized at that momentthe pulse occurs at a random point in' the picture lineand an incorrect d.c. voltage level results. It is onlyduring the first line period of the field blanking that the500.8-kHz square-wave' signal is received during the

pulse in every case. After this has happened, the correctd.c. voltage level is set within about two line periods.An undistorted continuous 500.8-kHz square-wavesignal then reaches the phase-locked loop; the oscil-lator is now synchronized with the incoming signal,again within about two line periods. As a general rule

Vid

I1 I

1

.filL _ -"1

.l ---------------_1 JlJL--j

.1------- -I-JUL-~------- IJIIL - - - - - - - - - - - - - - - - - - -1- JIfL -j.filL -- - - - - - - - - - - - -=- -=- -=--_l-JlJL-'

--- J -'1~_jjit- -- - - - - - --,-JUL_~1.1_ 11 ----------+-JUL_JI.1_ - - - - n- - - - - - - - - - 1- JUL I.JlIU _ - - - - -ii - -- - - -- - __J_ _-'_ l.11 - - - - -H - - - - - - - - __ _J _ JIIl -l,------+t---JUL I I-~

I I I I I IS S' I BI IB21

Fig.2. The signals in the blanking interval of an even field. BItwelve bits for bit-clock synchronization. B2 eight bits for soundtransmission. S 32-bit synchronization word. S' four bits fordialling information. The long series '0101 ... Ol' in the first fivelines are for fast acquisition of synchronization at the beginningof a call. The pulses after S' can without difficulty be replaced byinformation-carrying signals. Vid video signal with a voltageswing up to ten times that of the pulses.

the correct d.c. voltage is thus restored and the bit-clock oscillator synchronized with the received clocksignal within five line periods after the start of the firstfield blanking.The conditions for detection of the synchronization

word are now satisfied and this word follows in thesixth line period.

Synchronization word, matched filter

Once the clock oscillator of the receiver has beensynchronized, the numerous signals derived from it byfrequency division are at the correct frequency, buttheir phase relative to the corresponding signals at thetransmitter is still indeterminate. As we saw, the func-tion of the synchroriization word is to establish thecorrect phase relations.

Philips tech. Rev. 36, No. 8 VIDEO TELEPHONE, II 237

Fig. 3. The matched filter (shown simplified). This is a shift register; all the outputs (Q and Q)of the succession of flipflops that carry a 'high' voltage when an even synchronization word isstored in the shift register are connected via resistors to output Vl. The other outputs thencarry a low voltage and are connected to output V2. The received signal is shifted through thematched filter. The voltage at output Vl shows a positive peak when the even synchronizationword passes through the filter, and the voltage at output V2 a positive peak when the oddsynchronization word (the logical inverse of the even word) passes through the filter.

These words consist of a digital signal that can berecognized clearly by the receiver even when there isnoise or interference in the transmission. They havebeen given the form of a special sequence of 'O's and'1 's which is detected in the receiver with an appro-priate matched filter.

This matched filter is a shift register with 31 stages,each ofwhich has two outputs, Q and Q; seefig. 3. Ifa stage is in state' 1', a high voltage occurs at output Qand a low voltage at output Q; for a stage in state '0'the situation is reversed. All the outputs are distributedvia resistors in such a way over two common lines thatwhen the shift register contains exactly one of the syn-chronization words all the outputs at which there is ahigh voltage are connected together. A maximumpositive voltage then becomes available on one of thecommon lines, e.g. Vi.The other outputs then all carrythe low voltage and an equally large but negative volt-age occurs on V2. With the other synchronization word,the polarities are exactly the opposite because thisword is the logical inverse of the first. If the syn-chronization word in the matched filter is shifted oneor more places, the voltages are smaller. The maximumvoltage corresponds to the maximum correlation be-tween the received signal and the bit pattern designedinto the matched filter.The signal entering the receiver is stepped through

the matched filter after voltage limiting. If the outputvoltage from the filter exceeds a certain threshold, adetection pulse is generated. For optimum synchroniza-tion it is essential that this detection pulse should occuronly when the matched filter contains the completesynchronization word. This word has therefore beenchosen such that the output voltage from the matchedfilter is very much smaller at every other moment; the

cross-correlation function of the pattern built into thematched filter with the received signal - the syn-chronization word in a context of 'O's and 'l's - hasa peak standing out clearly from its surroundings.This requirement for unambiguous detection ismore

easily met the longer the synchronization word. There

are, however, practical limits; in the form of signalchosen the synchronization word has to :fit into half aline period (see fig. 2), part of which has already beenreserved for the twenty bits required for clock syn-chronization and speech. Furthermore, the size of thesynchronization word generator and the matched filterincreaseswith the length oftheword. Wechose the 32-bitlength mentioned above, i.e. a quarter of a line period.The synchronization word consists of a single cycle

of a 'maximum-length sequence' with a length of31 bits, plus the first bit repeated. A characteristic ofmaximum-length sequences is that when they areperiodically repeated their autocorrelation function hasa definite peak in each cycle [2]. The choice of a maxi-mum-length sequence means that of the approximatelyone thousand million possible 31-bit words only 186need be considered. These are found to be cyclicalshifts of six basic forms, three of which are the timeinverses of the other three.The most suitable of these 186serieswas chosen with

the aid of a computer program, with an 'environment'of alternating 'O'sand' 1's as a precondition. The mostsuitable word was found to be:

'10110011111000110111010100001001'.

This word is used as the synchronization word in theeven field and its logical inverse in the odd :fields.

[2) S. W. Golomb, Digital communications with space applica-tions, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1964.

238 H. P. J. BOUDEWIJNS et al. Philips tech. Rev. 36, No. 8

The cross-correlation functions of these two wordsin an environment of alternating 'O's and '1 's with thebit pattern of the matched filter are shown in jig. 4.This figure also shows the threshold levels that thepeak must exceed to cause generation of the detectionpulse. This pulse resets the counters in the frequencydivider chain (DivCh, fig. 1) and thus ensures that thephase of the signals resulting from the divisions iscorrect.

Verification

Verification consists in exammmg every detectionpulse to see whether it occurs at the moment at whichit can be expected on the basis of the preceding pulses.If it does, the reading of an up-and-down counter isincreased by one; the maximum reading to which thecounter can advance is 31. If a detection pulse fails tooccur when expected, the reading of the counter isreduced by one.The up-and-down counter determines whether the

receiver is set for acquisition or maintenance of syn-chronization. If the counter is in any position between16 and 31, the receiver is set for maintenance of syn-chronization; the signal is supplied to the phase-lockedloop for only twelve bits during the line-blanking inter-val during which the low threshold level applies to thedetection pulses. If because there are no detectionpulses the counter comes to occupy any of the posi-tions 1 to 15, the receiver has to acquire synchroniza-tion again, but the connection is maintained; thereceived signal is then continuously supplied to thephase-locked loop, the detection threshold is raisedand the speech signals are suppressed. Only after15 failures to restore synchronization does the counteroccupy the position 0, which breaks the connection.

Speech transmission

Digitally controlled de/ta modulation

The choice of delta modulation (fig. 5) for thedigital coding of the speech was determined by thecomparative insensitivity of this modulation system tonoise and interference. It is better in this respect thanthe pulse-code modulation much used in telephony, inwhich the magnitude of successive signal samples isexpressed in digital words consisting of several bits. Inpulse-code modulation the first bit of each word has aweight of half the maximum signal value, so that anerror in that bit causes considerable disturbance. Indelta modulation, on the other hand, an incorrect bitcan never produce a deviation exceeding a singlequantization unit [3]. .

The difference between the actual signal and the'echelon curve' 'obtained with delta modulation is ex-

pressed in the effect known as quantization noise. Asfig. 5 shows, the strength of this noise is independentof the signal strength; the ratio therefore does notfavour weak signals. The situation can be improved byadapting the size of the quantization steps to the signalstrength. This is done by compressing the signal-strength differences at the transmitter and expandingthem at the receiver - the combined arrangement isknown as 'companding'.In the video telephone digitally controlled compand-

ing [4] is introduced by means of the circuit shown infig. 6. In addition to the normal feedback loop to thepulse modulator PM, the delta modulator on the leftof the figure has a second loop in which a modulation-level analyser MLA and a pulse-amplitude modulatorPAM are inserted. The first loop operates as follows.The pulse modulator PM generates a '1' or a '0',depending on whether the echelon curve (fig. 5) has togo up or down one unit; this information is derivedfrom the difference signal e originating in the differenceamplifierD from a comparison ofthe generated echeloncurve with the input signal Vi. The echelon curve is

ThroThr

15 48 63bits--·t

31

ThroThr

. V,

Fig.4. The voltages at outputs Vi and V2 of the matched filterwhile the synchronization word is being received. It is assumedthat the word is surrounded entirely by the '0101 ... Ol' series.Top: even-field blanking. Bottom: odd-field blanking . .t time in bitunits after the start of the synchronization word. Thr thresholdwhich the peak must exceed before the synchronization word isaccepted. Thrs raised threshold in .force during acquisiton.

Philips tech. Rev. 36, No. 8 VIDEO TELEPHONE, II

provided by an integrator for this comparison. Thesame integration takes place in the demodulator, whereit gives the output signal.The pulse-amplitude modulator PAM can now affect

the size of the steps, however. It does not do so as longas the '1's are not being transmitted in groups exceedingthree in succession. Then every three 'l's are followedby at least one '0', which means that in four time unitsthe maximum increase is equal to two steps. The mod-

if, conversely, 'O's are transmitted in groups of morethan three at a time. The demodulator also includesa pulse-amplitude modulator; the step size here willaccurately follow that at the transmitter since both aredriven by the same signal - transmission errors ex-cepted; the result is therefore expansion, When Vc iszero, a minimum step size is maintained at both endsbecause a small d.c. voltage Vo (~0.01 Vc,max) isinjected.

---t1 1 1 1 1 1 101 101 0 1 000 0 0 0 0 0 000 1 000 101

Fig. 5. Delta modulation. A 'I' is transmitted when the 'echelon curve' by which the signal isapproximated has to rise one step, a '0' when it has to fall one step.

Fig. 6. Simplified block diagram of a delta modulator (left) and demodulator (right) withdigitally controlled companding. This is achieved with the aid of the extra control loop in-corporating modulation-level analyser MLA and pulse-amplitude modulator PAM. D dif-ferential amplifier. G pulse generator. PM pulse modulator. Vi input voltage. e differencesignal. Vc control voltage. Vo d.c. voltage for maintaining a minimum step size. Va outputvoltage.

ulation index therefore does not exceed l However, assoon as more than three 'l's arrive in succession, themodulation-level analyser produces a pulse. Aftersmoothing, these pulses result in a control voltage Vc,

whose effect is to make PAM increase the size of thesteps. The tendency to form groups of more thanthree 'l's will therefore in general decrease again andthe system will thus try to maintain the modulation-index value near to t over a large range of input levels;in other words there is compression. The same applies

The improvement in the signal-to-noise ratio result-ing from the companding can be seen from the dif-ference between the two curves infig. 7, where curve Arepresents the signal-to-noise ratio without and curve Bwith companding, both as a function of the input level.

(3] J. F. Schouten, F. de Jager and J. A. Greefkes, Delta modula-tion, a new modulation system for telecommunication, PhiIipstech. Rev. 13,237-245, 1951/52.

(4] J. A. Greefkes and K. Riemens, Code modulation withdigitally controlled companding for speech transmission,Philips tech. Rev. 31, 335-353, 1970. .

239

240 H. P. J. BOUDEWIJNS et al. Philips tech:Rev. 36, No. 8

Voice switchThe interruption of the closed electro-acoustic circuit

needed to prevent feedback on the loudspeaking tele-phone is controlled by a comparison of the strength ofthe outgoing and incoming signals in the two sets. Thedigitally controlled companding offers an excellentfacility for this purpose, making it only necessary tocompare the two control voltages Vc from the trans-mitter and receiver.This is done in a differential ampli-fier. The output voltage from the amplifier controls aswitch that during each picture line passes either theeight sound bits or the standard series of alternating'O's and 'l's (corresponding to a d.c. voltage of zerofor the delta demodulator).

Measures have to be taken to prevent too frequentswitching. The sound from the loudspeaker is alwayslouder than that of a natural speaking voice. So muchcrosstalk can occur on the microphone of the sub-scriber who is not speaking as to cause his voice switchto close and consequently that at the other end to open.The echo in the room at the receiving end can alsoproduce a microphone signal when a pause occurs inthe received signal. To ensure that switchover does notoccur too readily or too rapidly, the voice switch doesnot react until the microphone signal has been domin-ant for at least 40milliseconds. In addition, the controlvoltage derived from the transmitter is attenuated bya factor of 2; seefig. 8. For a connection to be made, asound must now be produced that is coded in stepstwice as large as the received signal, which means thatit must have almost twice the amplitude.

One disadvantage of this asymmetry in the controlsignals is that during breaks in the outgoing speechsignal the control voltage of the receiver always pre-dominates and the outgoing connection is easily inter-rupted. To prevent this a hold circuit is incorporated,whose function is to ensure that a d.c. voltage Vh issubtracted from the control signal from the receiverwhen the voice switch is closed (fig. 8).

If the ordinary handset is used, the voice switch isshort-circuited; there is then an uninterrupted connec-tion.

Possibilities of other applications

A welcome additional facility is the conference cir-cuit, i.e. a simultaneous interconnection between threeor more video-telephone sets. In the conference modethe otherwise independent bit-clock oscillators in thetransmitters of these sets are all synchronized with asingle centraloscillator. A voice-actuated switch en-sures that the picture of the participant who is speak-ing appears on all the other screens. Use of the con-ference circuit necessitates a special conference unit

SIN(SIN)max

I -20

-30

-40

-50

-600 -10 -20 -30 -40 -50 -60 -70 -BOdB-Lj

Fig. 7. Ratio of signal S to modulation noise N as a function ofthe input level L1. Curve A: ordinary delta modulation. Curve B:delta modulation with digitally controlled companding.

'0101....

L-~____JI--------.-:_Ït-iS iJ·o-IIII

----III

l-----<~\.1,

~~--------------------~1--Fig, 8. Voice switch S in the outgoing sound channel is controlledby the differential amplifier D, which compares the control sig-nals Vc from the delta modulator DMod and delta demodulatorD'Demod. To ensure more reliable operation the control voltageat the transmitting end is reduced by a half and a constantamount Vil is subtracted from that at the receiving end when thevoice switch is closed.

which contains a number of auxiliary circuits and isbest installed in the exchange.The extra signalling capacity of the digital video-

telephone system described above will prove of greatvalue in this kind of application in the future. Morethan four line periods in the field-blanking interval areavailable for the transmission of additional informa-tion; see fig. 2. It is also conceivable that once thereceivers have been synchronized the first five lines ofthe field-blanking interval could be used in whole orpart for information other than the continuous '0101'series.The extra capacity might be used to eliminate the

objectionable phase jump that occurs on switching overto another participant. To achieve this result the phaseof the picture to which the change-over has to be madewould have to be registered just when the currentpicture phase was reaching zero, and passed on to the

Philips tech. Rev. 36, No. 8 VIDEO TELEPHONE, II 241

participants before the change-over. The change-overwould then take place at the next zero crossing and thecounters in the receivers would be set not to zero butto the phase that had just been indicated. Special ar-rangements are generally necessary in the monitor toensure that the field deflection can follow the discon-tinuity without any difficulty.The extra signalling capacity might also be used to

pass on messages from the participants to the con-ference unit. A participant might, for example, ask fora picture that was different from the one allocated bythe voice-actuated switch.Conference telephony is by no means the only extra

facility that might be added to the video-telephonesystem. For example, a video-telephone set alreadycontains many of the elements needed in a computervideo terminal, such as the digital connection with bit-clock synchronization and the screen. The signals from

an added keyboard could be accommodated in signal-ling time not as yet fully occupied. Another possibilityis the use of the wideband video-telephone circuits forthe transmission of data and facsimile.

Summary. There are various possible ways of arranging a video-telephone system, with or without separate transmission lines forsound and signalling. Separate lines are not required in the ex-perimental system at Philips Research Laboratories. Sound andsignalling are digitally coded and transmitted during the blankingintervals of the video signals. Each set has two bit-clock oscilla-tors (1.0016 MHz); that of the receiver is synchronized by theother party's transmitter. Numerous control signals, includingthose for picture scanning in camera and monitor, are derived byfrequency division from these oscillators. Series of 'OlOl's togetherwith a synchronization word in each field are used for synchroni-zation; the word is detected by a matched filter. Delta modula-tion with digitally controlled companding is used for coding thespeech signals. The control signal employed in this process alsocontrols the voice switch; this prevents oscillation due to electro-acoustic feedback. Other potential applications include con-ferences taking in more than two sets and use as a computer videoterminal.

242 Philips tech. Rev. 36, No. 8

Recent scientific publicationsThese publications are contributed by staff of laboratories and plants which form part ofor cooperate with enterprises of the Philips group of companies, particularly by staff ofthe following research laboratories:

Philips Research Laboratories, Eindhoven, The Netherlands EMullard Research Laboratories, Redhill, Surrey, England MLaboratoires d'Electronique et de Physique Appliquée, 3 avenue Descartes,94450 Limeil-Brévannes, France L

.Philips GmbH Forschungslaboratorium Aachen, WeiBhausstraBe, 51Aachen,~~ . A

Philips GmbH Forschungslaboratorium Hamburg, Vogt-Kölln-Straûe 30,2000 Hamburg 54, Germany H

MBLE Laboratoire de Recherches, 2 avenue Van Becelaere, 1170 Brussels(Boitsfort), Belgium B

Philips Laboratories, 345 Scarborough Road, Briarcliff Manor, N.Y. 10510,U.S.A. (by contract with the North American Philips Corp.) N

Reprints of most of these publications will be available in the near future. Requests forreprints should be addressed to the respective laboratories (see the code letter) or to PhilipsResearch Laboratories, Eindhoven, The Netherlands.

H. Bex: Automatic measurement of crossmodulation.Nachrichtentechn. Z. 28, 327-329, 1975 (No. 9). A

K. Board, J. M. Shannon & A. Gill: C.C.F.E.T.: anactive charge-coupled device.Electronics Letters 11, 452-453, 1975(No. 19). M

G.-A. Boutry: Photoémission et vision nocturne.Sciences et Techniques No. 18, 7-11, 1974. L

P. Branquart, J.-P. Cardinael, J. Lewi, J.-P. Delescaille&M. Vanbegin: Data structure handling in ALGOL 68compilation.Proc. Int. Conf. on ALGOL 68, Winnipeg 1974, pp.~~ B

J. W. Broer: Your interim report - did you time itrightly?J. tech. Writ. Comm. 5, 11-15, 1975 (No. I). E

H. H. Brongersma: Structuuronderzoek van vastestof-oppervlakken.Chem. Weekblad 71, No. 12, 16-20, 1975. E

K. H. J. Buschow, J. F. Olijhoek & A. R. Miedema:Extremely large heat capacities between 4 and 10K.Cryogenics 15, 261-264, 1975 (No. 5). E

K. L. Bye: Optische Beobachtung ferroelektrischerDomänen in elektrooptischer PLZT-Keramik.Ber. Dtsch. Keram. Ges. 52, 234-235, 1975 (No. 7). M

F. Caillaud, G.Durand, C. Le Can &G.LeFloch :Circuitsintégrés pour télécommande en technologie C-MOS.C.R. ColI. Int. sur les Circuits intégrés complexes,Paris 1974, pp. 295-303. L

F. M. A. Carpay, S. Mahajan (Bell Laboratories, Mur-ray Hill, N.J.), G. Y. Chin (Bell Labs., M.H.) & J. J.Rubin (Bell Labs., M.H.): Slip-induced cracking inmolybdenum single crystals:Scripta metall. 9, 451-457, 1975 (No. 5). B

T. A. C. M. Claasen, W. F. G. Mecklenbräuker &J. B. H. Peek: Remarks on the zero-input behavior ofsecond-order digital filters designed with one magni-tude-truncation quantizer.IEEE Trans. ASSP-23, 240-242, 1975 (No. 2). E

G. Clérnent, C. Loty, J. P. Roux & C. Chancel (Corn-missariat à l'Energie Atomique, Limeil): The design ofa new electron optics for a picosecond streak camera.Proc. 11th Int. Congress on High speed photography,London 1974, pp. 130-135; 1975. L

J. Cornet & D. Rossier: Les verres semi-conducteurs:dispositifs nouveaux et technologie.Coli. Int. sur les Matériaux pour les composantsélectroniques, Paris 1975, pp. 194-202. L

P. Devijver: Sur la signification de certains critères deI'analyse discriminante appliqués au problème de lareconnaissance des formes.C.R. Séminaire IRIA Classification automatique etperception par ordinateur, 1974, pp. 285-304. B

H. Durand: Solar energy, from science fiction to reality?or: Acta Electronica, 15 years later.Acta Electronica 18, 261-266, 1975 (No. 4). (Also inFrench.) L

F. L. Engel (Institute for Perception Research, Eind-hoven): Visibility, conspicuousness and attention.Ophthalmologica 171, 41-42, 1975 (No. I).

D. den Engelsen & E. P. Honig: Bouwen met mono-moleculaire lagen.Chem. Weekblad 71, No. 12, 28-32, 1975. E

G. Eschard: The microchannel plates: a survey of thepossibilities of channel multipliers arrays in nano andpicosecond detection and imaging.Proc. 1lth Int. Congress on "High speed photography,London 1974, pp. 163-169; 1975. L

Philips tech. Rev. 36, No. 8 RECENT SCIENTIFIC PUBLICATIONS 243

E. Fabre & M. Mautref: Caractérisation physique descellules solaires au silicium par l'étude des réponsesspectrales.Acta Electronica 18, 331-338, 1975 (No. 4). L

Á. J. Fox & T. M. Bruton: Electro-optic effects in theoptically.active compounds BÏ)2Ti02oand Bi4oGa2063.Appl. Phys. Letters 27, 360-362, 1975 (No. 6). M

M. J. C. van Gemert & Á. Suggett (Unilever ResearchColworth/Welwyn, Sharnbrook, Beds., England):Multiple reflection time domain spectroscopy, Il. Alumped element approach leading to an analyticalsolution for the complex permittivity.J. chem. Phys. 62, 2720-2726, 1975 (No. 7). E

J. J. Goedbloed: Investigation of parasitic oscillationsin IMPATT-diode oscillators by a simple locus chart.Electronics Letters 11, 54-56, 1975 (No. 3). E

J. M. Goethals & J. J. Seidel (Eindhoven Universityof Technology) : The regular two-graph on 276 vertices.Discrete Math. 12, 143-158, 1975 (No. 2). B

P. Hansen, J. Schuldt, B. Hoekstra & J. P. M. Damen:Anisotropy and magnetostriction of ruthenium-substi-tuted lithium ferrite and nickel ferrite.Phys. Stat. sol. (a) 30, 289-298, 1975 (No. I). H, E

D. Hennings: Mikrostruktur und mechanische Stabili-tät von dotierten BIeititanat-Keramiken.Ber. Dtsch. Keram. Ges. 52,220-222,1975 (No. 7). A

W. J. van den Hoek & G. Rouweier (Philips LightingDivision, Eindhoven): On thermodynamic calculationsof chemical transport in halogen incandescent lamps.Philips Res. Repts. 31, 23-34, 1976 (No. I).

W. K. Hofker, D. P. Oosthoek, N. J. Koeman (all withPhilips Research Labs., Amsterdam Division) &H. Á. M. deGrefte: Concentration profiles of boron im-plantations in amorphous and polycrystalline silicon.Radiation Effects 24, 223-231, 1975 (No. 4). E

E. Kauer, R. Kersten & F. Mahdjuri: Photothermal con-version.Acta Electronica 18, 295-304, 1975 (No. 4). A

E. T. Keve: Röntgenuntersuchungen zu Phasenum-wandlungen in PLZT-Keramik.Ber. Dtsch. Keram. Ges. 52, 232-234,1975 (No. 7). M

F. M. Klaassen: Device physics of integrated injectionlogic.IEEE Trans. ED-22, 145-152, 1975 (No. 3). E

F. M. Klaassen: A MOS model for computer-aideddesign.Philips Res. Repts. 31, 71-83, 1976 (No. I). E

F. M. Klaassen, W. de Groot & F. L. van de Markt:Computer algorithm to determine MOS processparameters.Philips Res. Repts. 31, 84-92, 1976 (No. I). E

W. L. Konijnendijk & J. H. J. M. Buster: Raman-scat-tering measurements of arsenic-containing oxide glasses.J. non-cryst. Solids 17, 293-297, 1975 (No. 2). E

J.-P. Krumme, P. Hansen, G. Bartels & D. Mateika:Control of the uniaxial magnetic anisotropy in LPE-grown iron garnet films by small ruthenium substitu-tions.J. appl. Phys. 46, 2801-2803, 1975 (No. 6). H

J.-P. Krumme, B. Hill, J. Krüger & K. Witter: A highlysensitive reversible and nonvolatile hybrid photocon-ductive/magneto-optic storage material.J. appl. Phys. 46, 2733-2736, 1975 (No. 6). H

J. Lemmrich: Winkelregelungen von Gleichstrom-maschinen mit frequenzanaloger Signalverarbeitung.Elektrotechn. Z. B 27, 401-404, 1975 (No. 15). H

J. Michel & A. Mircea: Simulation de cellules solairesau silicium et comparaison avec des résultats expéri-mentaux.Acta Electronica 18, 311-330, 1975 (No. 4). L

M. Monneraye: Conducteurs au cuivre sérigraphiés encouches épaisses.Coil. Int. sur les Matériaux pour les composantsélectroniques, Paris 1975, p. 132. L

B. H. Newton: M.I.C. TRAPATT oscillator for efficientS band operation.Electronics Letters 11, 299-300, 1975 (No. 14). M

Á. E. Pannenborg (Philips Board of Management, Eind-hoven): Technology push versus market pull - thedesigner's dilemma.Electronics & Power 21, 563-566, 1975 (15 May).

L. J. van der Pauw: The radiation and propagation ofelectromagnetic power by a microstrip transmissionline.Philips Res. Repts. 31, 35-70, 1976 (No. I). E

G. Piétri: Design and performances of fast phototubesfor photon counting or for high speed imaging.Proc. 6th Int. Symp. IMEKO TC on Photon detectors,Siofok (Hungary) 1974, pp. 142-161. L

R. Polaert & J. Rodière: Improvement of the perform-ances of high speed cinematography through the use ofa proximity focusing microchannel image intensifier.Proc. 11th Int. Congress on High speed photography,London 1974, pp. 170-177; 1975. L

L. J. Poldervaart (Eindhoven University of Technology)& C. G. Sluijter: Photonics: the profile of a new dis-cipline.Proc. 11th Int. Congress on High speed photography,London 1974, pp. 585-589; 1975. E

W. Rey: On least p-th power methods in multipleregressions and location estimations.BIT 15, 174-184, 1975 (No. 2). B

C. J. M. Rooymans: Recent trends in magnetic bubbledevices.Rev. Phys. appl. 10, 179-181, 1975 (No. 3). E

J. Á. J. Roufs (Institute for Perception Research, Eind-hoven): The Standard Observer: a controversial subject.Ophthalmologica 171, 43-44, 1975 (No. I).,

244 RECENT .SCIENTIFIC PUBLICATIONS Philips tech. Rev. 36, No. 8

J. M. S. Schofield: The physics of gas discharge dis-play cells.Proc. 12th Int. Conf. on Phenomena in ionized gases,Eindhoven 1975, Part 1, p. 73. M

P. J. Severin, H. Bulle, G. Poodt & J. D. Wasscher:On the technique and evaluation of angle-bevelingsilicon epi taxi al layers.J. Electrochem. Soc. 122,440-443, 1975 (No. 3). E

J. Smith: Experimental storage display panels usingde gas discharges without resistors.IEEE Trans. ED-22, 642-649, 1975 (No. 9). M

M. J. Sparnaay: Oppervlakte onderzoek bij Philips.Chem. Weekblad 71, No. 12, 13-15, 1975. E

J. L. Spoormaker: Design of reliable plastic assem-blies.Proc. 1975 Ann. Reliability and Maintainability Symp.,Washington D.C., pp. 498-503. E

A. L. N. Stevels & A. D. M. Schrama-de Pauw: On theluminescence, excitation and UV-absorption spectra ofCsI activated with Na.Philips Res. Repts. 31, 1-22, 1976 (No. I). E

T. Thalhammer: Future energy demand and the roleof solar energy.Acta Electronica 18, 267-273, 1975 (No. 4). E

J. A. T. Verhoeven & J. J. Vrakking: Analyse vanoppervlakken met Auger-elektronenspectroscopie.Chem. Weekblad 71, No. 12, 20-24, 1975. E

J. M. P. J. Verstegen, D. Radielovié & L. E. Vrenken(Philips Lighting Division, Eindhoven): A new genera-tion deluxe fluorescent lamp - combining an efficacygreater than 80 lm/W with a color rendering index ofabout 85.J. IlIum. Engng. Soc. 4, 90-98, 1975 (No. 2).

H. J. Vink: Food-production and bioenergy.Acta Electronica 18, 305-310, 1975 (No. 4). E

H. W. Werner & G. E. Thomas: Emissie van ionen envan licht door beschieting van oppervlakken met snelleionen: SIMS en BLE.Chem. Weekblad 71, No. 12, 24-28, 1975. E

W. Yu, R. R. Alfano (both with City College of NewYork), C. L. Sam & R. J. Seymour: Spectral broaden-ing of picosecond 1.06 fl. pulse in KBr.Optics Comm. 14, 344-347, 1975 (No. 3). N

Contents of Philips Telecommunication Review 34, No. 1, 1976:

F. L. van den Berg: The Philips Doppler VOR beacon RN 200 (pp. 1-10).F. L. Jansen & G. W. Versteeg: The quality of reed contact units (pp. 11-21).W. F. Njio & C. J. Verkooijen: Frequency generating equipment for FDM transmission systems (pp. 22-37).C. A. Schipper: Thin film circuits in transmission equipment (pp. 38-41).A. Potuit: Belgium chooses Philips coaxial line equipment for 60 MHz network (p. 42).

Contents of Electronic Applications Bulletin 33, No. 3, 1976:

Switched-mode power supply transformer design nomograms (pp. 95-121).Transformer and choke design for forward-converter switched-mode power supplies (pp. 125-143).Components for switched-mode power supplies (pp. 145-147).

Contents of Mullard Technical Communications 13, No. 129, 1976:

F. J. Burgum: Switched-mode power supply transformer design nomograms (pp. 354-378).M. C. BaselI: 50W multiple-output switched-mode power supply (pp. 379-387).CL8880 series radar traffic sensors (pp. 388-392).

Contents ofValvo Berichte 20, No. 1, 1976:

G. Tauchen: Bildröhrenheizung aus der Horizontalendstufe (pp. 1-12).J. Koch: Permanentmagnetische Polfühligkeit in Gleichstrommotoren mit Ferroxdure-Segmenten (pp. 13-22).J. Koch: Entmagnetisierungsenergie und Anziehungskraft von Permanentmagneten (pp. 23-32).W. Hetterscheid: Schaltverhalten bei Transistoren für hohe Spannungen (pp. 33-44).

Volume 36, 1976, No. 8 Published 21st January 1977pages 217-244