PH I LI PS

TECHNICAL VOLUME

REVIEW

36, 1976, No. 4

An experimental video-telephone system E. A. Aagaard, P. M. van den Avoort and F. W. de Vrijer

'Mr. Watson, come here, I want you.' Those were the first words ever transmitted loudly and clearly by telephone. It was the 10th of March 1876 and Alexander Graham Bell was calling to his assistant for help, having splashed battery acid over his clothes while installing a new type of microphone. A hundred years have passed since then: the telephone is now commonplace and thought and interest have moved on - in the direction of a combination of speech and vision. The video telephone is still in an experimental stage; not because the necessary techniques are unknown but because of a desire to assess the value to the user of the various facilities it offers. A small experimental system has been in operation at Philips Research Laboratories for a number of years. Now the Netherlands Post Office and Philips have jointly set up a 'national' experimental network and linked it up with the original system. The article is mainly concerned with the system used at the Research Laboratories and gives a general description. A subsequent article will examine in greater detail the digital means used for synchronization and sound transmission.

Video telephone The video telephone or videophone enables users phone exchanges are not suitable for switching signals not only to hear but also to see each other. The call with such a large bandwidth; either they will have to gains considerably by this addition: users can see each be modified or special exchanges must be built. other's facial expressions and show each other drawIn view of this situation it has been suggested that ings, documents, etc. Contact is thus closer and concompletely new broadband systems should be built, versations are possible which could not be conducted which could also be used for the transmission of data by ordinary telephone. The video telephone can even and facsimile and for the distribution of broadcast be used for conferences, so that participants do not television. The existing telephone cables, however, have to travel. could still be used for local video-telephone circuits. The television picture accompanying the speech To enable the technical problems associated with a forms a valuable addition to the telephone call even if video-telephone service to be studied and the utility of only the head and shoulders of the user are clearly such a service to subscribers to be evaluated, a small imaged. Naturalness is still further enhanced ifthe norexperimental system of 20 stations was set up at mal handset is replaced by a loudspeaking telephone. Philips Research Laboratories in Eindhoven. Some of A video telephone of this kind can be achieved with these stations are located in video-telephone kiosks for existing techniques. For sufficient sharpness the picture general use. The system, which went into operation in has to have about three hundred lines; the video signal 1972, has now been incorporated in a larger experimenemployed will then have a bandwidth of about 1 MHz. tal network with trunk circuits between The Hague, The transmission of signals with that bandwidth over : Leidschendam, Hilversum and Eindhoven (fig. 1), set existing telephone cables is possible over short disup jointly by the Netherlands Post Office and Philips tances. As in the existing telephone service, the connecTelecommunication Industries [11. This larger system, tions for a video-telephone service have to be estabcomprising a total of 65 stations, went into use in lished through an exchange. Existing automatic teleMarch 1974. Ir E. A. Aagaard and Ir P. M. van den Avoort are with Philips Research Laboratories, Eindhoven; Dr F. W. de Vrijer is with Philips Research Laboratories, as a Scientific Adviser.

[1]

L. J. w. van Loon, H. van der Hoff and S. J. A. Knijnenburg, An experimental video telephone network, Philips Telecomm. Rev. 32, 11-23, 1974.

E. A. AAGAARD

86

This system is also intended for experimental purposes only and does not imply any decision concerning the future introduetion of an actual service. It is quite possible that the video telephone will first go into normal service in government departments, commerce and industry, and not become available in the private sector until very much later. It is difficult to predict the extent to which the need to travel will be affected by the existence of a reliable

o

el al.

Philips tech. Rev. 36, No. 4

video-telephone service, but the question is an interesting one in view of the expected increase in personal travel. This is why the experiments include the holding of conferences by video telephone (fig. 2). At any rate users report that they find a video-telephone call comes somewhere between an actual conversation and a telephone call; perhaps there is some significanee in the fact that people with a choice like using the video telephone for longer ca11s. The account of the video telephone given here is a technical one. It deals with the subscriber's station and, briefly, with the method of sound transmission and synchronization. First, however, we shall discuss the signal standard chosen for the video telephone. Choice of signal parameters Picture The bandwidth of a television signal is determined by-the num ber of picture elements per second which it is desired to transmit. This number increases with the number of lines per picture and the number of pictures per second; for a given pictu re height it is also proportional to the picture width. The bandwidth

B is given by: B = 1- n2

where n is the number height. The visible portion

pn occur

I-p

I-m

afK,

of lines and a the ratio of the width

picture

consists

of fewer than n lines;

in the field-flyback

time.

there are n2( I - p)2a elements per picture. mitred Fig. 1. The 'national' experimental video telephone network. Subscriber sets are installed at the Netherlands Post Office establishments at The Hague and Leidschendam and on Philips premises at Hilversum and Eindhoven, as well as Philips Research Laboratories (Nat, Lab.). The longer distances are spanned by radio links and coaxial cables. The total number of terminals is 65.

f

times

per second.

is not continuous;

m of every line time (line-flyback transmitted. This increases (I - p)-1(1 - 111)-1. Finally, perceptual

reasons.

therefore,

These have to be trans-

The time available

for a fraction

Initially,

to the a pro-

for this, however,

p of the field time and a fraction

information

is

the bandwidth by a factor a correction has to be made

time) no picture

of for

The line structure

makes the resolving

Fig.2. Conference by video telephone. Conference stations have been installed at several points in the experimental network. In the experimental station shown here (at Philips Research Laboratories) only a single large monitor is used; the other conference stations employ several ordinary video-telephone sets.

power

Philips tech. Rev. 36, No. 4

of the eye in the vertical alent

reduction

in

the

direction horizontal

this is taken into account than unity.

VIDEO TELEPHONE

We have taken

smaller. direction

in the Keil factor

A subjectively

equiv-

is also permitted; K, which is smaller

a value of 0.65 for K.

A picture rate of 25 per second is an obvious choice, since it is the same as that employed by TV broadcast stations. One advantage of this picture rate is that it is compatible with the use of fluorescent lamps, whose light emission has a ripple synchronous with the 50 Hz of the mains voltage. As in television, interlaced scanning, i.e. the scanning offirst the odd lines and then the even lines, is used to avoid flicker. To ensure that a

A picture consisting of about 300 lines ensures a good head-and-shoulders likeness of the speaker. The associated bandwidth is over 1 MHz. Such a picture can be compared with a quarter of an ordinary television picture (fig. 3). Unrepeatered transmission over existing local telephone cables is possible up to 1-1-2 kilometres; beyond that amplification will be necessary, especially for the highest frequencies. We opted for 313 lines because this number makes conversion to the broadcasting standard (625 lines) and vice versa relatively straightforward [2]. This decision was welcomed internationally for the same reason. A 625-line

Fig.3. A picture of about 300 lines, as used for the video quarter of the picture used for broadcast television.

sideways movement does not make the speaker disappear immediately from the screen, the width of the picture is greater than its height; as in broadcast television the 'aspect ratio' is 4 : 3. There is little freedom of choice for these various parameters, and it is consequently the number of lines which mainly deterrnines the bandwidth. Here a cornpromise has to be struck between the picture quality and the transmission requirements. If existing telephone cables are used locally for the video telephone, two problems arise because of the high frequencies: attenuation, and crosstalk on other pairs. The permissible attenuation is limited by crosstalk, noise and other interference. To reduce these effects repeaters have to be introduced into the cable at regular intervals.

87

telephone,

is equivalent

to one-

picture has a bandwidth of 4.8 M Hz and can usually only be conveyed over a short distance on a telephone cable. Sound The limited bandwidth customary in telephony (300 to 3400 Hz) does not yield very satisfactory reproduction over a loudspeaking telephone. To improve the quality of the sound we extended the bandwidth to 5 kHz, so that the sound is now comparable for quality with that of medium-wave sound broadcasts. This [2]

M. C. W. van Buul and L. J. van de Polder, Standards conversion of a TV signal with 625 lines into a videophone signal with 313 lines, Philips Res. Repts. 28, 377-390, 1973, and: Standards conversion of a videophone signal with 313 lines into a TV signal with 625 lines, Philips Res. Repts. 29,413428, 1974.

88

E. A. AAGAARD

et al.

Philips

tech. Rev. 36, No. 4

Fig. 4. Video-telephone set developed and in operation at Philips Research Laboratories. It consists of a picture unit, a control unit and a service unit. The picture unit incorporates both the camera (top centre) and the picture tube, as well as the loudspeaker. The control unit contains the microphone of the loudspeaking telephone system in addition to the push buttons for 'dialling' calls, and several switches for a variety of functions; if necessary, the loudspeaking telephone can be replaced by the handset. The service unit contains various supply circuits and also provides the matching to the telephone lines.

increase in bandwidth was possible because the sound is not transmitted along a telephone line in the normal way but is digitally encoded and then combined with the video signal.

The video-telephone set The design of a video-telephone set requires a study of both the surroundings in which it is used (lighting, site, sound level, etc.) and the human factors that play a part in relaxed and natural conversation [31. A video-telephone set for the experimental network was developed at the Research Laboratories (fig. 4); this was followed later by a design by Philips Telecomrnunication fndustries (PTl) for the 'national' network (fig. 5). The sets differ in a number of minor technical features. They both consist of three units: the actual picture unit, the control unit and a service unit. The picture unit comprises the picture monitor, the camera and also the loudspeaker or loudspeakers. The control unit carries the pushbuttons for selecting the desired subscriber, a number of switches and the microphone of the loudspeaking telephone; it also incorporates a handset which can be used instead of the loudspeaking telephone. The service unit, which can be stored out of sight, houses the power supplies for the other two units and networks for matching to the telephone lines. Location of the video-telephone set When two people are seated at a table talking to each other their eyes are at approxirnately the same level and 120 cm to 2 metres apart. It is not practical to simulate this situation exactly with the video telephone; the picture unit would get in the way. In practice it will have to stand on the corner of a table or desk, and the viewing distance will then be 90 cm to 1 metre.

The picture size is related to this distance. It has been chosen such that the 300 or so picture lines are only just visible; larger dimensions would not help and could even be a nuisance. The picture height has to be one-sixth to one-tenth of the viewing distance [41, i.e. 9 to 17 cm. The picture height of existing 9-inch monitor tubes comes within this range and these have been used for the video telephone. The picture is seen with minimum distortion if the picture screen is set at right angles to the direction of viewing, i.e. with a slight backward slope. This means, however, that annoying reflections from the ceiling lighting can occur in the slightly convex screen; the slope has therefore been decreased slightly (in the PT! version the entire unit can be tilted backwards a little). Brightness and contrast Unlike a television receiver the video telephone is used in very brightly illuminated surroundings. Bright lighting is in fact necessary for the camera. Without suitable precautions the brighter ambient light would lead to reflections in the picture screen, direct reflection on the smooth face of the picture tube and diffuse reflections on the phosphor layer. The images reflected from the picture screen are annoying and the phosphor reflections give a diffuse background light, reducing the contrast between dark and light parts of the picture. To counteract the diffuse reflection from the phosphor layer the picture tube is made of dark-tinted glass which attenuates the light. The reflected light passes through the glass twice and the light from the picture only once so that the relative brightness of the picture is improved. The direct reflections are suppressed by an optical filter system consisting of a polarizing filter with a birefringent layer, in front of the picture. The diffusely

Philips

tech. Rev. 36, No. 4

VIDEO

TELEPHONE

89

Fig. 5. Video-telephone station set designed by Philips Telecommunication Industries (PTI) for use in the national experimental network. Like the set developed at Philips Research Laboratories (fig. 4) it consists of three units. The technical differences are only minor.

reflected light is also attenuated twice by a factor of 2, i.e. in total by a factor of 4, by the polarizing filter. Reflections from the filter itself are prevented by an anti-reflection coating. The method

used to suppress

in jig. 6. The polarizer particular

direction

birefringent direction

of polarization The

axes are at angles polarized

through

along

the layer than

light with a In the

in the figure).

of 45° to this

incident

as the sum of two equal components

these axes. Light polarized rapidly

only the incident (vertical

layer B the optical of polarization.

considered

is illustrated

the direct reflections

P transmits

light

can

polarized

the SS axis is propagated

the other component.

be

along less

The thick-

ness of the layer is such that on leaving

the layer the first com-

ponent

in relation

has a phase

component. After circularly

lag of a quarter-cycle

The emergent

reflection polarized,

from

light is thus right-circularly the front

becoming

through the birefringent the vertical polarizer.

of the picture

horizontal

layer. It therefore

to the other polarized.

tube it is left-

after its second passage cannot

pass through

Fig.6. Suppression of mirror (direct) reflections on the picture tube. A screen consisting of a polarization filter P and a birefringent layer B is placed in front of the picture tube. On passing through Pand B, the incident light is circularly polarized. The direction of rotation of the polarized light is reversed on reflection at the picture tube. After its second passage through the birefringent layer the polarization is linear, but now in a direction perpendicular to that of the incident light; the polarizing filter therefore does not transmit the reflected light.

Eye contact People engaged in an ordinary conversation look each other in the eye from time to time. With the video telephone the eyes are imaged slightly above the centre of the screen, which is where the lens of the camera should therefore be sited for eye contact. It has however been found that a deviation of the direction of viewing in the vertical plane goes unnoticed up to an angle of 3° and does not seem unusual until it reaches 12°. The camera lens is located just above the screen, in the middle. This is the direction in which any deviation is most readily tolerated; the deviation here is in fact 7.5°. If the lens were placed below the screen, the deviation would be less favourable, while the camera would also 'see' more of the brighter ceiling lighting. Image plane, depth offocus The camera lens ensures that an image plane situated about 1 metre in front of the camera is imaged sharply on the target of the miniature 'Plurnbicon' camera tube used in the video telephone. The dimensions of the image on the target are about 8 X 6 mm. In the PTI stations the lens can be focused on objects at a greater distance to show a wall panel or blackboard. The camera is equipped with a lens of continuously variable focal length (zoom lens). The picture area is largest at the smallest focal length. It must be sufficiently large to allow the user some freedom for small movements but it must not be unnecessarily large, either, because the face would then appear too small. The image plane can be shifted vertically by tilting the camera, over a range of ± 4°. In the PTI station the camera is not tilted but the scanned part of the target (3J

[4J

R. R. Stokes, Human factors and appearance design considerations of the Mod II 'Picturephone' station set, Conf. Rec. 1968 IEEE Int. Conf. on Communications, Philadelphia, pp. 130-135. R. Theile, Fernsehtechnik, Springer, Berlin 1973.

E. A.