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Review of Philips Projection Television

Wireless World, October, 1950
    
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Front view; from left to right the controls are Brightness, Focus, Volume and Contrast.

It is obvious that the great merit of projection television is the large size of the picture which is obtainable from apparatus of relatively small dimensions. Equally obviously, it has the advantage over many directly-viewed systems of providing flat picture. In the Philips Type 600 A the picture obtained measures 13½ in by 10 in. This is about the size obtainable on a 16 in tube but it appears on a flat surface whereas the face of a 16 in tube would almost inevitably have a considerable curvature. As shown in the photographs the viewing screen is fixed to the cabinet and the picture is projected on to its back by an optical system. The tube is of some 2½ in diameter and operates with a final-anode supply of 25 kV.

The picture obtained is bright enough for daylight viewing and, when viewed on the centre line, the screen is fairly evenly illuminated. The screen has directional characteristics in both planes in order to increase the efficiency and this has the inevitable result of restricting the viewing angle. As one moves away from the centre line not only does the picture as a whole lose brightness but it becomes unevenly illuminated and the parts of the screen nearer to one are brighter than those farther away.

The effect is not marked for deviations up to about ±3o° from the centre line and the viewing angle is thus adequate for most purposes. it is, however, undoubtedly smaller than with a directly viewed tube and this does remove some of the advantage of the flat screen.

The minimum satisfactory viewing distance appears to be relatively greater than with direct viewing. It is, of course, very much a matter of personal preference, but in some tests most observers placed it at 8-10 ft - the optimum distance being about 12-15 ft. Since the picture height is 10 in this makes the ratio viewing - distance / picture - height around 12:1, which is about double that for direct viewing.

It is not easy to see why this preferred distance is greater. It is not because the line structure is more prominent; in fact, the lines are much less noticeable than one would expect, even on close inspection. It is not because of any lack of detail, for careful examination reveals that the detail is not inferior to that with direct viewing.

It is probably connected with the entirely different character of the picture which, in itself, is almost certainly due to the viewing screen. The picture quality may be likened to that of a photographic print on matt or rough paper whereas the quality with a directly viewed tube is nearer that of a glossy print. Which is considered the better is a matter of personal preference and depends very much also upon the subject matter of the picture.

The somewhat restricted viewing angle and the rather large optimum viewing distance limit the domestic application of the set to those possessing large rooms. The size of the picture is undoubtedly too great for a small room.

On the electrical side the adoption of projection has only a small effect on the design and the receiver is in most respects conventional. It is a superheterodyne and can be adapted for London or Birmingham reception by changing the RF and oscillator coils and certain resistors. In spite of the high operating voltage of the tube the scanning requirements are little different from normal because the tube is relatively longer than its big brothers. A single pentode with a 'damping diode', providing HT boost, is adequate for the line scan.

Fig. 1. Circuit diagram of 25-kV EHT supply.

The 25 kV EHT supply is obtained through a voltage-tripler rectifier from a 9 kV damped sine wave developed in a ringing transformer which is connected in the anode circuit of a pentode. The circuit is shown in Fig. 1, the ringing transformer being T2 and the rectifiers V2, V4 and V2. A blocking oscillator V2 with transformer T1 develops a saw-tooth wave of repetition frequency 1 kHz across C2. This is applied to the pentode V2. It is cut off on the fly-back of the saw-tooth wave and for a period thereafter but is brought up gradually to full conduction during the intervals between successive fly-backs. It is the sudden interruption of current on fly-back which shock-excites T2 to develop a damped sine wave.

A winding on T2 supplies a portion of the voltage wave to the diode in V2 which rectifies it and develops a steady voltage across R2. This is applied as bias to V2 and acts rather like an AGC system to reduce the peak current in V2 if the output voltage increases. It tends, therefore, to maintain the output constant and so provides a regulated EHT supply.

The whole of the EHT circuit, including the rectifiers, but not the other valves, is housed in a metal container and is so adequately protected from accidental contact.

Safeguards against time-base failure, which would otherwise prove disastrous to the tube, are provided. The tube is normally heavily over-biased. Diode circuits develop a counter-bias from the outputs of both line and frame time bases and unless both are operating the tube remains blacked out.

There are four panel controls: Brightness, Contrast, Focus and Volume with which is combined the on-off switch. At the rear there are controls for Line and Frame Hold, Height and Interference Suppression. There is no width control.

The adjustment of the panel controls is more difficult than usual because it is not possible to get far enough from the screen to observe their effect accurately. They would be much easier to adjust if they were fitted to a 10 ft flexible cable instead of on the set itself.

Fig. 2. Layout of the optical unit which includes the tube and its associated coils.

The optical system is of the Schmidt type and is folded to occupy a minimum of space as shown in Fig. 2. The tube faces a spherical mirror B which reflects the light on to an inclined mirror A and thence through a correcting plate C to a second inclined mirror mounted in the back of the top part of the cabinet. This mirror passes the light to the viewing screen and its angle is adjustable for positioning the picture in the vertical direction. The first mirror has a central hole through which the tube passes.

Rear vxew of the receaver showing the inclined mirror in the upper part.

The tube-mounting with its focus and deflector coils, the spherical mirror, first plane mirror and corrector plate form a unit which can be removed as a whole. The tube base and the mechanical adjustments are reached through a removable panel in the side of the cabinet. The receiver chassis can be removed without disturbing the optical system. The cabinet is of wood and measures 21 in Wide by 22¼ in high by 18½ in deep. The set is for AC operation only and costs £88 14s 6d including purchase tax. A stand is available at £21 5s 2d extra. The makers are Philips Electrical, Ltd., Century House, Shaftesbury Avenue, London, WC2.

On test the receiver gave an excellent picture. The linearity was good and the synchronizing excellent. As already mentioned the picture detail was as good as with a directly viewed tube but the quality of the picture was of a different character.

In spite of the use of a regulated EHT supply, changes of picture size and focus with large changes of picture brightness were noticed. The contrast and brightness controls were rather fierce and it would be better if the panel controls had a more restricted range and were supplemented by pre-set controls.

Close-up of the receiver chassis with separating plate removed to show the optical unit. The 'porthole' is the corrector plate.

See also The Optics of Projection Television & MW6/2

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