Details of Talkie Reproduction

Radio Record, Volume III, Issue 43, 9 May 1930, Page 29

Details of Talkie Reproduction

From Film to Loud-Speaker

(By

MEGOHM

ITH the advent of the moving pictures, the public had to | ‘become accustomed to seeing | and enjoying dramatic pre- | Sentations stripped of what had previously been considered an: indispensable por-tion-the speech. Certainly the accompaniment of good orchestral music, the frequent’ helpingout of the plot by paragraphs of lettering, and at the same time a rapid improvement in the all-round technique of witture production, made the loss of anlimportant item less noticeable, ‘The result was that audiences soon settled down to accepting conditions as they were. But such a state of things would not continue indefinitely, and so the problem of adding sound to the films has engaged the attention of inventive minds for many years. First Performances. AS early as 1908, a few public performances were given in which a gramophone behind the screen was syn¢chronised as far. as possible with -the projected picture, but practically no further progress resulted from this experimenting. At that time only a single gramophone could be used, so that the volume was small, and quality extremely bad, judging by present-day standards. The méans of recording sounds «eortainly existed, but the means of reoducing those sounds with sufficient volume and quality for purposes of entertainment was lacking. With the advent of radio, a few keen minds quickly realised that the means they hoped to discover had arrived. But in its early days the radio amplifier gave no more quality or volume than the gramophone, and thus the production of sound films had to await drastie improvements in the radio amplifier and accompanying loudspeaker. A certain stage of development was reached, and the talking picture appeared everywhere’ with almost startling suddenness. Installations in General.: [THAT an old-established and pioneer firm in matters pertaining to sound reproduction for telephones should have become deeply concerned in sound reproguction for moving pictures is not su@jrrising, and for that reason a visit wasy paid to the head office of the Western DBlectriec Co. (N.Z.). Ltd. in Wellington, to learn as much as poss sible about the portion of talkie reproduction that is of most interest to radio enthusiasts. There are already 61 installations of this system now operating in New Zealand. 5 The equipment directly associated with the picture projector consists of a special turn-table, an optical system, together with a photo-electrie soundreproducing unit and its accompanying amplifier, a film take-up mechanism, and an electric motor. A constant + Speéd of 1200 r.p.m. is required in the motor, which is kept steady by a vacum tube regulator. To ensure accurate synchronism and absence of vibration. all gear wheels are cut by a highlyaccurate method, but even then more drastic means must be employed to prevent interference, chiefly mechan-

ical, from reaching the amplifier or dise reproducer. To effect this there are flexible couplings, mechnical filters of felt-covered springs, and a hydraulic damping action on the sound sprocket drive. The pick-up. pivot is cushioned on rubber, and in addition the whole turn-table assembly is mounted on a soft rubber pad. The whole of the apparatus mentioned is built compactly into the lower part of a metal stand, to the upper part of which any standard picture projector and arc-lamp may be attached. In moving-picture reproduction it is necessary for the film to make a brief stop for each picture during the frac-

tion of a second in which it appears on the screen. whilst for sound reproduction a continuous steady forward movement is required. The recorded sound occupies a nartow strip about 1/10in. wide at one side of the film, but instead of the sounds belonging to a particular "frame" of the picture being situated in the adjacent margin, the sound record is 144 inches ahead of the corresponding picture. Thus there is a small amount of slack film between the sound-reproducer and the projec:

tor, so that the jerky motion of projecting does not affect the steady movement of the sound reproducer, to which the film is fed by a. smoothlyrunning sprocket-wheel. A special turn-table incorporated in the machine has already been mentioned.. This turn-table runs the "synchronous". dise records, which may be used when necessary to supply sound in place. of a sound track record on the film. These disc records are different in many respects. from the familiar type of record. They run at a speed of 88 1-3 revolutions per minute instead of the usual 78. They are 16 inches in diameter, and contrary to the ordinary custom, the record commencés near the centre, terminating at the outside edge, Ordinary or "non-synchronous" gramophone records are played upon a separate unit suited to the purpose, and wired to the controlling switch. Two film-operating machines are invariably installed, so that there is no delay for the changing of spools, Photo-Electric Track. [THE essential portion of the optical sound record is a track of varying photographie intensity, about onetenth of an inch wide, running along oné side of the film inside the sprocket holes, through its entire length. Owing to the number of pictures passing per second (24), the sound record is well stretched out, fin. being thus only required to register the sound occurring during 1-24th of a second. This lengthening of the sound record favours great accuracy. Actually, the sound vibrations are represented by fine ‘ines or bands across the recording strip. A diagram illustrates roughly the appearance of the sound-track when. magnified, but there is actually a more gradual shading than can be shown diagrammatically-a_ gradual photographic shading interspersed with dark bands. It should be noted that the lighter spaces represent the louder sounds, and that dark bands represent brief intervals of silence, because the photo-electric cell only passes current when there is at least a small amount of light. Inspection of a numb. of enlarge: ments shows a wide variety in the general appearance of the records. Single notes are represente¢. by even-ly-spaced-dark lines between each vibration or cycle, with gradations in between representing the modulation. When a strong light shines through the record and into a photo-electric cell, the latter passes current in proportion to the intensity of the light. so that when the currént passed is amplified, the sound vibrations are reproduced, ‘At this point it would be as well to describe briefly the photoelectric ¢ell, . ‘

The Photo-Electric Cell, THIS cell is a glass’ vacuum tube somewhat larger -(for this puarticular purpose) than an ordinary radio receiving valve. In the -centre is the anode, which is a circular ring of metal supported by two wires, and having a contact through the base. The other electrode or cathode is formed by first coating the inside of the bulb with magnesium, and over this a deposit of potassium salt, which is photosensitive, ° ' Other light-sensitive alkali metals are lithium, sodium, rubidium, and cacsium, ‘A platinum connection through the upper part of the bulb contacts the silvered coating and thus the whole of the potassium lining. This electrode is connected to the negative of the battery supply, and the anode to the positive, in this case 90 volts, At one side of the bulb a clear glass space is left through which the operating-light may enter. When no light enters the cell, no current passes, and when light enters, the current passed is directly proportional to the inténsity of the light, so that doubling the strength of the light doubles the amount of current passed. It is this’ property that renders the céll so very useful for a number of purposes. : The current actually passing with a light of great intensity is extremely. small-far less than that génerated by a pick-up and too small to b~ heard in ordinary ’phonés. Great amplification is theréfore necessary, and iu this particular system amounts to betweén one and two million times when it is delivered from the loudspéakers. The précise amount of amplification is decided by the size of the auditorium. The action of the photo-electric dell depends upon the fact that when the anode or plate is positively charged, and the light-sensitive surface (cathode) on the inside of the bulb is negatively charged by connection to the respective poles of a battery, the negative electrons with which the cathode is charged cannot leave it until illuminated. The presence of light. has. the same effect as the heating of a valve filament. so that when the cathode receives a ray of light ,electrons are driven off and rush to the positively-charged anode. ‘This electron stream is exactly proportioned to the amount of light entering the cell, Operating the Photo-Electrie Cell. AN exciting lamp is used to provide the light that passés through the sound récord to the photo-eléctric cell. This lamp is specially made with a short, straight filament, and ‘is rated at 50 candle-power. A miniature projecting system of lenses is provided in a tube about oné inch in diameétém Near to the exciting lamp is the usual condensing lens which concentrates the light of the filament upon a narrow horizontal slit néar the céntre of the tube, The exciting lamp has adjustments in three directions, so that its correct position in relation to the slit may be easily found. In front of the slit is a small objective-lens which

focuses’ the image of the. illuminated slit upon the ‘sound-track on the film, which in turn communicates the sound vibrations to the photo-electric cell in the form of intermittent light rays of varying intensity and. duration. The feeble current passing even when the cell receives maximum illumination has already been mentioned, and now the process of amplification begins. The high impedance of the ploto-electric cell works ..best with’ a resistance coupling followed by one stage of transformer coupling.’ © This compact amplifier styled 49A, is fitted with non-microphonie sockets, while the ‘rubberised base is suspended on four spiral springs in its metal hous-. ing, which rests on rubber, the whole. being compactly stowed away in the lower portion of the machine stand. Stepping-up the Amplification. LL that has so far been described takes place in the "machine" itself, but further amplification is carried out upon a convenient metal rack upon which’ the subsequent amplifiers and regulating gear are placed. The output from the amplifier on the ‘machine next passes through an "attenuator" or ‘balancing resistances (wire-wound), their purpose being to balance the output. of the two machines that are always installed, so that there will be no jump in volume when changing over from one machine to the other. s There is also a "fader," which is merely an electrical potentiometer, and ig the main volume control for the loudspeakers. ‘Then there is what 1s known as the "non-synchronous" panel, providing for switching on to the amplifier either the ordinary gramophone turntable or the switch on the machine which changes from synchronous disc to film record or vice-versa. The first amplifier on the rack consists of three stages of resistance-coup-ling with "239" valves, having an impedance of 15,000 to’20,000 ohms and a 4-volt filament. A 12-volt accumulator . is included in the plant, so the three

valves in this: amplifier are wired in series to utilise the 12-volt supply and also obtain grid-bias from it, whilst the necessary plate current is drawn from the succeeding amplifier, which is entirely a.c. operated. Here follows the "main gain control" which is. really a.volume control

with switch studs. Its use is to "calibrate" the theatre, or fix the available volume so that the "fader" has a range suited to the requirements of the building. Once set correctly, this control is not altered. The next amplifier is known as the "494." and consists of one stage of ‘push-pull with a pair of 205D valves,

5-watt. rating, with an impedence of 6000. to 7000 ohms (a valve approximately similar’ to the UX210).- Two of these valves aré also’ used as recti‘fiers, supplying a total of 57° m.a. to the two plates at 400 volts. The filaments. take 1.60 amps. at 4 to 5 volts, or a mean of 4.4'volts. Special-precau-tions in the form of filters and resistances are included to eliminate motorboating and hum. This and succeeding stages are all transformer coupled, .the transformer cores being made of "permaloid," a specially high-grade «alloy suited to the purpose, and adding, to the excellence of reproduction, whilst a further refinement is provided by al-. lowing only. the alternating component of the signal to traverse the primary windings of transformers, a coupling condenser being employed to divert the direct current through an audio choke or’ suitable resistance. This method applies to each push-pull stage. The power output of this stage is 2.4 watts, and for a theatre’ containing 700 to 800 seats, is the final amplifier, completing the "2S" system. For larger auditoriums a further push-pull amplifier known as. the "438A" is required. ‘This is. also completely. a.c. operated, arid contains a pair of 211E valves as amplifiers, and another pair of rectifiers. The plate current taken by this stage is 125 m.a. The output now available to operate loudspeakers is 12 watts, and sufficient for the average theatre, being kiiown as the "2SX" system. ..: Yor the largest theatres the 2SX system is used, with the addition of another 43A amplifier, which is wired in parallel with the one already provided, the output now. being 24 watts;.which operates six loudspeakers: at full‘-vol-ume. All the apparatus used is manufactured to suit a standard supply of 110 volts, so that where the supply is 230 or other voltage, a transformer is-em-ployed to supply 110 volts to the amplifiers. An impedance-matching panel takes the output of the last amplifier in the system, which passes through an autotransformer with tappings taken to switches on the panel. Bach speaker unit is also connected to a switch, so that by selecting the correct tappings the impedance of each unit-all units are in parallel-is matched against the impedance of the system. ‘ This feature of matching impedances throughout the circuit from photoelectric cell to loudspeakers is a large factor in the success of the system. In the next issue the writer will describe in detail the huge loudspeakers that complete the talkie equipment.