Home Movies and Home Talkies (Jun 1933-May 1934)

HOME MOVIES & HOME TALKIES 65 It is scarcely j^ossible to giv^e positive information on the constructional dimensions, etc., but Fig. 44 may help. It should be emphasised that all we are endeavouring to do is to throw a thin rectangle of extremely brilliant light iipon the edge of the sound track. The width of this rectangle sl:ould certainly not exceed one thousandth of an inch, and providing brilliancy can be retained is better still smaller. This will be vmderstood when, as we have already mentioned, we compare the width of the light rectangle to the point diameter of an ordinar\^ gramophone needle. Optical System Preferred How we attain this liglit rectangle is really unimportant, although, by experience, the optical sy.«'tem has been foimd preferable to the direct use of a slit without a lens. Referring to Fig. 44, we find we have three unknown dimensions. A, B, and C, which depend upon the characteristics of the lenses we employ. The distance A obviously depends upon the focus of the condenser and, naturally, this must be such that there is at least a small clearance between the lens and the glass of the exciter lamp. Distance B depends \ipon the focus of the objective lens and C upon the characteristics of both lenses. The dimensions shown in the figure are, however, roughly what is followed in practice, i.e., the lens assembly is round about two and a-half inches long and the tube diameter is about half an inch. Using these as a ba.sis the experimenter should be able to obtain suitable lenses from an optical manufacturer. Perhaps it may again be emphasised that the slit is merely a slot cut in a thin strip of metal and is the image-former of the light rectangle, which is optically reduced by the objective and projected on to the soimd track. Sound Unit Drive I'p to the present separate soimd units for sub-standard film are not marketed, and thus for pm-poses of illustration we must of necessity SOuNC GATE ASSEMBLY EXCIILR LAM IMPEDANCE ROLLER CONSTANT SPEED SPROCKFT TAKE-UP SPROCKET Fig. 45a. Another R.C.A. Sound Un {Courtesy R.C.A. Photophone.) turn to the standard .S5-mm. projectors. We have already shown one .small R.C.A. somid tuiit, and in Fig. 45a present another, in which the details are particularly clear. It will be noted that the exciter lamp possesses a. sliielding which prevents stray reflection affecting reproduction. In Fig. 45b will be seen the exterior view of the Western Electric Sound Unit such as is employed in thousands of cinema theatres. In basic principles the unit is similar to those of R.C.A. but does indeed follow even more closely the schematic illustrations already given. An intere.sting point in cormection with the Western Electric sou-nd imit is that it is completely free from moving mechanism, but fits over the continuous or movietone sprocket. The white patch seen through the glass of the centie compartment is really a hole through the back of the casting — through this hole comes the continiious sprocket wheel. In both R.C.A. and Western Electric systems it will be noted that an ammeter is fitted to the unit. The reason for this is that it is almost essential to know that the exciter lamp is always working to a fixed degree of brilliancy, and this is achieved by means of a rheostat set to bring the filament current to a predetermined value. The continuous running sprocket must rota.te at a speed which will di-aw 16-mm. films through the pro Fig. 45b. Exterior View of Western Electric Sound Unit. {Courtesy Western Electric Co.) jector at twenty-four frames per second, this being the standard. Last month a reader inquii-ed for some information comparing .35-mra. and ] 6-mm. sound film, and we can do no better than quote a table prepaied by the British Thomson -Houston Company in connection with their own sub-standard sound-on-film projector. ?,5-mm. Film. Inflammable. 900 feet gives lO-minute run. 1,000 feet weighs 5 lb. l(\-mm. Film. Non-inflammable. 360 feet gives 10-minute run. Equivalent of 1,000 feet of 35-mm. film weighs 14 oz. A heavy metal case is required for transit. Can be sent through the post. 3.5-wm. Film. Film speed : 24 frames per second ; 90 feet per minute. Width of sound track : 0.070 inches. Width of picture, 0.895 inches. Depth of picture, 0.748 inches. 16-?wm. Film. Film speed : 24 frames per second ; 35 feet per minute. \\'idth of sound track, 0.055 inches. Width of picture, 0.348 inches. Dej3th of jiicture, 0.300 inches. There appears to be some confusion as to the two film speeds of 16 and 24 pictures per second ; 16 pictures per second was the accepted standard for 35-mm. and 16-mm. film (14 pictures per second was used for 9-mm.). \Ahen sound-on-film was introduced it was found that this speed of 16 pictures per second was insufficient to obtain the high notes and thus it was increased by 50 per cent., thus becoming 24 pictures per second or 90 feet per minute. This new higher speed was adopted for all talking picture work, for disc as well as film, although in the case of the former it was not actually necessary. Although we have been speaking of pictures per second, really this is not the criterion, for it is the speed of film travel and not the number of pictures which count as regards reproduction. 16-mm. fi'm running at 24 frames per (Continued on page 69)