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

HOME MOVIES & HOME TALKIES 23 sound luiit of one of the R.C'.A. small size soimd reproducing equipments. It should, however, be pointed out that this luiit is designed for use with standard projector heads, using :{5-mm. film, but the design is somewhat simple since it is to cater for very small theatres having a maximum capacity of 500 seats. Those readers who have studied the home talkie question will appreciate that this .size auditorium can be covered by some of the more powerful substandard equipments. RefeiTLng now to the photograi^h. we find the exciter lamp, optical system, sound gate (styled film gate here) and the photo-electric cell, which in this instance is mounted in a sideways fashion. The constant speed sprocket and the take-up sprocket are also shown as are the transformers for the exciter lamiJ and the photoelectric cell. A close examination of the photograph will also indicate the film path. In the professional outfits space is of little importance, but it will be seen from this sound imit that much of this is to spare and that if it were arranged for the exciter lamp to be swung through an angle of 'M) degrees into line with the axis of the optical system the unit could be made very much more compact, especially if the photo-electric cell were treated in a similar manner. Of coiuse, both exciter lamp and cell would have to be redesigned for this especial purpose. Functions of Parts To explain the mechanical functions of the various parts handling the passage of the film to the sound unit a transverse section of the mechanism is shown diagrammatically in Fig. 41. Here we have the film running downwards and passing in succession over the top guide roller through the sotmd gate, past the movietone or constant speed sprocket, over the take-up sprocket and romid the takeup spool. What is shown in this diagram is practically equivalent to that seen by taking a section across Fig. 38 down the path of the film. It might be thought somewhat curious that a single guide roller is given so much prominence, but actually this small detail plays an important part in soimd-on-film reproduction and unless it is set exactly right our programme is not likely to be very enjoyable, its name is self explanatory for by virtue of its flanges it constrains the film and thus the sound track to pass exacth' before the slit in the sound gate upon which the light from the lens assembly is imj)inged. A little consideration will show that if the film wanders either to the right or to the left the small light pencil will be overlapping either the edge of the film itself or the pictures. When using standard film wrong setting of the top guide roller frequently pioduced what was known as "Sprocket hole noise" which, coming from the loud speaker, resembled several dozen aeroplane engines running at open throttle. Top guide rollers are always made adjustable so that small variations in film and disjiosition of sountl track can then be catered for. .As a matter of fact different tyjies of" sound track sometimes require sj)ccial setting of HERMETICALLY SEALED OPTICAL SYSTEM e.xciter lamp switched on. If the iihn is allowed to remain stationary for about twenty seconds and then turned on a little further the light slit prints itself on the film and a comparison with the position of this with the standarfl sound tiack will indicate PHOTOCELL EXCITER LAMP TRANSFORMER CONSTANT SPEED SPROCKET TAKE UP SPROCKET Fig. ^0. Small R.C.A. Sound Unit PHOTOCELL rRANSEORMER l'liot(i)>hmie the guide rollers although not to the extent nowadays as was experienced in the beginning when sound tracks were not standardised. It is possible Track f/^AA Cast. ' Z^' Sound ,jJGAr^ /ifFAVY fi. yiVHEBL For DamP//^g Fig. 41. Showing the relation of pans to set the roller while the film is in motion, but normally this procedure is not to be encouraged since an overturn to the right or the left may mar reproduction. It is very easy to set the position of the roller with a piece of old exposed negative film. This is threaded through the machine and the whether or no the tojj guide roller j)Osition is as required. The soimd gate is simi)ly a smaller version of the picture gate of the main projection system. In Fig. 41 the letter X shows the optica! sotmd axis, the gajj shown on the sotmd gate is not to be confused with the slit which has been mentioned in this and preceding articles. In most sovmd-on-film equipments the light pencil thrown upon the sotmd track is a fraction of a thousandth of an inch in width, thtis practically eliminating the possibility of forming it by purely mechanical means. Imagine a slit of onehalf a tliousandth of an inch wide cut in a piece of metal ! After the first few feet of film this will become clogged with small dust particles and each one of these would affect the reproduction. Normally the fineness of the slit is reproduced by optical means. Reverting again to Fig. ."^S it will be seen that between the condenser and the objective lens in the lens assembly is the diajihragm having a small opening. This opening is the stencil, as it were, through which the light prints the size and shape of the pencil which is further reduced by the objective lens and thrown on to the track. There are numerous types of sound gate fitted to the machines, not the least ingenious being one vyhich works without springs simply \>y bowing the film rotmd a series of rollers and a curved piece of metal. In another system the slit is nctually a mechanical one, but is kept clean since it is formed between two constantly rotating rollers.