The motion picture projectionist (Nov 1931-Jan 1933)

August, 1932 Motion Picture Projectionist 23 Sound Head Troubles By Lloyd Harding IN a recent tour of motion picture theaters, the writer was impressed with the fact that a goodly proportion of troubles in the sound equipment originate in the sound heads of the projection equipment. That this is so is somewhat of a reflection on someone, we will not say who, for this type of trouble is the easiest of all to clear. In several cases the sound reproduction was marred by a distinct 96 cycle tone which came and went intermittently. One has but to hear this particular tone to recognize that it is caused by one thing only, — weaving of the film as it passes before the light beam in the sound head. The fact that the tone is one of 96 cycles denotes that it is caused by 24 frames of four sprocket holes per frame passing the light beam each second. The ensuing interruption of the beam 96 times per second produces the tell-tale tone in the speakers. If the tone were constant the projectionist would know that the fault was not with film weaving, but that the lateral adjustment between the light beam and the sound track was at fault. When the tone is heard constantly it indicates that the beam is being cut continuously by the sprocket holes. In the first case, that of intermittent 96 cycle tone, the remedy is to inspect the sound gate thoroughly and to ascertain what it is that is causing the film to weave. A bent flange on the guide roller is often found to be the cause, while in most cases it will be found that the bearings on which the guide roller runs are not tightened up snugly. Another common cause of weaving is that the spring, adjustment of the guide roller becomes weak and thus fails to exert pressure laterally on the film as it passes the beam. The second case, where the interrupting tone is constant, means that the lateral adjustment is so far wrong that the light beam is being cut by either the sprocket holes on one side or by the frame lines and picture on the other. All sound heads in use at the present time are provided with means for making lateral adjustment between sound track and light beam by one of the two following methods. In one case, the adjustment is made by moving the optical system laterally (from operating to drive side of projector and vice versa) and in the other case the same result is attained by allowing the optical system to remain stationary while the sound gate guide roller is moved laterally the required amount. In either case, the correction can be made while the film is being run through the sound heard during re hearsal. Never do it during the running of a show. Now while the film is running through the sound gate and the 96 cycle note is being heard from the horn in the booth, move the optical system, by means of its adjustment screw towards the drive side of the projector. When the tone disappears from the sound the correct adjustment has been reached. Now tighten up the locking arrangement on the adjusting screw and listen further for the disturbing sound. If none is heard the trouble is cleared. If however the tone is again heard continue the adjustment until it is eliminated. In the case of apparatus in which the optical system is stationary and the guide roller adjustable, move the guide roller by means of its adjusting screws toward the operating side of the machine until the tone disappears. When the trouble is with the film weaving back and forth before the light beam the remedy is usually not so simple. If the flange of the guide roller is bent it is usually best to replace it with a new part, although in an emergency the flange may be straightened with a pair of pliers. A weak tension spring should be replaced. In the case of loose bearings holding the guide roller it is only necessary to tighten them to a snug adjustment and then go through the process of lining up the sound track and the beam by means of the adjustment method provided with that particular installation. There are, of course, other troubles besides sprocket hole hum that originate in the sound head. When trouble has definitely been traced to the sound head, the following tests will help to locate the source of trouble. The Exciter Lamp If this becomes blackened through age, or is dirty it will cut down the volume, but not the quality of the reproduced sound. To bring the volume up to normal, it will then be necessary to run the volume control near its maximum value, whereupon the unavoidable hum from the amplifier will become objectionable. Therefore if the sound is weak or if there is no sound at all from film on one machine, look to see if the exciter lamp is lit at all. and if so, whether it is furnishing enough light to the optical system. If not, replace with a new lamp. The Exciter Lamp Socket Often the above conditions arise even though the exciter lamp is lit. In this case it sometimes happens that the spring pin contact in the center of the socket has pushed the lamp up in the socket. This may be due to the fact that the set screw contact became loose and results in the filament failing to be focussed on the slit. The card test will reveal this condition immediately. Place an ordinary white card between the optical system and the photocell. If the exciter lamp is in the correct position the light from the optical system will show up on the card as a round sharp disc of light. If the exciter is too high or too low the circle of light will not be perfect but will be shaded on the bottom or on the top as the case may be. If the exciter lamp is not in the correct position laterally, the shadow will appear on the light circle either to the left or to the right. Optical System Dirty condenser and objective lenses will cut down the volume, but not the quality. However, if there are globules of oil on or between the lenses of the objective, their refractive effect will be spoiled, and the slit will be improperly focussed on the film, with consequent distortion of the sound. If the optical system is out of focus, the sound will be drummy — such sounds as that of "S" will be missing, and music will lack brilliancy. If the slit is out of rotational adjustment, or is partly blocked, the sound will be distorted and speech garbled. However, it is to be noted that unless the light is entirely cut off by a totally blocked up slit, or total impediment in front ot one of the lenses, there will still be sound, only it will be of poor quality. Sound Gate This is one of the most important parts of the sound head, for much of the quality of sound depends upon" this and the way in which the film is pulled through it. It goes without saying that the surface of the gate must be clean and smooth — unmarred by any scratches. Also the ' teeth of the constant speed sprocket, or the surface of the impedance roller, must be clean and not worn. In the latter case, be sure that not only does the roller turn, but that it rotates at the same speed as that of the film passing around it, that is, there must be no slippage. Any unevenness of motion of the film through the sound gate, due to any of the above causes, or due to the viscous damping being too tight, results in poor sound. The tones may sound harsh and fluttery, especially at the higher frequencies and the background noise may be increased. If the sprocket or roller is eccentric, or the shaft bent, "wows" will (Continued on page 34)