SYNCHRONIZED SOUND PICTURES 185 



Speed Regulation Requirements 



A good musical ear while having a sense of absolute pitch of only 

 about 3 per cent is extremely sensitive to sudden changes in pitch. 

 It has been found that a sudden change in pitch as small as one half 

 of 1 per cent may be noticed if made abruptly. In order to properly 

 take care of this requirement, therefore, the speed regulation or 

 change in speed of the motor drive over normal variations in line 

 voltage and load should be held within 2/10 of 1 per cent. 



The absolute speed must also be held near these limits since at 

 the end of a film it is necessary to switch from one projector to another 

 with minimum change in the pitch of the sound reproduction. 



Voltage, Frequency and Load Variations 



A study of the voltage variations in power supply systems indicated 

 a range from 100 to 125 volts. At a pa'ticular location the normal 

 variation of voltage was found to be 5 per cent above or below the 

 mean value with occasional momentary variations of as much as 

 10 per cent above and below mean value. 



An investigation of variations in frequency of the supply voltage 

 showed that in the large cities the frequency was held very accurately 

 at 60 cycles. In New York City for example the frequency stays 

 within one quarter of 1 cycle and does not change rapidly. However, 

 in some small power systems the frequency varied as much as 5 cycles 

 and in some cases was subject to rapid changes in frequency. 



The load of the motor is due mainly to mechanical friction in the 

 projector and take-up mechanism. This load was found to be on 

 the average 1/10 of a horse power but subject to wide variations. 

 In the case of a new machine with a stiff adjustment of the take-up 

 mechanism, the load was found to be as high as one-fifth of a horse 

 power. 



Speed Control Circuit 



A consideration of the variables just discussed imposes rather 

 severe requirements of speed control, the two extremes being (1) the 

 combination of low line voltage, low frequency and heavy load, 

 (2) the combination of high line voltage, high frequency and light load. 

 Ordinarily it might be possible to compromise and not provide for 

 such an extremely unfavorable combination of requirements. How- 

 ever, it must be borne in mind that in the case of a musical program 

 the failure of the speed regulating system for even as short a time as a 

 fraction of a second would be a very serious matter causing the music 

 to sound off pitch similar to a phonograph which has run down while 

 in operation. 

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