69. General Requirements.- The aim in any type of sound reproduction is that the reproduced sound be
as nearly like the original sound as it is possible to make it. The discs, when properly made, carry a
nearly perfect record of the sound, but this is not enough for good sound reproduction because no
matter how nearly perfect the record, it would not give satisfactory reproduction unless it was used with
suitable reproducing equipment properly operated.
The frequencies of the reproduced sounds should coincide with the frequency of the original sound (or
there will be a change of pitch). This condition can be obtained by running the disc record during
reproduction at exactly the same speed as the wax disc was run when the sound was recorded. For
synchronous records this speed is thirty-three and one-third revolutions per minute. For nonsynchronous records the proper speed is seventy-eight revolutions per minute.
This is accomplished by making the speed of the reproducer turntable correspond with the speed of the
recorder. For synchronous disc reproduction this is taken care of in the design of the projector, and
consredt speed motors are used for projector drive. When DC motors are used for projector drive the
speed must be adjusted by the operator as outlined in section 27. The speed of the non-synchronous
turntables must also be adjusted by the operator. The method of doing this is described in section 79.
A second consideration is that the record must be rotated at an absolutely consredt speed as even
slight variations in speed will result in "wows." To prevent any such slight variations of speed a
mechanical "filtering" device is incorporated in the synchronous turntable drive.
For synchronous reproduction of sound the record and picture must coincide at all times. Since the
turntable mechanism is mechanically geared to the sprocket which pulls the film through the machine,
the proper relation of speed between film and disc is maintained at all times. Barring mishaps, such as
the needle jumping the groove, or breaking of the film, the sound and picture will remain synchronized
throughout the run, if properly started.
The device for translating the sound record of the disc into sound itself must be capable of doing so
without introducing distortion. The operator's responsibility in regard to this lies mostly in seeing to it
that the record is clean and that a good needle is used in the pick-up. As stated in section 63, the record
is purposely made abrasive, so as to wear the needle quickly to a good fit. The needle continues to wear
after a good fit is secured, therefore, a new needle should be used every time a record is changed. Only
a good grade of needles should be used.