61. Fundamentals of "Talking Movie" Sound Recording.-No matter which method of recording is used there are certain fundamental rules which must be followed if satisfactory results are to be obtained.

The record must be as nearly an exact representation of the original sound waves as it is possible to make. All the frequencies of the original sound should be present and in the same proportions as in the original sound; that is, there should be no discrimination against some frequencies as compared to others within the band of frequencies necessary for natural reproduction. Of still more importance is the necessity of preventing the addition to the record of frequencies not in the original sound.

The sound must be so recorded that its reproduction corresponds to the picture at all times; that is, the sound must be recorded so that when it is reproduced the sounds corresponding to the action of the picture occur at the same time that the action causing the sound is shown on the screen. The process involved in doing this is called "synchronizing."

62. Picking Up the Sound Energy and Converting It to Electrical Energy.-The first step of recording, the "picking up" of the sound energy and converting it into electrical energy is essentially the same in all modern recording systems. This is done by means of a sensitive microphone. The RCA Photophone, the Western Electric and Fox Movietone all use a condenser microphone similar to those used in radio broadcasting. The RCA Photophone microphone consists of two metal plates separated 0.0015 of an inch by an insulating material. One of the plates is made of duralumin rolled to a thickness of about 0.0015 inches and stretched to a very high tension on a frame. These two plates and the insulation between them form a simple condenser.

A condenser consists of two plates, or two groups of plates, separated by an insulating material. If an electrical potential (voltage) is applied between the plates they will become charged. When the plates are charged or discharged, the charge flows into or out of the condenser. This flow of a charge, as stated in section 13, is an electric current. If the capacity (ability to- hold a charge) of a condenser is changed while a DC voltage is applied to the plates, the amount of charge which the condenser will hold will change in accordance to the change of capacity, and a current will flow in the leads connecting the source of the DC voltage and the plates of the condenser. The capacity of the condenser can be changed by changing the distance between the plates. Therefore if one plate is vibrated, an alternating current will flow in the leads because as the distance between the plates decreases a current will flow into the condenser, and when the distance between the plates is increasing a current will flow out of the condenser. If a resistor is placed in one of the leads connecting the source of DC to the condenser, an alternating current will be produced across the resistor due to the current flowing through it.

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