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348 CINEMATOGRAPHIC ANNUAL
recent and although the earlier experimenters were handicapped by lack of modern equipment, some of the ideas they advanced and worked on are being made use of in our present methods.
At the present time, there are two distinct types of sound recording. There is a type with which we have been familiar for several years and no doubt each of you has in his home a great many examples of it in the form of phonograph records. This type of recording is generally referred to as the disc or wax type. This latter term, in view of the present recording methods, is somewhat of a misnomer, because the material on which the original record is engraved is not wax at all, but an insoluble soap. Other Academy papers will describe in detail this method of recording and I will merely refer to it now as being a method whereby a sapphire stylus attached to the vibrating armature of a cutting head, cuts a logrithmic involute in the master blank. This disc is then graphited and electroplated with copper to obtain what is known as a master. This master is plated to produce a mother and the mother is plated to produce stampers. The stamper is then nickel faced and used in a hydraulic press to make impressions in a biscuit of a black compound known as record stock. This results in the record we all know — a record which may be, and usually is, a very accurate sound picture of the original.
The other type of recording is usually referred to as film recording and may be divided roughly into two general systems. One is known as variable density recording. Major exponents of this system are Western Electric Company and Fox Movietone Company. The other division is known as variable area recording. It was developed and is used by RCA Photophone. In the next few pages a brief discussion of the RCA Photophone system and apparatus will be presented.
Up to a certain point, the RCA system is essentially the same as the systems used by Western Electric Company and the Fox Movietone Company. The first unit in the system is a microphone which intercepts sound waves and converts them into electrical waves. In order that there be no loss, it is necessary for the shape of the electrical wave produced by a microphone to be the same as the sound wave which caused the diaphragm in the microphone to move.
In brief, the microphone consists of two metal plates. The diaphragm is one plate and behind this at a distance of