Movie Makers (Jan-Dec 1953)

MOVIE MAKERS 271 magnet which produces a magnetic field, a coil which is mounted in the magnetic field in such a manner that it is free to move in the field, and a cone, the inner edge of which is rigidly fastened to the coil and the outer edge to the speaker body. When the currents from the amplifier are passed through the coil winding, a reaction takes place which causes the coil to move. Motion of the coil is back and forth, depending on the currents through it which are of the same wave form as the original sound. The movement of the coil pushes the cone back and forth, compressing and decompressing the adjacent air to produce the sounds which we hear. Moving coil speakers may have an electromagnet instead of the permanent magnet mentioned previously, but the operation is identical. ELEMENTS OF A RECORD PLAYER When the movie maker wishes to provide music for his pictures, or to dub in recorded sound effects, he uses a turntable, phonograph pickup, amplifier and speaker. These are connected as shown in Fig. 8. The turntable consists of a plate mounted on a motor, usually electrically driven, the speed of rotation of which can be exactly controlled to 78, 45 or 33 1/3 revolutions per minute, depending on the recording to be reproduced. At whatever rate of rotation, the turntable should run at a constant speed to prevent "wows," or changes in the pitch of the music. Several types of pickup are available, such as the magnetic and the crystal types. For the highest quality of reproduction, a magnetic pickup of especially lightweight construction is used. The most commonly used pickup is the crystal type. Those which have a minimum needle pressure are preferable, since they will create the least amount of harmful wear on the record grooves. Let us now look at the construction of one such pickup. The fundamental parts of this pickup (see Fig. 9) are a crystal, a torque wire, a needle or stylus, leads and a housing. The crystal is much the same as the one used in the microphone, except that only a single Rochelle salt plate is used. It is mounted in a housing with one end rigidly fastened. The torque wire is secured to the free end. A permanent sapphire stylus or a means for holding a removable needle is attached at the outer end of the torque wire. The outer or stylus end of the torque wire is held in a bearing so that it can be turned only about its axis. When the point of the needle or stylus is moved from side to side, the torque wire is twisted. The twist is transmitted to the free end of the crystal, causing it to be strained. The strain generates a voltage in the crystal. This voltage is conducted from the crystal to the amplifier by means of the pickup leads and the wires connecting the pickup and the amplifier. The operation of the amplifier and speaker is the same as in the setup using the microphone. Magnetic pickups generate their output voltage by changing, with the movement of the stylus, the relative position of a magnet and a fine coil of wire. In one of the more popular types — a variable reluctance pickup — the movement of the stylus changes the strength of a magnetic field which includes the coil of wire. Magnetic pickup heads exert very little pressure on the tiny record grooves and can be made quite small. Their output voltage is likewise small, and more amplification is generally necessary to increase the audio signal strength sufficiently to operate the power amplifiers. This additional amplification is supplied by a record preamplifier which, additionally, equalizes, or compensates for, a particular variation of the recorded frequencies necessary in the manufacture of commercial recordings. High quality of music reproduction and long life of recordings are obtained by using good magnetic pickups together with their accompanying equalizing preamplifiers. 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