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About mechanical sound recording

A mechanical sound recording is based on cutting (or extruding) grooves in the material of a recording medium using a sound recording device — a recorder. When playing along the groove, the needle of the reproducing device – the pickup – moves (moves).

In the process of developing mechanical recording, two main methods of modulating the groove were proposed: transverse, in which the cutter oscillates left and right from the neutral position and the groove of constant width acquires twists (Fig. 1a), and the deep, in which the cutter moves up and down from neutral position by cutting a groove of variable depth and width (Fig. 1, b).

About mechanical sound recording (mechanical sound recording)
Fig. 1. The type of modulated groove in the transverse (a) and deep (b) recording

All preliminary operations for recording and editing phonograms are carried out using magnetic recording. The finally mounted magnetic phonogram is transferred to a mechanical one on a mechanical recording machine (Fig. 2). It has a massive disk (faceplate) 1, rotated by the engine 2. Recorder 4 using the mechanism 3 moves in the radial direction. Recording medium 5 (copper or lacquer disc) is firmly fixed to the faceplate. The diamond cutter of the recorder gradually moves in the radial direction from the edge of the disc to the center. In the absence of a signal, a groove without gyrus is obtained, unmodulated.

About mechanical sound recording (mechanical sound recording)
Fig. 2. The scheme of the machine mechanical recording

When a signal is applied to the recorder, the cutter begins to oscillate and cut a groove of complex shape. Such a groove is called modulated.

After the recording is finished, a silver layer is applied to the surface of the varnish disk, which further serves as a current path during the electrochemical process of applying the nickel layer. The nickel layer reaches 0.3 … 0.4 mm. The resulting copy (first original) is carefully separated from the lacquer disk. From the first original again, several (up to ten) new copies (second originals) are made using the electrochemical (galvanic) method. After separating the second original, they listen to him like a regular record. The detected defects are eliminated by engraving, controlling this work under a microscope. Third originals with a thickness of about 0.25 mm are removed from the second originals, the matrices are coated with a layer of chromium to reduce the wear of the matrices (although the quality of the phonogram is deteriorated) and used as matrices when pressing plates. One matrix without significant deterioration of quality is pressed up to a thousand records. The material for the plates is vinylite – a copolymer of vinyl chloride with 15% vinyl acetate with a small addition of softening and coloring substances.

Features of recording stereo signals

As mentioned above, in the process of developing mechanical sound recording, two methods for modulating a groove were proposed: transverse and deep. . A combination of these two methods is used for two-channel stereo recording. However, it is impractical to control the movement of the cutter in the horizontal and vertical directions directly by the signals of the left and right channels due to some differences in the properties of the recorded phonograms and the difficulty of separating the signals in the pickup. therefore, both directions of the displacement of the cutter are rotated by 45 °, and the signals of each channel are recorded by simultaneous movement of the cutter in the horizontal and vertical directions. Then the movement of the needle reproducing the signal of one channel occurs in the direction from the top left – to the right down (and in the opposite direction), of the other channel – from the top right – to the left down (and in the opposite direction). So, in order to get a two-channel stereo phonogram, the cutter is informed of movements in two mutually perpendicular directions, both of which are at an angle of 45 ° to the surface of the carrier, as shown in Fig. 3. Since the angle of the walls of the groove is 90 °, one signal is recorded on each wall of the groove: on the external (closer to the edge of the plate) signal of the right channel, on the internal (facing the center of the plate) signal of the left channel.

The phasing of signals in channels (tracks) is standardized. The pickup output voltages are in phase if the needle feeds up for the left channel and down for the right (or vice versa). This procedure has been adopted for the following reasons:
When the disk rotates, vertical needle movements occur due to warping of the plate and beating of the disk due to an inaccurate nozzle on the axis (the plane of the disk is not perpendicular to the axis). If the signal of one channel was recorded by the transverse recording method and the other by the deep recording method, then the interference caused by the vertical movements of the needle would be reproduced in only one channel. By shifting the recording directions by 45 °, an out-of-phase and approximately equal in amplitude distribution of the voltage of these interference along both channels is achieved. As a result, the sound of interference is not localized, which reduces its visibility.

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