a device or circuit for the conversion of information into a signal or an aggregate of signals according to a particular code. Information can be coded only if it is presented in the form of discrete signals. If it is contained in a continuous signal, the signal must first be converted by the coder into a series of discrete signals. Voltages, time intervals between pulses, or linear or angular mechanical displacements are usually coded. The purpose of coding usually is long-distance transmission, improved noise immunity, or conversion of the parameter being measured. All other physical values are usually converted into one of the forms of presentation mentioned above.

Coders are used in automatic control systems and in digital measuring instruments, as well as for recording the parameters being measured. Coding can be accomplished by coding tubes and disks, matrix circuits, or mechanical devices similar to those used in telegraph apparatus.

A cathode-ray coding tube is used for conversion of a continuous electrical signal into a series of successive pulses of a particular code system. Tubes of this type are used in automation, remote control, and computer technology for coding and signal conversion, as well as in television and communications for reduction of the frequency band in the transmission of information. A cathode-ray coding tube functions as a contactless switch in which the luminescent screen is replaced by a special output device. Such a tube consists of three main parts (Figure 1): an electron gun, a deflecting system, and the output device. The first two parts are similar to the corresponding parts of a conventional cathode-ray tube used in oscillographs, television receivers, and so on. Coding and output plates or a collector of secondary electrons whose signals are fed to an electronic amplifier may be used as the output device in such a tube. The operating parameters of the tube and amplifier (horizontal sweep, separation of useful signals, and so on) are determined by a control unit.

Figure 1. Diagram of a cathode-ray coding tube unit: (1) input of signal to be coded, (2) electron gun, (3) sweep plates, (4) coding plate, (5) output plate, (6) electronic amplifier, (7) output of coded signal, (8) control unit, (9) output plate load, (10) coupling capacitors, (11) collector load, (12) collector of secondary electrons, (13) deflecting plates

The main distinguishing component of cathode-ray coding tubes is the coding plate, which is a flat metal sheet on which the coding elements, in the form of retangular or (infrequently) other types of slits, are located in an arrangement determined by the coding required. The arrangement of coding elements is chosen such that along the horizontal axis they are located according to the code established for each line, and along the vertical axis perpendicular to the lines they are located according to the coding system and the scale established for amplitude conversion of the input signal voltage.

The principle of operation of a cathode-ray coding tube (for example, in pulse-code modulation) is as follows. Electrons strike the output plate after passing through the apertures of the coding plate. In passing through the coding plate the electron beam generates a series of pulses in the output plate circuit. The code represented by these pulses corresponds to the input signal voltage established for the coding system chosen. Correcting and control devices are used to ensure correct coding of signals that change in real time. The main advantages of tubes are very high conversion speed (up to 10⁶ pulses per sec) and absence of intermediate conversion of the amplitude of a voltage signal to a time interval and absence of special comparing devices used in other coding apparatus for identifying the signal.

Coding disks are used for conversion of angular displacements into a digital, mainly binary, code. Code digits, which correspond to rotation of the shaft through a certain angle, are represented by geometric configurations and are marked on the surface of the disk as concentric tracks (Figure 2). The disk is mechanically connected to a shaft, whose position is coded. Sections or elements of the code tracks are made from corresponding pairs of materials, such as a conductor and dielectric, transparent and opaque substances, or magnetic and nonmagnetic materials, depending on the method of reading the code (contact, photoelectric, electromagnetic, and so on). As the shaft turns, individual sections of the tracks having differing properties are examined by a reading device (a photoelectric cell, magnetic head, or contact brushes); the transition from one section of track to another is equivalent to changing the code at the output of the reading device by one unit in the corresponding digit (the extreme outer track represents the least significant digit of the code). In some cases a drum mounted on the shaft is used instead of the disk.

Figure 2. Coding disk with an ordinary binary code

The circuit shown in Figure 3 is an example of a matrix coder. Upon closing any of the switches the signal from a generator is fed to the corresponding input buses of the matrix, which are inductively coupled to output buses. The distribution of signals among the matrix outputs corresponds to the binary code number of the closed switch.

Figure 3. Matrix coding device with magnetic cores: (G) pulse generator; (K) keys, (M) magnetic cores; (2°), (2¹), and (2²) matrix outputs (in binary code)

The main coding components in telegraph apparatus are combinatory slides, whose profile at their intersection with key-operated levers determines the direction of displacement of the slides when the keys are depressed. The displacement is transmitted by blocking arms to a contact group of the coder. A displacement to the left corresponds to closing contact (current is transmitted); a displacement to the right blocks the contacts (no current transmitted).