A Companded One-Bit Coder for Television Transmission

Encoding an analog signal to digital form entails quantization of amplitude. This process introduces a noise into the analog signal that is recovered from the digits. The magnitude of the noise, relative to the signal, is determined by the bit rate in the digital representation and the spectrum of the signal. Successful coder designs make efficient use of the digits, avoiding worthless redundancies, and shape the noise to be subjectively least noticeable. Delta modulation is one of the simplest and best known coding methods. 1 It changes its analog output positively or negatively by a fixed increment at regular instants, as illustrated by V in Fig. 3. Differential coding is a related method where, at regular instants, the output changes by any one of a set of prescribed values. Delta modulation is regarded as one-bit differential coding because at each sampling 1459 1460 T H E BELL SYSTEM T E C H N I C A L J O U R N A L , M A Y - J U N E 19G9 time it transmits either of two codes, a pulse or a space, representing a positive or a negative step, respectively. In general, an ?n-bit coder transmits one of 2 m codes at each sample time. The advantages of one-bit coding are simplicity of circuitry and a high sampling rate. Thus, for a given bit rate in the digit channel, its sampling rate is ?w-times greater than t h a t of a corresponding ra-bit coder. Although the total noise power from a one-bit coder is greater than that from a multibit coder, much of the power occurs at higher frequencies where it is out of the signal band.