Pulse-code modulation

Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony, and other digital audio applications. In a PCM stream, the amplitude of the analog signal is sampled regularly at uniform intervals, and each sample is quantized to the nearest value within a range of digital steps.

Overview

PCM involves three basic steps: sampling, quantization, and encoding. During the sampling process, the amplitude of the analog signal is measured at regular intervals, known as the sampling rate, and these measurements are then quantized, meaning they are mapped to the nearest value within a set of discrete steps. This quantization process introduces a small amount of noise, known as quantization noise, but this can be minimized with a sufficiently high number of steps. Finally, each quantized sample is encoded into a digital format for storage or transmission.

History

The concept of PCM was invented by Alec Reeves in 1937 as a method for transmitting voice audio over digital communication channels. It was later developed and expanded in the 1960s and 1970s for digital telephony by Bell Labs, and it became the foundation for the digital audio revolution in the late 20th century.

Applications

PCM is widely used in digital audio applications, including CD audio format, where it serves as the foundation for the digital audio compact disc. It is also used in digital telephony systems, where it has been standardized in the ITU-T G.711 standard, and in many other areas of digital audio processing.

Technical Details

The quality of a PCM recording depends on the sampling rate, the number of bits used to represent each sample (bit depth), and the total number of channels recorded. The standard CD audio format, for example, uses a sampling rate of 44.1 kHz, a bit depth of 16 bits, and two channels for stereo sound.

Advantages and Disadvantages

The main advantage of PCM is its simplicity and ease of digital manipulation. Since PCM signals are digital, they can be easily processed with digital signal processing techniques. However, PCM requires a relatively high bandwidth compared to analog signals and other forms of digital audio encoding, which can be a disadvantage in bandwidth-limited applications.

See Also

• Analog-to-digital converter
• Digital-to-analog converter
• Quantization (signal processing)
• Sampling (signal processing)
• Digital audio