Pulse-code modulation
Alec Reeves, Claude Shannon, Barney Oliver and John R. Pierce are credited with its invention.
Electrical engineer W. M. Miner, in 1903, used an electro-mechanical commutator for time-division multiplexing multiple telegraph signals; he also applied this technology to telephony.
In 1920, the Bartlane cable picture transmission system used telegraph signaling of characters punched in paper tape to send samples of images quantized to 5 levels.
[9] In 1926, Paul M. Rainey of Western Electric patented a facsimile machine that transmitted its signal using 5-bit PCM, encoded by an opto-mechanical analog-to-digital converter.
[11] The first transmission of speech by digital techniques, the SIGSALY encryption equipment, conveyed high-level Allied communications during World War II.
In 1943 the Bell Labs researchers who designed the SIGSALY system became aware of the use of PCM binary coding as already proposed by Reeves.
[13] PCM in the late 1940s and early 1950s used a cathode-ray coding tube with a plate electrode having encoding perforations.
The plate collected or passed the beam, producing current variations in binary code, one bit at a time.
[19] The T-carrier system, introduced in 1961, uses two twisted-pair transmission lines to carry 24 PCM telephone calls sampled at 8 kHz and 8-bit resolution.
This development improved capacity and call quality compared to the previous frequency-division multiplexing schemes.
In 1973, adaptive differential pulse-code modulation (ADPCM) was developed, by P. Cummiskey, Nikil Jayant and James L.
[21] The 30 kHz 12-bit device used a compander (similar to DBX Noise Reduction) to extend the dynamic range, and stored the signals on a video tape recorder.
[23][24] The silicon-gate CMOS (complementary MOS) PCM codec-filter chip, developed by David A. Hodges and W.C. Black in 1980,[23] has since been the industry standard for digital telephony.
The PCM process is commonly implemented on a single integrated circuit called an analog-to-digital converter (ADC).
This produces a fully discrete representation of the input signal (blue points) that can be easily encoded as digital data for storage or manipulation.
One technique is called time-division multiplexing (TDM) and is widely used, notably in the modern public telephone system.
They produce a voltage or current (depending on type) that represents the value presented on their digital inputs.
To recover the original signal from the sampled data, a demodulator can apply the procedure of modulation in reverse.
As a result of these transitions, the signal retains a significant amount of high-frequency energy due to imaging effects.
Support for multichannel audio depends on file format and relies on synchronization of multiple LPCM streams.
Advanced compression techniques, such as modified discrete cosine transform (MDCT) and linear predictive coding (LPC), are now widely used in mobile phones, voice over IP (VoIP) and streaming media.
