![]() ![]() June 1950: Differential pulse-code modulation (DPCM) developed by C.The twelve transmission points were retired after the war. 1943: Bell Telephone Laboratories develops the first PCM-based digital scrambled speech transmission system, SIGSALY, in response to German interception of military telephone traffic during World War II.It was first proposed as a telephony technology. On November 22, 1939, Reeves files also in the US. October 3, 1938: British telephone engineer Alec Harley Reeves files at the French Patent Office the first patent describing the technique known today as pulse-code modulation (PCM).Because the signal is stored digitally, assuming proper error detection and correction, the recording is not degraded by copying, storage or interference. In a properly matched analog-to-digital converter (ADC) and digital-to-analog converter (DAC) pair the analog signal is accurately reconstructed per the constraints of the Nyquist–Shannon sampling theorem dependent on the sampling rate and quantization error dependent on the audio or video bit depth. ![]() To play back a digital recording, the numbers are retrieved and converted back into their original analog audio or video forms so that they can be heard or seen. This number stream is saved to a storage device. In digital recording, an audio or video signal is converted into a stream of discrete numbers representing the changes over time in air pressure for audio, or chroma and luminance values for video. Audio levels display on a digital audio recorder ( Zoom H4n) ![]()
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