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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. Linear pulse-code modulation (LPCM) is a specific type of PCM where the quantization levels are linearly uniform. This is in contrast to PCM encodings where quantization levels vary as a function of amplitude (as with the A-law algorithm or the μ-law algorithm). Though PCM is a more general term, it is often used to describe data encoded as LPCM. A PCM stream has two basic properties that determine the stream's fidelity to the original analog signal: the sampling rate, which is the number of times per second that samples are taken; and the bit depth, which determines the number of possible digital values that can be used to represent each sample. ==History== Early electrical communications started to sample signals in order to interlace samples from multiple telegraphy sources and to convey them over a single telegraph cable. The American inventor Moses G. Farmer conveyed telegraph time-division multiplexing (TDM) as early as 1853. 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. He obtained intelligible speech from channels sampled at a rate above 3500–4300 Hz; lower rates proved unsatisfactory. This was TDM, but pulse-amplitude modulation (PAM) rather than PCM. In 1920 the Bartlane cable picture transmission system, named after its inventors Harry G. Bartholomew and Maynard D. McFarlane, used telegraph signaling of characters punched in paper tape to send samples of images quantized to 5 levels; whether this is considered PCM or not depends on how one interprets "pulse code", but it involved transmission of quantized samples. In 1926, Paul M. Rainey of Western Electric patented a facsimile machine which transmitted its signal using 5-bit PCM, encoded by an opto-mechanical analog-to-digital converter.〔U.S. patent number 1,608,527; also see p. 8, ''Data conversion handbook'', Walter Allan Kester, ed., Newnes, 2005, ISBN 0-7506-7841-0.〕 The machine did not go into production. British engineer Alec Reeves, unaware of previous work, conceived the use of PCM for voice communication in 1937 while working for International Telephone and Telegraph in France. He described the theory and advantages, but no practical application resulted. Reeves filed for a French patent in 1938, and his US patent was granted in 1943. By this time Reeves had started working at the Telecommunications Research Establishment (TRE).〔 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 Alec Reeves. In 1949 for the Canadian Navy's DATAR system, Ferranti Canada built a working PCM radio system that was able to transmit digitized radar data over long distances.〔Porter, Arthur. So Many Hills to Climb (2004) Beckham Publications Group〕 PCM in the late 1940s and early 1950s used a cathode-ray coding tube with a plate electrode having encoding perforations.〔R. W. Sears, "Electron Beam Deflection Tube for Pulse Code Modulation," Bell Sys. Tech. J., Vol. 27 pp. 44–57〕〔W. M. Goodall, "Television by Pulse Code Modulation," Bell Sys. Tech. J., Vol. 30 pp. 33–49, 1951.〕 As in an oscilloscope, the beam was swept horizontally at the sample rate while the vertical deflection was controlled by the input analog signal, causing the beam to pass through higher or lower portions of the perforated plate. The plate collected or passed the beam, producing current variations in binary code, one bit at a time. Rather than natural binary, the grid of Goodall's later tube was perforated to produce a glitch-free Gray code, and produced all bits simultaneously by using a fan beam instead of a scanning beam. In the United States, the National Inventors Hall of Fame has honored Bernard M. Oliver〔 〕 and Claude Shannon〔 〕 as the inventors of PCM,〔 〕 as described in "Communication System Employing Pulse Code Modulation", filed in 1946 and 1952, granted in 1956. Another patent by the same title was filed by John R. Pierce in 1945, and issued in 1948: . The three of them published "The Philosophy of PCM" in 1948.〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pulse-code modulation」の詳細全文を読む スポンサード リンク
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