| 单词 | PCM (Pulse Code Modulation) |
| 释义 | PCM (Pulse Code Modulation) A modulation technique in which an analog signal is encoded, i.e., converted from analog to digital format. The term generally is applied to the conversion of voice from a continuous analog waveform to digital pulses as specified in ITU-T Recommendation G.711, which is based on the Nyquist theorem. That theorem states that an analog signal waveform can be converted to digital format and be reconstructed without error from samples taken at equal time intervals, if the sampling rate is equal to, or greater than, twice the highest frequency component in the analog signal. As the voice band is 0–4,000 Hz, the highest frequency component is 4,000 cycles per second (cps), and the sampling rate is 8,000 per second. At that rate, a codec measures the amplitude of the audio sine wave and encodes (i.e., quantizes) that value as a 14-bit number, which it compresses by assigning each sample to an eight-bit binary (byte) approximate value, based on a table of 256 (28) standard values of amplitude according to the non-linear PCM scale. The standard values are closer together at low volume, or loudness, levels and spread further apart for high loudness.The process yields a digital voice signal at a rate of 64 kbps (8,000 cps × 8 bits = 64,000 bps).The individual byte values then are transmitted over a digital circuit, such as a channelized E-1 or T1. If a T1, a time division multiplexer (TDM mux) byte interleaves the eight-bit (byte) samples with those of 23 other conversations into a frame, and repeats that process at the very precise pace of one each 125µs (8,000 times a second).The process is reversed on the receiving end of the connection as the encoded amplitude samples are expanded (i.e., decoded, or decompressed), to reconstitute an approximation of the original analog voice signal. A sampling rate that is too low, i.e., below the Nyquist rate, results in a phenomenon known as aliasing, in which the reconstructed signal is inaccurate, or even unintelligible.The twin processes of compressing and expanding the signal are known as companding, and are illustrated in Figure P-2. Recommendation G.711 specifies two companding techniques that define the assignment of 14-bit values to 8-bit values: µ-law (mu-law), which is used in the North American T-carrier systems, and A-law, which is used in the European E-carrier systems and the Japanese J-carrier systems. PCM was invented and patented in 1925 by P. M. Rainey of Western Electric, the manufacturing arm of the Bell System. See also ADPCM, A-law, analog, byte, byte interleaving, codec, digital, DPCM, E-carrier, frequency, ITU-T, J-carrier, µ-law, modulation, Nyquist theorem, quantizing noise, sine wave, T-carrier, TDM, voice band, and waveform. Original signal Reconstructed signal 10101001 |
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