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Pulse Code Modulation (PCM)
Pulse-code modulation (PCM) is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a numeric (usually binary) code. PCM has been used in digital telephone systems and is also the standard form for digital audio in computers and the compact disc red book format. It is also standard in digital video, for example, using ITU-R BT.601. However, straight PCM is not typically used for video in standard definition consumer applications such as DVD or DVR because the bit rate required is far too high. Very frequently, PCM encoding facilitates digital transmission from one point to another (within a given system, or geographically) in serial form.
The form of modulation in which the information signals are sampled at regular intervals and a series of pulses in coded form are transmitted representing the amplitude of the information signal at that time. For T1 applications, a method of converting successive (every 125 us) analog samples of a voice waveform to successive 8-bit codes, to be transmitted in an 8-bit timeslot of a T1 frame. In "robbed bit" frames, only the most significant 7 bits are used to encode the sample. The total bit rate for such a channel is (8000 samples/sec) x (8-bits/sample) = 64000 bits/sec.