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Pulse Code Modulation

Pulse Code Modulation (PCM) forms the heart of the modern telephone system. To understand PCM, let’s consider how multiple analog voice signals are digitized and combined on to a single digital trunk.

The analog signals are digitized in the end office by a device called a codec (coder-decoder) producing a 7 or 8 bit number. The coder makes 8000 samples per second (125µsec/sample) because the Nyquist theorem say that thit is sufficient to capture all the information from the 4-KHz telephone channel bandwidth. At a lower sampling rate, information would be generated/gained. This technique is called PCM (pulse Code Modulation). As a consequence, virtually all time intervals within the telephone system are multiplies of 125µsec.

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