Equivalent pulse code modulation noise
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In telecommunication, equivalent pulse code modulation noise (PCM) is the amount of thermal noise power on a frequency-division multiplexed (FDM) or wire channel necessary to approximate the same judgment of speech quality created by quantizing noise in a PCM channel.
Note 1: The speech quality judgment is based on comparative tests.
Note 2: Generally, 33.5 dBrnC ±2.5 dB is considered the approximate equivalent PCM noise of a 7-bit PCM system.
Source: from Federal Standard 1037C and from MIL-STD-188