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Callmeamanda (talk | contribs) m A minor change for clarity. |
Kevinblong (talk | contribs) m Updated URL to current in use by publisher. Old one did not function. New URL comes from publisher's site which offers a permalink URL. |
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{{See also|T-carrier|E-carrier}}
PCM can be either [[return-to-zero]] (RZ) or [[non-return-to-zero]] (NRZ). For a NRZ system to be synchronized using in-band information, there must not be long sequences of identical symbols, such as ones or zeroes. For binary PCM systems, the density of 1-symbols is called ''ones-density''.<ref>Stallings, William, [
Ones-density is often controlled using precoding techniques such as [[run-length limited]] encoding, where the PCM code is expanded into a slightly longer code with a guaranteed bound on ones-density before modulation into the channel. In other cases, extra [[framing bit]]s are added into the stream, which guarantees at least occasional symbol transitions.
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