What are sign bits?

What are sign bits?

Sign bits are reported in 3Com V.90 modems I11 status line.

Sign bits can be used to either transfer user data or they can be used for spectral shaping.

56k modems create frames of multiple PCM words - with V.90 and x2 there are 6 PCM words per frame - and thus 6 sign bits. A PCM word is 8 bits, but instead of coding from 0-256, they are coded as 7 bits plus 1 sign bit to give values from -128 to +127. If, for instance, only 1 sign bit was needed for spectral shaping, the remaining 5 can be used to transmit user data. The exact number of bits used for spectral shaping is determined by the client modem during training, and is transmitted back to the server modem. [Spectral shaping is needed for the analog portion of the telephone call to limit power to that required by the country the modem is operating in.]

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