VoIP Voice and Fax Signal Processing

14.3 FAX MODULATION RATES

In group-3 facsimile equipment, messages and data are sent as modulations in the voice band. The messages are sent as V.21 frequency-shift keying (FSK) binary modulations. The picture or scanned lines information is modulated using one of V.29, V.27ter, V.17, and V.34 modulations as given in Table 14.1. The modulation scheme is negotiated in the beginning of the fax call or in the middle of the call for any fall back to lower rate requirements based on the interoperability, line conditions, and errors.

Baud Rate and Bit Rate Differences. In this note, an overview on bit rate and baud rate is given. Bits per second (bps) or bit rate is the number of bits transmitted or received in a one-second time interval. For representing higher modem bit rates, a unit of kilobit per second (kbps) is used. A fax rate of 14,400bps is represented as 14.4 kbps. Baud is the number of symbols per second [URL (Linux-docs)]. Baud is named after Emile Baudot, the inventor of the asynchronous telegraph printer [URL (ATIS)]. In the early development of modem and fax, the modulations used were FSK, which also used to be called two tones, and PSK. In FSK and PSK, one bit is used per symbol. Hence, the baud rate and bit rate names are used interchangeably. In V.21, baud is 300, which means symbols are 300 and the number of bits sent on FSK is 300. In FSK, the baud and bit rate in bps are the same and each symbol sends just one bit. In advanced modulation methods, each symbol sends ...