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VoIP Voice and Fax Signal Processing by Sivannarayana Nagireddi

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14.9 V.27ter FAX MODEM

V.27ter is a half-duplex voice band modem used in fax applications. The suffix “ter” or “Ter” along with V.27 conveys that it is the third revision. Some ITU recommendations were marked with the suffix “bis” and “ter.” In French, bis means second and ter means third. When recommendations with “bis” and “ter” suffix are available, it is required to use “ter” as the latest. V.27ter is supported as a mandatory modem in fax. For simplification in naming, V.27ter is referred to as V.27 or V27 in representation. A summary on V.27 features are given below:

Baud rate and modulation: 1200 symbols at 2 bits per symbol (4-DPSK), making a total bit rate of 2400 bps as per V.26 Alternative A [ITU-T-V.26 (1988)] and 1600 symbols at 3 bits per symbol (8-DPSK), making a total bit a rate of 4800 bps

Center frequency: 1800 Hz and no separate pilot tones are used

Power levels: Power output should not exceed 1 mW, and nominal level −13 dBm as per V.2

3-dB bandwidth frequencies: 1000 and 2600 Hz for 4800 bps; 1200 and 2400 Hz for 2400 bps

Signal detection: Bit or signal is detected if signal level is greater than −43 dBm, and no signal detection if signal level of <−48 dBm. When transmission conditions are good, the signal detectors can be made less sensitive.

All amplitudes for 4-phase or 8-phase are the same. V.27 is supported in most fax machines. Because of a low rate and slow symbol rate, V.27 tolerates more distortions and it takes more time to send a fax page in V.27. ...

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