If only mono-directional capture is available, the following demodulation constraints apply:
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Fax
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T.30 (V.21 modulated) – Signals from recorded direction demodulated; opposite direction demodulated if echo level is sufficient. |
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Data
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V.22/V.22bis – Signals from recorded direction demodulated; opposite direction demodulated if echo level is sufficient. V.32/V.32bis – Signals from recorded direction demodulated if echo from opposite direction is sufficiently low. V.34/V.90 – Demodulation impossible.
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The recording must include all signaling from the very beginning of a transmission otherwise demodulation and decoding may not be possible. Carrier lost or reception of fax DCN signal is considered as the end of communication.
If a fax sample stream is missing the initial handshake and thus the fax parameters are absent, the signal may still be decoded by manually entering the value of the missing parameters.
For data signals without the initial handshake, the user may force the modulation type in the case of V.22, V.26, V.27, V.29 and V.32 signals with a bit rate lower than 4800 bps.