Training - Canon FACSIMILE BASIC 2000 Service Manual

G3 FACSIMILE COMMUNICATIONS
6.5

Training

The compensation of line characteristics such as attenuation distortion or
group delay distortion that occurs on telephone lines is "equalizing."
"Training" is the procedure where equalizing is performed to reliably
transmit image signals. To be more precise, various pattern signals are out-
put from the transmitting side at the same transmission speed as the image
signals before the image signals are transmitted. This is to start the auto-
matic equalizer built into the MODEM on the receiver so that the image
signals can be transmitted successfully. An "automatic equalizer" is in
effect a circuit that performs equalizing automatically. "Equalizing" refers
to the compensation of line characteristics such as attenuation distortion or
group delay distortion that occurs on telephone lines. The discovery of the
optimum value in equalizing during training is referred to as "MODEM
convergence," while the failure to discover the optimum value is referred
to as "MODEM divergence."
6.5.1 Training check (TCF)
Training is performed to transmit image signals correctly. If image signals
are transmitted immediately after training, errors may occur in the image
signals when training is not performed properly.
Following training, "TCF" (a check as to whether training has been per-
formed correctly) is transmitted at the same transmission speed as the
image signals. A TCF is a signal of continuous "0" lasting about 1.5 sec-
onds.
6.5.2 When TCF can be successfully received
The receiving side judges that training is normal when TCF has been
received successfully. The receiving side transmits CFR and notifies the
transmitting side that TCF was received successfully to prepare for image
reception. After the transmitting side has received CFR, training is per-
formed again (called "re-training") and then the image signal is transmit-
ted.
6.5.3 When TCF cannot be successfully received
Training is judged to have failed when TCF cannot be successfully
received due to noise in the line state. FTT is then transmitted to notify the
transmitting side that TCF could not be received successfully. After receiv-
ing FTT, the transmitting side outputs DCS again to instruct the receiving
side to receive at a delayed transmission speed. In this case, the next train-
ing operation is performed at a transmission speed that was communicated
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