A telephone line modem presents a significant challenge for a zero
ISI design as the transmission channel itself may well
introduce significant filter distortion which will compromise any carefully engineered
Nyquist response between the sending and receiving hardware. In this scenario, it is
necessary to try to correct for the channel distortion, or reconfigure the TX/RX filters
so that the overall response is Nyquist.
Usually, adaptive channel equalizers are employed
which attempt to flatten the channel transfer function so that the symbol shaping in the
sending and receiving units predominates. For zero ISI performance, it is necessary to
make sure that the overall transfer function of these compound filters
is Nyquist. Most modern telephone modems operating at speeds above 4.8 kbps employ
adaptive equalization using a 'training sequence' in the handshaking at the start of each
call to measure the non-ideal channel response.
A good text on the subject of adaptive filters and equalizers is Ifeachor and Jervis (1993).
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