## Achieving a Nyquist channel response

#### It is very unlikely that the communications channel will inherently exhibit a Nyquist transfer response. This means that the system designer must add compensating filtering to achieve the desired response.

#### 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).