Abstract: A start-stop synchronous communication speed detecting apparatus includes a counter, a speed determining unit, a clock switching unit, a shift register, a character determining unit, a code generator, and a controller. The counter counts a time period, in which received data is a space polarity, in start-stop synchronous communication. The speed determining unit compares the count value with a time per bit of a specified communication speed. The clock switching unit selects a clock synchronized with the start bit of the received data and having the same frequency as that of the specified communication speed. The shift register stores the received data. The character determining unit compares a received character from the register with a predetermined character. The code generator supplies codes of the first to Nth characters to the character determining unit.

Abstract: An echo canceller in a modem cancels echoes from hybrid transformers on both the near end and the far end. The echo canceller has a variable delay which may be adjusted to conform to a round trip of an echo. An adaptive filter has a number of delay circuits, each adding an increment of delay. A number of these delay circuits are selected in order to provide a selected delay time.

Abstract: The invention relates to an automatic retrain method applicable to full duplex data transmission modems which shortens the conventional protocol for retraining in the instance where signal reception can be improved by retraining only the automatic equalizers of the linked modems. If the retrain-initiating modem fails to detect a retrain signal from the linked modem within a predetermined time after transmitting its own retrain signal, the conventional retraining protocol is then adopted in order to retrain both the echo cancellers and the automatic equalizers of the linked modems.