An electronic device for converting between serial data (typically RS-232) from a computer and an audio signal suitable for transmission over telephone lines. The audio signal is usually composed of silence (no data) or one of two frequencies representing 0 and 1. Modems are distinguished primarily by the baud rates they support which can range from 75 baud up to 56000 and beyond. Various data compression and error algorithms are required to support the highest speeds. Other optional features are auto-dial (auto-call) and auto-answer which allow the computer to initiate and accept calls without human intervention.