Abstract: An all digital modem includes a clock divider for generating four waveforms at a desired transmission frequency but mutually phase shifted by 90 degrees, a first microprocessor programmed to receive digital data at its input, to convert the data into dibits and to select as a modem output one of the four waveforms phase shifted 90 degrees with respect to each other. To demodulate incoming signals, a zero crossing detector detects zero crossings of an incoming modulated signal, a second microprocessor coupled receives the zero crossing information along with parity information and converts the information into dibits, and a third microprocessor receives dibit outputs from the second microprocessor and converts the dibits into a serial stream of data, at essentially constant frequency.