Storing Digital Data on Audio Tape

We have implemented a modem for digital storage onto standard audio tape. The primary goal of this project was to get a prototype system to function with very low probability of bit error. The prototype system is \soft" in that it is all done within Ptolemy except D/A and A/D conversion and, of course, recording is done on a cassette deck. The transmitter takes an input bit stream and modulates raised cosine pulses for di erentially encoded 4-QAM passband transmission at 256 baud, where di erential encoding is used to overcome carrier phase ambiguity. Time-compression distortion of the channel necessitates both timing and carrier recovery as will be explained. Therefore, timing recovery, carrier recovery, and adaptive fractionally spaced equalizer were all crucial subsystems of the of the receiver.