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edit: you and me both :) I,v been semi obsessed with writing my own soft modem ever since I saw Oonas sound of dialup writeup.

Keep in mind, if you plan on doing this to dial into other modems, many phone contracts explicitly prohibit using a phone call for anything other than voice.

That doesn't mean I'm not gonna try hooking up my old hayes compatible, but I'm on prepaid so I don't have to care.

You might be interested in minimodem: http://www.whence.com/minimodem/

The starting point is probably V.21 / Bell 103. Check out the datasheet for the AMI S3531 single-chip modem, it's got useful background and timing info.