C64 Tape Port

[spirantho]

To be precise, the cassette port samples at 1000000 (more or less)  ticks per second, allowing a theoretical throughput of (1000000/8) bytes/s = 125KB/s.

That's a completely theoretical limit, of course - no loader gets even close (the ROM loader being 50 bytes/s) but in theory if you could sample reliably that to work out if the flux had changed or not you could get that rate. If you wrote a routine to check for changes in flux that often and connected it to a digital input, you could get that rate if your tape routine consisted of one instruction - which of course it won't.

In reality, I believe the fastest loaders were probably running at about 300 B/s (2400 baud) but that's just a guess from what I've observed.

[spirantho]

If my maths is right - doesnt that mean it should be possible to record an 8bit 22khz sample via the tape port - with some headroom?

Not a chance, I'm afraid. The tape port is digital, not analogue, and is serial. You can write a '1' to it, and you can write a '0' to it, but that's all.

When you read the tape on a C64, the downward transition triggers an interrupt via the VIA (if you see what I mean). You don't have full control of the wave, just transitions from +ve to -ve.

Sorry.

@PSXPhill
That's basically what I was trying to get across, yes.