Increase cassette speed

rjhelms / compe6502

Homebrew 6502 system

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Increase cassette speed #31

Closed rjhelms closed 2 years ago

rjhelms commented 2 years ago

300bps is painful! Theoretically possible to go up to 1200bps without any hardware changes, by reducing bit time from 4 1200Hz cycles to 1.

Any faster requires increasing carrier frequencies, either by increasing system clock or changing divider on VIA card.

rjhelms commented 2 years ago

Increasing cassette speed should be as easy as modifying CHALFWAIT in io_card.s - for 300bps, it's 327 times through the loop. In image tests 82 (#$52) worked for 1200bps.

Also need to modify bin2tape.py - just change SPACE_CYCLES and MARK_CYCLES.

rjhelms commented 2 years ago

1200bps read works OK with files generated PC side - but things output 6502-side can't be read back. Trying 600bps at 163 times through the loop in CHALFWAIT

rjhelms commented 2 years ago

600 bps works.

I think 1200bps is possible, but there's no wiggle room on writes.

1200bps gives 833 cycles to process a bit:

two JSR and two RTS in the call to CWAIT takes 24
20 cycles setting up CHALFWAIT twice (phy/ldy/ply, less the cycles gained from final bne branching)
11 cycles in the loop of CPUTBYTE
leaving 778 to be chewed up by the loop - so ideal delay 77 or 78? 77 with two NOPs to make it exactly 77.8?

Also a question about phase - cassette output seems phase inverted from what I expect, but I don't know if that matters at all.

Going to set this aside for now, but leave issue open as it would be nice to get 1200bps.

rjhelms commented 2 years ago

With timing as above, 1200bps mostly works - can reliably read PC-generated recordings, can save and read back but is very particular around phase and bandwidth. See 055c7579538378650354119f13585ee5e0b08096.

A few options:

double carrier frequency - this could be accomplished by shifting all the outputs of U3 on the I/O card. This would allow 1200bps while still having multiple clock cycles per bit (which I think is the reason for these problems)
add logic to specify bitrate in header - read header at 600bps, then if specified increment speed for data reads to 1200bps. Output from save routine would stay at 600bps.
Leave it at 600bps, and get SD support working.

rjhelms commented 2 years ago

Used second approach there, implemented in 5f4873eddd523b7377f6abd29f7ff1c41d057ffe.