I was wondering if someone already tried to optimize the file lenght decreasing some of those values (4 and 9 pulses, 150us periods, 1300us space) still trying to be compatible with the standard load routine.The tapes for the ZX81 seems to be a different kettle of fish. A "0" bit consists of four high pulses, a "1" bit of nine high pulses. Both are followed by a silence period. Each pulse is split into a 150us high period, and a 150us low period. The duration of the silence between each bit is 1300us. The baud rate is thus 400 bps (for a "0" filled area) down to 250 bps (for a "1" filled area). Average medium transfer rate is approximately 307 bps (38 bytes/sec) for files that contain 50% of "0" and "1" bits each.
Surely there should be some tolerance, but to which level it can be resonably ported?
In case someone want to do some tests I've "enhanced" the zx81 tape converter. Attached the new source and linux and window executable files. I've tested just Linux one, and with some extreme(?) parameters like "./p2wav.elf -wo7 -wz3 -wh100 -wi800 " it resulted almost half time!