This is an old post, preserved for reference.
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The Future Was 8 Bit has recently begun releasing software on cassette. The first was Pentagorat for the VIC20, which with then followed by Pilot Attack for the ZX Spectrum, both by Misfit.
DivMMC programmer. There is some extra circuitry added to that to get the level right. The level is always a tricky thing with the Spectrum, unless you are using a Spectrum +2 where the level are preset with the built in tape deck.
Update:This turned out to be quite a useful thing to have, so after I shipped that one off to the TFW8B Mansion, I decided to build another one for myself. I'll probably do a PCB for this in slow time, but for the moment, I went for a quick solution, I chose an Arduino Ethernet board I had lying around. That was bought for a project a while ago that went in a different direction.
We found that we had some issues with the tapes duplicated from the mater tape on the tape duplicator machines. These are designed for audio tapes, so have some additional processing in there to low pass filter the audio and automatically level things down, which degrades the quality of the data on there. I have sketched out some ideas to modify those units to work better with data, but for the moment, the master tape generator has turned into the production tape generator.
Update # 2
pre-order now from The Future Was 8 bit.