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Comment by jedbrooke | Hacker News Reader

YES! I remember now. I have a notes file where all this is written down somewhere. A stock tape recorder can usually only read two channels at a time (stereo L and R), but then you’d still have the “flip it over” side A plus side B runtime.

Thanks for the link, definitely saving that one

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Note also that the linked article stops at QAM. This is a previously solved problem for phoneline communication; in the 1980s modems went from 9.6kbps to 33.6kbps[1] after adopting trellis coding. It's not impossible that a similar technique would work for audio cassettes.

1: https://www.itu.int/rec/T-REC-V.34-199802-I/en

Players with auto-reverse have 4-track heads. Using one of those, you can read all 4 tracks at the same time. But the best would be a rotating head (helical recording) like VCR or D/CAS I mentioned. That can use the whole width of the tape, not just 4 tracks.

In theory, helical recording does not affect total capacity. A track will take up a certain area of tape regardless of whether the tracks are linear or helical. Helical tracking is used in VCRs because it lets you increase the speed of the head relative to the speed of the tape by a factor related to the width of the tape, thus allowing for a single track with very high bandwidth relative to the speed that the tape moves at.

FWIW, modern LTO uses linear, not helical tracking (but with 1000s of tracks).

Don't for get actual 4-track recorders, a tape deck that recorded and played back all 4 channels simultaneously. They were cheap and enabled a low cost "poor mans recording studio" where you could record 4 different channels to mix down later. Some were portable and featured a built in mixer which made bedroom and live gig recording affordable.

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