One of the tricks many of the early machines allowed was the ability to record an audio track on one of the tape channels, and then turn it on or off using the deck control. I believe the Ataris may have been the first to use this "in production" but I also saw it on the CoCo and I believe the C64 could do it too.

So that got me thinking... did anyone use the second channel for storage? Looking over what I can find, the answer appears to be "no". None of the early S-100 systems did, KCS or CUPS et all, nor any of the microcomputer systems I can find.

And why not? I can imagine that machines that used normal user-supplied decks might have mono input, but surely even then one could find a suitable portable device with stereo in, or even use a nice Akai or something. These weren't exactly the most complex circuits, I can't believe it would add anything material to the cost of the AD/DA circuitry at least.

And those with digital interfaces, like the Atari and Commodore decks, one could send data to the deck at a higher rate, split the bits in the AD, and write them out two at a time.

And then I thought, why not use FSK at several frequencies? Looking over the standards I see frequencies from around 600 to 6000 being used, often at one end or the other. For instance, Atari used relatively high-end frequencies at 3995 and 5327 Hz, while KCS was 1200 and 2400. This implies one has safe usage from 1200 to 5300 at a minimum. So why not both, or several?

Does anyone have some insight here? And were there any really advanced systems that did these sorts of things?

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    The problem would be cross-talk between the two channels. Cheap consumer cassette players and recorders were not precision mechanical devices. They often used a single tape head for both recording and playback, and if that was grossly misaligned it made no difference when playing back recordings made on the same machine, but it would have been a disaster trying to swap digital data recordings between difference machines, or trying to accurately read a properly aligned factory-produced tape. – alephzero Nov 8 at 20:11
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    Well gross misalignment still kills you in mono, which I can state as fact because my Atari deck could easily read and write my cassettes but no one else's. Turns out there is an alignment screw... But that doesn't really address the question, alignment is either on of off, and does nothing to using multiple frequencies. – Maury Markowitz Nov 8 at 20:14
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    The physical separation of stereo channels on the tape was about 0.3mm, or about 1/100 of an inch - too small to correct the alignment with a screw adjustment without a test tape and proper measuring equipment. – alephzero Nov 8 at 20:18
  • Stereo track crosstalk wasn't actually that bad - Bleed-through between the two sides of the cassette is normally worse. I think the simple answer is that most cassette-based home computers only had one CPU ;) and this was busy enough with single-track recordings... – tofro Nov 8 at 21:41
  • @alephzero - what can I tell you, I did this. I turned the screw, tried to load the tape. Repeat. Four turns later, and I'm up and running. But again, this would effect both channels equally, so it can't be the reason. – Maury Markowitz Nov 8 at 21:47

So that got me thinking... did anyone use the second channel for storage?

Well yes, everyone - They simply used the same signals for both channels. But that was obviously not your question.

  1. You simply could not assume everyone had a stereo recorder. While it might seem "normal" today, it wasn't in the 80ies
  2. Coding and decoding one channel already max out a typical 6502 or Z80 of the time1. Doing two in parallel, (and for cost reasons, the de-/encoding is typically done by the CPU only) would not be possible without much more complex electronics
  3. Increasing information density decreases reliability and also compatibility across and between a wide range of recorders and tape material.
  4. Market erosion for more complex storage devices. Tape storage was low-end, low-cost technology. It had to stay there in order to still be able to sell floppy drives.

1: While not a standard cassette drive: The Sinclair microdrives actually used both tracks on the drives to increase data density and speed. In order to still be able to use relatively simple hardware, even there the tracks were not used in parallel, but rather alternating.

The closest you are looking for is maybe the Atari 410 - This actually used one of the stereo tracks for separate audio recording.

  • All of these are good answers, BUT... in the case of digital drives like the Atari and Commodore, all of this could be done right in the adaptor, the CPU is not involved. It seems odd they would not do this. And as the frequencies used were all ostensibly "safe", there seems to be no reason not to use more than one base carrier. – Maury Markowitz Nov 9 at 17:01
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    There's not much "digital" stuff in a Commodore Datasette (in fact, exactly nothing). The modulation is entirely done by the CPU, only level shifting takes place in the tape drive. The Atari tape drive was different, and actually did use both left and right tape tracks separately - But one for audio only. – tofro Nov 9 at 18:01
  • The C64 can actually be connected to a "standard" cassette deck if you connect it through a number of 4089 CMOS inverters as amplifiers and a resistor in the data-out line.. – tofro Nov 9 at 18:37
  • Re (2) only: I keep wondering though whether two channels would actually cost more processing time to decode; if you made one the clock and the other the data then it feels like you could double the data rate over the usual single-channel self-clocked solution without undue extra processing. You're still watching one for transitions, but now sampling the other on cue rather than deriving from transition lengths. Maybe I'm being over-confident about resilience to phase errors? (and/or failing in some other regard at logical reasoning) – Tommy Nov 9 at 22:11
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    There were tape loaders which did complex things like run minigames while the game was loading, with no obvious increase in loading times, so I don't think that the CPU was really the bottleneck. – user3570736 Nov 11 at 0:58

Does anyone have some insight here?

Preface: Since it hasn't been done, everything that follows is of course only a list of possible reasons. Each may have been overcome at some point, so there is no reason to pile possible solutions to the points called here.

This is basically two questions in one:

Why weren't not both channels used?

There are several factors

  • Double the hardware. Even if it's not much, it adds to cost and complexity
  • Way more complex software. It did not only need to decode two channels simultaneously, but also check for errors on either side, and separate error handling
  • More prone to misalignment
  • Slow CPUs. Keep in mid, coding was done in software in early machines. There is not much time left to do two channels simultaneously.

Less important but still valid reasons might have been:

  • inability to use cheap mono recorders
  • inability to use cheap tapes

Considering that most cassette formats didn't even use mono recording to it's full ability, using both channels seems far fetched.

Even a cheap recorder could go up to 12 kHz. That would have enabled a recording speed of about 4800 bit/s (using just two tones of 9600 and 4800 Hz). Sure, maybe a bit more error correction might have been useful, still, it wouldn't be so far fetched. And with a single channel this could have been handled with an 8 bit CPU alone.

Why were only two straight frequencies used?

For one all of the above also applies here. It would have needed way more complex hardware and/or even more software and thus more powerful CPUs. So again, there is no need to go for more complex encoding when single frequency recording wasn't fully used.

The most important reason...

... might have been that there was no need to do so. Cassette recording was only important for a very short period. When floppies became available, they where so much more convenient with their maned and random access, not to mention the way higher speeds, that cassette wasn't worth any further investment.

A good hint here is that several faster cassette systems were developed and offered around 1980 (not at least the stringy floppies :)) and none of them caught on. The manufacturers had no intention to speed up cassettes beyond the initial speed, as they wanted to sell their floppies, and third party manufacturers couldn't sell their add on devices, as the price was rather high when compared with the more expensive but also way more convenient floppies.


There is also one misconception, which is that any 'digital' system would change the situation, as the hardware is still the same, no matter if it's installed within the drive or the computer.

  • Agreed, a computer that could store data in stereo on an audio cassette would be a bit of an anachronism. It would need to use audio cassettes for storage (probably an 8-bit computer) and it would also need to have built-in stereo. The first computer with built-in stereo was the 16/32-bit Amiga 1000 in 1985, but it didn't use cassettes. – traal Nov 10 at 0:47
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    @traal The first computer that used both stereo tracks of audio cassettes for different purposes was the Atari 400/800. You could use one track to store data, and the other for audio. Atari even sold some language learning programs in that format. – tofro Nov 10 at 22:42
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    @traal - the Amstrad CPCs had stereo audio before the Amiga. Admittedly not very good stereo: 1 channel of an AY chip was left, another right, with channel two going to both left and right. Its tapes were sullenly monophonic. – scruss yesterday

I can imagine that machines that used normal user-supplied decks might have mono input, but surely even then one could find a suitable portable device with > stereo in, or even use a nice Akai or something.

Yes, almost all consumer tape recorders (except for expensive component stereo decks) had mono inputs.

The only reason for using cassette tape was to lower the cost of the computer, and being able to use a recorder that the customer already had was cheapest of the lot. Back then this was vitally important for breaking into the consumer market, because disk drives and controllers were hideously expensive. It made no sense to produce an interface for expensive stereo recorders because that wasn't the target market.

These weren't exactly the most complex circuits, I can't believe it would add anything material to the cost of the AD/DA circuitry at least.

Look at the tape interface circuit of a Sinclair ZX81 or Spectrum - do you see any AD/DA converter? These computers were designed to use the smallest number of components they could get away with, to keep costs as low as possible.

...why not use FSK at several frequencies? Looking over the standards I see frequencies from around 600 to 6000 being used, often at one end or the other. For instance, Atari used relatively high-end frequencies at 3995 and 5327 Hz, while KCS was 1200 and 2400. This implies one has safe usage from 1200 to 5300 at a minimum. So why not both, or several?

Conventional FSK is actually not a good choice for audio cassette recorders (I suspect it was only popular because engineers were trained to believe that's how you do it). Atari's use of high frequencies is bad, and any system that tries to generate sine waves or filter frequencies is counterproductive. To use multiple frequency bands at once you would need to separate them with filters, and then distortion and noise become even more of a problem.

Better schemes ignore individual frequencies and just time transitions, realizing that the data is recorded on the tape as flux transitions which become a series of spikes when played back. They don't try to resolve high frequencies which are attenuated by head azimuth alignment and tape-head gap, and they take into account the frequency changes and random jitter caused by motor speed variations and tape stretching.

And those with digital interfaces, like the Atari and Commodore decks, one could send data to the deck at a higher rate

The CPU in many retro computers is fast enough to encode and decode modified FSK at much higher rates than a consumer tape recorder can handle. Project O.T.L.A. creates audio files which load on a ZX Spectrum, Amstrad CPC or MSX at 12600 bps, but only when played by a device which has sufficient frequency stability and linearity to reproduce the signal accurately.

The problem with consumer tape recorders isn't just bandwidth, but also motor speed variations, phase noise, distortion, dropouts etc. Using a mono head or both sides of a stereo head improves signal-to-noise ratio and reduces dropouts compared to using one side of a stereo head. Using both sides of a stereo head independently would make it much worse. Cassette stereo is an awful format!

Two channel cassettes were quite common in the minicomputer and data acquisition fields. These “streamer cassettes” were single-sided, with data being recorded in mono on one full track and the other track (side 2 if the tape were flipped) contained an index timing track. The drives were servo-controlled, and could seek fairly quickly to read and write random blocks. One manufacturer was MFE Computer Access Systems of Salem, NH, USA. Here's a brochure on one of their transports: MFE Model 260. They were used in the Canadian MCM/800 systems. A similar control scheme was used in early desktops including the HP 9800.

So: not stereo, but definitely two tracks.

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