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Audio tape interface revives microcassettes as storage medium

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Re: Audio tape interface revives microcassettes as storage medium

#21

Post by femto »

p1mrx wrote: post #2

I wonder what the maximum bitrate of an audio cassette is, using modern encoding techniques like OFDM with error correction? I searched once, and saw estimates in the 25 kbps range, but no usable implementation.

S/N for a cassette is variously given as 50dB, 58dB or 67dB [1,2]. Bandwidth is given as 12kHz (standard), 16kH (medium Fi), 19kHz (HiFi) for different quality cassette systems [3]. Let's simplify by assuming the frequency cutoff is a brick wall and the S/N is constant across the band.

The maximum channel capacity for a cassette can be calculated using Shannon's Theorem [4]: C = B*log2(S/N+1)

Cheap and cheerful (S/N=50dB, B=12kHz): C= 199kbit/s

Mid range (S/N=58dB, B=16kHz): C= 297kbit/s

Hi Fi (S/N=67dB, B=19kHz): C= 422kbit/s

Modern coding (LPDC) can achieve within a fraction of a dB of the Shannon Bound, so those numbers will be close to what a state-of-the-art encoding could do with a cassette. (A new hobby for someone?)

[1] http://www.tapeheads.net/showthread.php?t=35390

[2] http://www.muzines.co.uk/articles/choosing-a-cassette-tape/1...

[3] https://arstechnica.com/civis/viewtopic.php?t=848978

[3] https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...

Edit: Here's a dose of reality from another forum:

https://dsp.stackexchange.com/questions/70700/why-can-so-lit...

My guess is the the Shannon based predictions are realistic, as they are lower than the same rates as achieved by specialised data recording systems. They are also a bit better than what a narrow band radio system can achieve, and a cassette will be a better channel than a typical radio channel.

Re: Audio tape interface revives microcassettes as storage medium

#22

Post by anyfoo »

LesserEvil665 wrote: post #4

I remember 6/7-year-old me begging my father for a tape deck to use with CoCo3. He refused, saying they were too expensive. Were they really in the early 90s? I doubt it. I lost interest in writing basic on that thing since I couldn't save programs. Anyway, I fully intend to try this out if only to close that loop.

I was also very young and did not have a good enough grasp on money yet, but for us, in Germany, I remember the 1541 disk drive for the Commodore not being cheap, but still everyone I knew with a C64 got one, so the cost must not have been too prohibitive. The "Datasette" tape drive was much cheaper as I recall, but because of the disk drive's propensity I only knew one person who had it (and probably barely used it).

Depended a lot on the region I guess. I hear that in Britain for some reason the tape decks were much more common. But Britain kind of got an "alternative history" of home computing anyway, owed largely due to their computer literacy program and (partly) resulting British micro industry. Many of those products barely made it to the continent, some not at all, so it's hard to compare.

Re: Audio tape interface revives microcassettes as storage medium

#23

Post by Osiris »

anyfoo wrote: post #22
LesserEvil665 wrote: post #4

I remember 6/7-year-old me begging my father for a tape deck to use with CoCo3. He refused, saying they were too expensive. Were they really in the early 90s? I doubt it. I lost interest in writing basic on that thing since I couldn't save programs. Anyway, I fully intend to try this out if only to close that loop.

I was also very young and did not have a good enough grasp on money yet, but for us, in Germany, I remember the 1541 disk drive for the Commodore not being cheap, but still everyone I knew with a C64 got one, so the cost must not have been too prohibitive. The "Datasette" tape drive was much cheaper as I recall, but because of the disk drive's propensity I only knew one person who had it (and probably barely used it)…

I had (probably still at my parent's house) a Commodore cassette deck, 2 1541s and a 1571. All modded of course.

I remember using the cassette deck but it was definitely a stop gap to a real disk drive.

Re: Audio tape interface revives microcassettes as storage medium

#24

Post by anyfoo »

hammock wrote: post #17

>common disturbances like speed variations (which cause wow and flutter) Had to look that up. What they call "slow wow" that is common in turntable records makes sense though as onomatopoeia. https://www.britannica.com/technology/flutter

Super common terms for analog media, tape in various forms (like reel to reel) is very old after all, and it's easy to imagine that the motors driving them exhibit significant speed variations during playback. There were also dedicated wow and flutter meters that measured both: You'd play a fixed frequency tone and it would display the frequency variation of what actually came out. Technically that's a form of frequency modulation, with wow and flutter being modulated with low and high frequencies respectively[1].

Later "quartz locked" high precision drives for example got rid of most of it, but as I recall that got widespread briefly before or even at the same time as CDs, and was far from ubiquitous.

[1] The "high" frequency here still being much lower than what you usually think about when you read "high frequency" even in an audio context.

Re: Audio tape interface revives microcassettes as storage medium

#25

Post by krallja »

tenebrisalietum wrote: post #19
Const-me wrote: post #12

Earlier quoted context omitted.

Not sure it's directly comparable. When modem flips one bit, the CRC checksum in IP packet becomes incorrect, someone drops the IP packet, the condition is detected and the packet is re-transmitted. This slows down things substantially but user still gets their data. I think tapes need more reliability because there's only 1 try.

The Commodore 64 (and possibly earlier Commodore systems such as the PET and VIC20) actually recorded each "block" twice on the tape, and could correct up to 31 errors per block as long as an error didn't affect the same exact byte in each block (it didn't do anything like ECC). This also made loading/saving from tape really slow - 50bps.

SiriusXM uses a similar method, plus buffering, to ensure your satellite radio keeps playing, even if you dare to drive under such rare things as traffic lights and tree branches.

Re: Audio tape interface revives microcassettes as storage medium

#26

Post by anyfoo »

Osiris wrote: post #23
anyfoo wrote: post #22

Earlier quoted context omitted.

I was also very young and did not have a good enough grasp on money yet, but for us, in Germany, I remember the 1541 disk drive for the Commodore not being cheap, but still everyone I knew with a C64 got one, so the cost must not have been too prohibitive. The "Datasette" tape drive was much cheaper as I recall, but because of the disk drive's propensity I only knew one person who had it (and probably barely used it)…

I had (probably still at my parent's house) a Commodore cassette deck, 2 1541s and a 1571. All modded of course. I remember using the cassette deck but it was definitely a stop gap to a real disk drive.

Ah, I was too young for multiple drives. While my parents probably saw the futility of not having a disk drive at all, shelling out the hundreds of DM for a second one was likely too much.

And with no own money I put up with what I had. I nowadays suspect that the grease in my then rather old 1541 must have hardened, because it needed the occasional hard slap on its case to continue loading, it would "hang" in the middle of a disk access until you did that.

Re: Audio tape interface revives microcassettes as storage medium

#27

Post by adrianmonk »

femto wrote: post #21
p1mrx wrote: post #2

I wonder what the maximum bitrate of an audio cassette is, using modern encoding techniques like OFDM with error correction? I searched once, and saw estimates in the 25 kbps range, but no usable implementation.

S/N for a cassette is variously given as 50dB, 58dB or 67dB [1,2]. Bandwidth is given as 12kHz (standard), 16kH (medium Fi), 19kHz (HiFi) for different quality cassette systems [3]. Let's simplify by assuming the frequency cutoff is a brick wall and the S/N is constant across the band. The maximum channel capacity for a cassette can be calculated using Shannon's Theorem [4]: C = B*log2(S/N+1) Cheap and cheerful (S/N=…

There's also stereo to consider, right? So you could get up to 2X bandwidth. (Channel separation issues might mean less than 2X in practice, though.)

If you want to stretch the definition of "cassette", tapes are two-sided, and there were 4-track recorders that used all 4 tracks in one direction. They weren't anything too exotic. Musicians used them for cheap recordings for demos and such.

Re: Audio tape interface revives microcassettes as storage medium

#29

Post by TedDoesntTalk »

p1mrx wrote: post #7
LeoPanthera wrote: post #6

Earlier quoted context omitted.

The sound quality of an audio cassette is at least as good, and almost certainly better, than the quality of an analogue telephone line. So I bet you could set a lower bound of at least 33.6kbit/s, since that was as fast as telephone modems went. 56k modems relied on your phone line being digital after the switchboard, but I would imagine that 56k should be doable as well.

I wouldn't automatically assume that tape is better than a phone line. For instance, a phone line will always clock at 1 second per second, because everything runs in real time with no storage.

Analog phone lines were limited to 4 KHz frequency range in the US. Compare that to tapes close to 20 KHz.

Re: Audio tape interface revives microcassettes as storage medium

#30

Post by akira2501 »

femto wrote: post #21
p1mrx wrote: post #2

I wonder what the maximum bitrate of an audio cassette is, using modern encoding techniques like OFDM with error correction? I searched once, and saw estimates in the 25 kbps range, but no usable implementation.

S/N for a cassette is variously given as 50dB, 58dB or 67dB [1,2]. Bandwidth is given as 12kHz (standard), 16kH (medium Fi), 19kHz (HiFi) for different quality cassette systems [3]. Let's simplify by assuming the frequency cutoff is a brick wall and the S/N is constant across the band. The maximum channel capacity for a cassette can be calculated using Shannon's Theorem [4]: C = B*log2(S/N+1) Cheap and cheerful (S/N=…

Another way to look at it, SQNR = 20 * log10(2**Q).

When Q=8, equivalent to 8bit PCM encoding, the maximum SNR would be 48db. So the cheap cassettes are like 8bit PCM at 12kHz.

Remaining values would be:

Q=9, SQNR=54db.

Q=10, SQNR=60db.

Q=11, SQNR=66db.

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