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  #1  
Old 01-13-2018, 03:51 PM
Winston Smith Winston Smith is online now
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Reelspin Digital Audio recording on Compact Cassettes

For video recorders PCM processors were already available in the 1970s, which enabled these analogue machines to record digital audio uncompressed. Nowadays, only a much smaller amount of data would have to be recorded. Very good quality is achieved with 128 -192 kbit/s AAC. Would it be possible to record such a data stream on a normal CC? I remember seeing a video a few years ago where someone recorded and played back successful the Digital Radio Mondiale stream on a CC. There was DCC with 384 kbit/s as a modified CC, but would it be possible to do something similar with a CC and the help of a modern Computer? So to speak as a hacker Project. :-) That would be very exiting!

Last edited by Winston Smith; 01-13-2018 at 03:54 PM.
  #2  
Old 01-13-2018, 04:54 PM
latreche34 latreche34 is offline
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As far as I know it is not possible with the analog stereo heads, Even on a dcc deck and an analog metal formulation cassette still not possible, This French youtube member tried it already.

Besides who would invest time and money on developing such lossy format when people are moving away from it due to storage availability and affordability.
  #3  
Old 01-13-2018, 07:41 PM
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It’s a thought but if it was that easy Philips hadn’t come up with the DCC system.

I’m not an engineer but I read once digitizing requires more storage than the same in analog, compression does help but prolly won’t be enough to compensate.

Last edited by Blink; 01-13-2018 at 07:46 PM.
  #4  
Old 01-14-2018, 03:29 AM
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Compact cassette was never used on a large scale for Digital recordings with the exception of
the World First Digital Still Camera and a series of prototypes shown at audio fairs.




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  #5  
Old 01-14-2018, 12:25 PM
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8-bit computers of the late 70s through to the late 80s usually stored data on mono audio cassettes using regular shoebox machines. The data rates were usually about 4000 bits per second or less. 4kpbs....not 192kbps. "Turbo loading" programs boosted this to 14.4kbps

The best that analogue modems ever managed was 56kpbs in the 90s. Even this was unreliable.

Now of course this is digital data converted into analogue audio signals. Perhaps with PCM recording the data rate could be improved.....but it took DCC with it's specialised 8-track head to achieve a decent data rate. With specially designed heads they got 96kbps per track.

The experimental systems for PCM recording onto cassette as demonstrated in the link provided by NEMOaudio all used specialised, multi track heads....a similar basic concept to DCC.
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  #6  
Old 01-14-2018, 01:57 PM
AndyC772 AndyC772 is offline
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Modems could achieve up to 56k on a phone channel with a bandwidth of 300-3400Hz or thereabouts, in the direction from ISP to consumer where the encoder is connected digitally and so has access to the master 8kHz sample clock. Upstream, where there's an analogue connection to the line, IIRC the limit was 33k6.

With up to, say, 15 kHz bandwidth, maybe it would theoretically be possible to get higher bit rates, and of course you get twice the bandwidth if there's good stereo separation. The maths involved is pretty horrendous, though,
  #7  
Old 01-14-2018, 07:46 PM
aamodtpw aamodtpw is offline
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Quote:
Originally Posted by absinthe_boy View Post
8-bit computers of the late 70s through to the late 80s usually stored data on mono audio cassettes using regular shoebox machines. The data rates were usually about 4000 bits per second or less. 4kpbs....not 192kbps. "Turbo loading" programs boosted this to 14.4kbps

The best that analogue modems ever managed was 56kpbs in the 90s. Even this was unreliable.

Now of course this is digital data converted into analogue audio signals. Perhaps with PCM recording the data rate could be improved.....but it took DCC with it's specialised 8-track head to achieve a decent data rate. With specially designed heads they got 96kbps per track.

The experimental systems for PCM recording onto cassette as demonstrated in the link provided by NEMOaudio all used specialised, multi track heads....a similar basic concept to DCC.
Exactly right about those early PCs. The Tandy TRS-80 came immediately to mind, as I remembered seeing the compact cassette peripheral drive in action.
  #8  
Old 01-14-2018, 08:07 PM
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I still have mine. And I wrote a routine to use the cassette data output port on the Model I to generate sound for all the games I wrote. It was waaaaaay cool!

Without a bunch of parallel data tracks a-la DCC, I just don't see how you could get anything useful out of a cassette deck.
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  #9  
Old 01-15-2018, 02:31 AM
AndyC772 AndyC772 is offline
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Back in the day (late 70's / early 80's), computers using cassettes for storage had very limited processing capability, so data was recorded as a series of tones. A high frequency would correspond to a '1', and a low frequency to a '0' (or some other very simple encoding scheme). An pair of analogue bandpass filters would be used to detect the two tones, and the CPU would read their outputs and shift in the data bits accordingly.

Modems were always very different. They had dedicated processors which could use much more complex, but bandwidth efficient, encoding / decoding schemes. The training sequence at the start of the call was used to determine the frequency and phase response of the line in each direction, to measure the SNR, and for the two partnering modems to agree the best choice of modulation and equalisation in each direction.

This technology was never applied to reading data off analogue tape, because tapes had long since been superseded by floppy discs by the time it was mature and cheap enough.

Today it certainly would be possible to apply DSP technology to get a reasonable data rate for audio off a good quality analogue tape. The processing involved is still used in digital radio communications, which have the same problem to solve: transmit large amounts of digital data as quickly and reliably as possible, over an analogue medium with limited bandwidth.

The issues are that: a) nobody strictly needs to do it, so it's very much in the realms of hobby projects that someone might choose to do for fun, and b) the implementation requires degree-level maths, which severely limits the number of people who could ever pull it off.
  #10  
Old 01-15-2018, 03:06 AM
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Velktron Velktron is offline
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Yup, in theory one could do much better than what the original TARBEL or KCS (Kansas City Standard) "data cassettes" could store, and certainly at least two-three times better than what V.92 modems could do on a noisy POTS line, without even considering stereo or 4-track layouts.

The tradeoff is that the DSP hardware required for writing/reading such complex encoded data would need to be quite powerful, so this approach wouldn't be economical even in the 90s, let alone the 80s. At best, with the most optimistic assumptions (4x bandwidth, both stereo channels used etc.) you would get a data rate comparable to DCC, ca. 400 kbps, but requiring a much better deck to take advantage of it, too. Don't forget that even the low-density ZX Spectrum etc. tapes with their simple modulation were often hard to interchange and very sensitive to level and azimuth variations between decks. So the resulting setup (expensive DSP unit, high-quality deck, quality media) wouldn't be necessarily cheaper or more reliable than a dedicated computer tape streamer with Quarter Inch cartridges, even in the 1990s, and it would lack the (relatively) fine control those streamers allowed, e.g. computer-controlled re-reads etc.

For a practical example of how actual UNCOMPRESSED digital audio was recorded on a standard (Metal tape only, though) CC, see the experimental Sharp Optonica RP-X4: it used the full width of the tape to record 16 (!) data tracks of 14-bit stereo PCM, 44100 Hz uncompressed digital audio, @3.5 ips. The head was developed using floppy disk technology. And that was in 1985...horribly heavy (10 kg) and likely prohibitively expensive for mass production. I wonder what happened to the (only?) prototype...

Last edited by Velktron; 01-15-2018 at 03:13 AM.
  #11  
Old 01-15-2018, 03:42 AM
Winston Smith Winston Smith is online now
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Already 10 years ago someone recorded DRM (Digital Radio Mondiale) signals on CC and played them successfully. Unfortunately, I don't know how he did it. Do you know any software that can be used to create a DRM stream and output it via the sound card? That might be a start.

The Video: https://www.youtube.com/watch?v=2mseiBwPKvk
  #12  
Old 01-15-2018, 03:52 AM
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Quote:
I managed to get a reliable 11.4 kilobit digital audio stream (HE-AAC) encoded on the tape.
Without wanting to demean the guy, that's a far cry from what even a V.90 modem can do on a POTS line, and in fact, not so far from what speedloading routines could achieve on the C64 or ZX Spectrum (ca. 9600 bps), using only the onboard hardware.

It just goes to show how precisely calibrated equipment and powerful DSP one needs to get a reliable signal. However, if you're interested in this line of research, a good starting point would be softmodem/winmodem drivers and hardware: those already do the necessary DSP in the software, some are even open source, and provided you can give them a POTS-like channel, they can deliver you at least 33.6 kbps per channel
  #13  
Old 01-15-2018, 12:30 PM
Winston Smith Winston Smith is online now
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I'm afraid I'm not qualified for the softmodem experiments. The DRM realtime modulator software is probably this one here: Spark DRM Signal Generator. But I think its expensive.
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