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Riging together a way to store data on an audio cassette
The greatest invention of all time was when you starting being able to play some simple game like space invaders WHILE the main game was loading...the minutes just flew by
Re: Rigging together a way to store data on an audio cassette
Originally posted by aneeshm
I can assume this idea is not original , it being so obvious . Yet I'm thinking of making it for my 12th grade project ( as a project common to physics ( the electronics parts ) and CS ( the programming parts ) ) . I'd say that the hardware is not too hard to rig together . The software could be pretty simple , too . But I don't know yet . Another thing is the maximum data density . I'd say that I can store data in octal or hex format to maximise data density .
To manage the input , I'm planning to use normal sound input and output ports . Then I can send the data to standard sound output so that I can use a cable to plug it into the mike port of a normal audio cassette recorder to make recording easier , and use the headphone/standard output port of a player and mike port of a computer to copy data from the tape back to the computer .
Can the resident experts here tell me if this is feasible ?
Yes it's feasible although I suspect you will need to learn some more about electronics before you can do it.
You'll have to decide which frequencies mean what in the backup, and design the circuits as much as the software to do it.
Back when hard drive sizes were 20Mb or so, companies sold kits to back up onto video tape - dedicated tape drives were very expensive and so home VCRs were used.
Haven't seen one of them for 12 years or so.
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iirc, we had ~20 games on a tape, so it makes something like, maximum 320K. But it is pure guess from my part. In fact, I have no idea...
I seem to remember on my Spectrum (Timex for you yanks), a 20 minute tape could hold a game on each 10 minutes side. So 48k (ish) = 10 minutes. So a single side of a 90 minute tape woud hold about 250k????
But as you say:
But it is pure guess from my part. In fact, I have no idea...
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This shouldn't be extremely difficult and shouldn't require additional hardware. As you say, all you would need to do is use an audio card with output and input. Everything could be done in software. An audio card like this...
Acclaimed audio interfaces, studio monitors, and keyboard controllers
...would provide 96 Khz recording from line in, or a theoretical maximum of 96k baud. Of course, you would need to store the data in binary, with a checksum of some sort every x bits. Actual bit density would be much less, because of the limitations of the audio tape drive. You would just have to do a testing regime. The error rate/density might be a good color chart for your teachers to be impressed with.
As a general rule, you can store a lot on tape. You can store several tens of gigabytes of data on a tape through an ordinary digital camcorder, for instance.
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Been there, done this. In the eighties, in East Germany. It was frigging difficult to get the DM for a home computer (Commodore 64 or Atari 800). To get the additional money for a datassette was impossible, at least for me. Everyone OTOH had an ordinary cassette recorder, so why not use it for data storage?
The problem with the storage is, that you have to create vibrations (to make the sound for storage). Obviously, if stored directly, the bit sequence 000000... would produce silence on the tape. But if you have a sequence of 111111..., it would also translate into silence, because there are no oscillating bits. Our solution was to have every bit represented by a triple of bits:
0 = 010
1 = 011
So only every third bit counts.
Since every bit starts with 0 and has an upflank, it translates into a vibration, which can be stored at the tape (it sounds abominable). There is no silence possible. We had a block size of 256 bytes. Each block had a header of "5A5A". That was important, because you have to catch the phase, as on some systems you got the vibrations inverted. So if the header was "A5A5" (the inverse of "5A5A"), we just inverted every bit of the block. The blocks also had counters, so if a block was rejected due to a checksum error, it was enough to rewind a little bit, usually the system caught up.
We called it "turbo" and reached a speed of 4000 baud. We made experiments up to 6000, it worked, but not safely. 4000 baud worked wonderful. It had a little adapter, but I can't tell anything about it (hardware embryo here). I just wrote the driver. It was fun. Those were the days.
Why not use different sound frequencies? That is, you could either have 16 different frequencies for each nibble (i.e. 4 bits). The result would be single tones. Or you could make it a little more complicated and instead use 8 frequencies for each byte. The result will be "chords" which you could decompose with some spectrum analysis.
... But I'm guessing that's sort of what you had in mind already, aneeshm, since you mentioned octal/hex and recording with a simple mike (edit: oh wait, you weren't).
Last edited by Mercator; February 10, 2005, 21:04.
You mean like DTMF dialing? You wouldn't be able to reach big data rates this way, because you need larger samples to measure a frequency (or a mix of frequencies like in DTMF). But it would be easy doing hardware-wise, because there are chips at the market, that do the decoding.
Originally posted by reds4ever
Mercador: we used to call it a nybble
Bah!
Originally posted by Sir Ralph
You mean like DTMF dialing? You wouldn't be able to reach big data rates this way, because you need larger samples to measure a frequency (or a mix of frequencies like in DTMF). But it would be easy doing hardware-wise, because there are chips at the market, that do the decoding.
I suppose something like that. I had never heard of that before. I was thinking 8 different frequencies each representing one bit in a byte. So silence would be 0x00, all 8 tones would be 0xFF. Higher frequencies would help a bit, but I guess you're right...
I don't know anything about this anyway. I just know my suggestion would sound way cooler!
You could try playing an audio tape and see what data it "generates".
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In the eighties, in East Germany. It was frigging difficult to get the DM for a home computer (Commodore 64 or Atari 800). To get the additional money for a datassette was impossible, at least for me. Everyone OTOH had an ordinary cassette recorder, so why not use it for data storage?
I suppose something like that. I had never heard of that before. I was thinking 8 different frequencies each representing one bit in a byte. So silence would be 0x00, all 8 tones would be 0xFF. Higher frequencies would help a bit, but I guess you're right...
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