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Martin Piper
Registered: Nov 2007
Posts: 699 |
Sample streaming from tape
Has anyone done four bit sample streaming from tape before? I'm thinking the time between pulses can be directly translated to four bit volume values as they are read from tape.
Practically useless, except for pure geekyness. But I wanted to know if it's been done, before I do it myself. |
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chatGPZ
Registered: Dec 2001
Posts: 11290 |
You can also abuse the "long gap" to make gaps with 1 cycle granularity... i'd certainly try that, use 8bit, and the mahoney playback. |
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Martin Piper
Registered: Nov 2007
Posts: 699 |
I might also try the Mahoney sample method. If I really want to remove the rest of my hair. |
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Bansai
Registered: Feb 2023
Posts: 40 |
Quoting MagerValpUse Macbeth's delta encoding instead (0/+1/-1), it only requires three different pulses. I take it that saturating arithmetic on the integrator equation would self-correct for tape dropouts where pulses are lost or corrupted? That is, at some point the value would incorrectly want to +1 the $d418 volume beyond 15 or -1 below 0, and saturating math would eventually clamp the value stream right back to where it should be given how overdriven 4-bit samples are anyway. |
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Hoogo
Registered: Jun 2002
Posts: 103 |
Staying within the restrictions of the TAP format is reasonable. But better check Slushload V3 to see minimum reliable pulse lengths.
16 pulse lengths for 4 bit are nice to handle defects on tape. But you will get a higher sample rate if you use 4 pulse lengths and write 2*2 bit.
Also your bit rate will be variable.
And finally: An analog magnetic tape may behave more weird than a TAP file suggests. |
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Martin Piper
Registered: Nov 2007
Posts: 699 |
Proof of concept ready. :)
Download this: https://github.com/martinpiper/C64Public/raw/master/IRQTape/Tap..
Run it in PAL Vice. It will wait on a black screen for a TAP file to be attached.
Download this TAP file: https://github.com/martinpiper/C64Public/raw/master/IRQTape/vic..
Use "File->Attach tape image..." in Vice (do not use auto-start!) and press play on the tape using "File->Datasette control".
You should see narrow colour bars and hear a sample from Space Ace.
If you want a command line for Vice then this should work: x64sc.exe -remotemonitor -dstapewobble 0 -dsspeedtuning 0 -1 vice.tap TapeStreamSamples.prg |
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Martin Piper
Registered: Nov 2007
Posts: 699 |
The sample rate is variable, since the 4-bit chunks from the tape for the volume are variable. However the average sample rate is 5.1KHz since the average pulse length is 192 cycles. |
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Martin Piper
Registered: Nov 2007
Posts: 699 |
The code changes for this example. https://github.com/martinpiper/C64Public/compare/e7dc2ef85d1c72..
Accuracy would be improved by using an IRQ instead of polling bit 4 on $dc0d. |
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tlr
Registered: Sep 2003
Posts: 1762 |
Would be interesting to see how this works on real HW. If you just translate pulse length to sample value, then any imperfections in detecting the length will translate to some noise which is probably unproblematic.
This encoding is basically frequency modulation (as in FM radio).
If you accept relying on precise pulse length detection, then you could employ a differential encoding instead, e.g 0, +1, -1, +2, -2... This way you could choose to have a higher sample rate for small changes, improving high frequency content of the sound. |
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Krill
Registered: Apr 2002
Posts: 2940 |
Quoting Martin PiperAccuracy would be improved by using an IRQ instead of polling bit 4 on $dc0d. Main thread running over a thousand NOPs, interrupt kicks in, does stuff, and jumps back to start of NOP desert.
Did any tape loaders use this to improve accuracy (= increase bit rate)? =) |
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Krill
Registered: Apr 2002
Posts: 2940 |
Quoting tlrIf you accept relying on precise pulse length detection, then you could employ a differential encoding instead, e.g 0, +1, -1, +2, -2... This way you could choose to have a higher sample rate for small changes, improving high frequency content of the sound. Hmm... wouldn't the larger diffs imply a steeper gradient on the time-domain wave, and thus should be encoded with the shorter pulses? |
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