This a real–time multimedia tape player for the unexpanded Commodore VIC-20. It plays pulse–width modulated audio from tape which you may stop, resume, rewind or fast-forward like a regular music cassette. Its output is roughly four bits at 4-6kHz sampling rate.
To make tapes audio recording equipment is required.
You'll need at least git, sbcl, mplayer and sox installed. Then download 'download-and-build.sh' and execute it.
The datasette isn't suited for analog signals. It can only detect if a signal from datassette goes low. Data is encoded by writing pulses of different lengths that are measured on playback. To record audio the samples are pulse–width modulated. A sample with the value 0 is converted to the shortest pulse, a sample of the maximum value of 15 is turned into the longest pulse. (The sample values are inverted on playback actually but that really doesn't matter.)
The recorded pulses look like this:
X cycles | 128 cycles max.
+----------------------+FEDCBA9876543210 <- sample values for each 8
| | variable | extra cycles
| Minimum pulse width | additional |
| for sample value 0. | width |
+----------------------+----------------+
+
| +
| +
| +
| +
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| +
|+
+ <- average sample value
+|
+ |
+ |
+ |
+ |
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+|
The minimum pulse width is required to a. make a pulse occur at all and b. to make the signal get through the datassette's high pass filter. That filter is the reason why the sampling rate is limited to 6kHz. Once could increase it to 7kHz or more but the resulting distortions are very audible and everything closing in to 8kHz is very likely to get filtered out.
One cannot assume that all tape drives have the same speed and we need some way to adjust the minimum pulse length somehow. That's done by constantly calculating the average pulse length to correct the reference timer in small steps. This is working out very well. On arthritic, wobbly datassettes the audio wobbles along in disturbing perfection.
Video resolution is 16x16 pixels with 16 grades of luminance. Audio and video samples are simply interleaved. An extra long pulse tells the player to reset the screen pointer and to start over with the following audio pulse.