Play audio using a ZX81 and a ZXpand classic or plus.
The software plays a song as 1-bit audio on the tape output and 4-bit logarithmic audio on the soundchip on the ZXpand or an additional ZON-X.
All the files are played in order and loops. Pressing BREAK jumps to the next file.
WARNING: the sound is full of high frequency noise, DO NOT listen to this at high volumes or your hearing might be damaged! This is not a hifi reproduction, merely a curiosity.
There is also a version that can use a simple R2R-DAC connected on port B of the sound chip to play raw 8-bit files. This sounds MUCH better!
The playlist can be changed easily to accomodate your own audio files. The player code is only 2 files and builds using PokeMon's utility ZX-IDE available at http://sinclairzxworld.com/viewtopic.php?f=6&t=1064
The small changes needed are in the file zaj.asm, located between the horizontal lines.
Note about file-names: FAT16 file-name rules apply with max 8 characters/numbers before the three-letter file extension. The ZX81 also does not like many of the "modern" characters one can use in a file-name. A good way to test this is to name some files on an SD-card and do CAT on the ZX81 to see what it actually thinks about the file-names.
The required audio files can be created by following this process:
-
Convert the input file test.mp3 to a raw 15700 Hz audio file, 8-bit signed, mono, named audio03.raw:
sox test.mp3 -t raw -r 15700 -b 8 -e signed-integer -c 1 audio03.raw -
Run the raw file through the python conversion utility:
python3 audio_convert.py audio00.raw audio00.zaj
The converter prints the number of blocks the audio file uses, which is needed in the ZX81 playlist.
It's also possible to preview the 4-bit audio on your PC:
python3 audio_convert.py -p 4 audio00.raw test.raw
play -t raw -c 1 -r 15700 -b 8 -e signed-integer test.raw
Or the 1-bit audio:
python3 audio_convert.py -p 1 audio00.raw test.raw
play -t raw -c 1 -r 15700 -b 8 -e signed-integer test.raw
The play command is part of sox.
There are command line options for changing the dither and error diffusion in the converter, try them out for different/better results.
The required audio files can be created by following this process:
-
Convert the input file test.mp3 to a raw 22109 Hz audio file, 8-bit unsigned, mono, named audio03.raw:
sox test.mp3 -t raw -r 22109 -b 8 -e unsigned-integer -c 1 audio03.raw -
Calculate the number of blocks to play by taking the file-size divided by 256. The integer part without rounding is the number of blocks to use in the playlist.
The replay rate of 22109 Hz is less than 0.27% faster than 22050 Hz, so it is OK to use that if converting sound by some other means.
- Open an audio file in Audacity.
- Convert to mono:
- Tracks / Mix / Mix stereo down to mono.
- Resample track to 22050Hz:
- Tracks / Resample / select 22050, press OK.
- Change Project Rate (Hz) to 22050, usually in the bottom row, left selector box.
- Export to RAW:
- File / Export / Export Audio
- Just above the Save button, change to Other uncompressed files.
- In Header select RAW (Header-less).
- In Encoding select Unsigned 8-bit PCM.
- Name the file something simple like TRACK01.RAW.
- Press the Save button.
- Calculate the number of blocks to play, two alternatives:
- In a file browser, get the file-size in bytes, divide this by 256, cut off the decimals.
- In Audacity, bottom row, change to Start and End of Selection and below that select samples instead of time. Select / All will give you a large number in the info box below the Start and End of Selection. Take this number, divide by 256, cut off the decimals.
- Edit the zajraw.asm file to use the file-name and number of blocks from the transcoding above.
My trusty go-to emulator EightyOne v1.29 plays the files, but the sound quality is even worse than the real thing.
I recently released a streaming video player for the ZX81 which made me realize that there was no audio player. This is my proof of concept.
The 1-bit tape output needs to be synchronized with the automatic hsyncs on the ZX81 to sound good, which is why 15700 Hz is used. It also looks nice!
A PSG-only player could play files at more than 20 kHz.
The ZX80 does not have automatic hsyncs, so the sample rate could be raised on a 1-bit player as well.
The quality of the converted sound files could be improved.
All this is work for someone else. I have shown it is possible to stream audio on the ZX81, now make something (better) with it!
The code I created for this project is licensed under Creative Commons CC0 1.0 Universal.
The music was released by Kevin MacLeod, licensed under Creative Commons Attribute 3.0. See the file music.txt for details.
Mail: [email protected]
Github: https://github.com/NollKollTroll/zeddy-audio-jukebox