Streaming music to YM2149F

To follow-up on my previous blog post about driving a YM2149F chip with an Arduino, I wrote an Arduino firmware that can be used together with a PC side Python script to send music to the YM2149F. All the code is available in the ym2149-streamer repository on GitHub.

Arduino Firmware

The first piece of code is the firmware. This is the code that runs in the Arduino and drives the YM2149 chip. Most of the code is in the ym2149 library, which exports functions such as send_data(char addr, char data) to write in the YM2149 registers, while complying with the signals timings specified in the YM2149 datasheet.

Basically, the firmware continuously reads the Arduino serial line. Each time it has received 16 bytes, the firmware uses them to set the 16 YM2149 registers (To be more precise only the first 14 registers are used, the 2 last registers are useless to play music).

Note that this code is specific to the circuit, and depends on how the Arduino is wired to the YM2149. It has been written for (and tested with) the circuit described in the blog post about driving the YM2149 from an Arduino

To flash the Arduino with the firmware:

A couple of libraries need to be installed in order for the firmware to compile and be uploaded to the Arduino. To setup your environment, one can follow the instruction of the Arduino Hello World without IDE blog post.

Playing music from a computer

What remains to be done, is sending to the Arduino, the values of the registers to be set in the YM2149. Here we can build on the YM file format created by Arnaud Carré. The file format is quite straight forward. It contains a header providing all the required information about the tune, then a snapshot of the YM2149's 16 registers at regular intervals (usually 50 times per second). To play the tune, we have to send back the content of the YM2149 registers (sometimes referred as the samples) at the appropriate frequency.

I wrote a small Python script, available in the ym2149-streamer repository, that parses a YM file and sends it to the Arduino board. The syntax is:

$ python streamer.py <output_device> <ym_filepath>

Note that streamer.py may work with other circuits driving a YM2149. The only thing required is that the circuit be able to receive a stream of YM2149's registers snapshots and send it to the chip each time it receives a sample.

To illustrate this work, I published a video of the YM2149F playing Lemmings.

A large YM tunes archive allows playing old school musics on the YM2149 chip. Note that the files in the zip archives are compressed with lha (which can be installed through the Fedora lha package). Therefore, one has to uncompress the ym files before playing them with streamer.py. I may try to handle the decompression directly in the script some day.

Besides, another large archive of Atari ST music is around there, with a more dynamic community. Though the tunes are in the sndh format, which is harder to interpret and stream to a YM2149 chip. A nice thing would be to find a way to have these sndh files played on the sound chip!

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