mt32-pi/README.md

🎹🎶 mt32-pi

A work-in-progress baremetal Roland MT-32 emulator for the Raspberry Pi 3 or above, based on Munt and Circle. Turn your Raspberry Pi into a dedicated emulation of the famous multi-timbre sound module used by countless classic MS-DOS and Sharp X68000 games, that starts up in seconds!

✔️ Project status

• Tested on Raspberry Pi 4 Model B and Raspberry Pi 3 Model B & B+.
  • Pi 2 works, but only with concessions on playback quality.
  • Pi 0 and 1 are unfortunately too slow, even with an overclock.
• PWM headphone jack audio.
  • Quality is known to be poor (aliasing/distortion on quieter sounds).
  • It is not currently known whether this can be improved or not.
• I2S Hi-Fi DAC support.
  • This is the recommended audio output method for the best quality audio.
• USB or GPIO MIDI interface.
• Config file for selecting hardware options and fine tuning.
• LCD status screen support (for MT-32 SysEx messages and status information) is in progress.

✨ Quick-start guide

• Download the latest release from the Releases section.
• Extract contents to a blank FAT32-formatted SD card.
  • If you are updating an old version, you can just replace the kernel*.img files. The other boot files will not change often; but keep an eye on the changelog just in case.
• Add MT32_CONTROL.ROM and MT32_PCM.ROM to the root of the SD card - you have to provide these for copyright reasons.
• Connect a USB MIDI interface or GPIO MIDI circuit to the Pi, and connect some speakers to the headphone jack.
• Connect your vintage PC's MIDI OUT to the Pi's MIDI IN and (optionally) vice versa.

🎹 MIDI connection examples

[ Pi ] --> [ USB/GPIO MIDI ] <===> [ USB MIDI ] <-- [ Modern PC ]
[ Pi ] --> [ USB/GPIO MIDI ] <===> [ Gameport MIDI cable ] <-- [ Vintage PC ]
[ Pi ] --> [ USB/GPIO MIDI ] <===> [ Atari ST or other machine with built-in MIDI ]
[ Pi ] --> [ USB/GPIO MIDI ] <===> [ Synthesizer keyboard or controller ]

📝 Configuration file

mt32-pi tries to read a configuration file from the root of the SD card named mt32-pi.cfg. Please read the file for a description of all the available options.

Note: Don't confuse this file with config.txt or cmdline.txt - they are for configuring the Raspberry Pi itself, and you should not need to alter these when using mt32-pi.

🎹 USB MIDI interfaces

Any class-compliant USB MIDI interface should work fine - if the interface works on Windows or Linux PCs without requiring any drivers, there's a high chance it will work with mt32-pi.

Beware: cheap no-name interfaces are not recommended; they have reliability issues not unique to this project [1, 2].

Compatibility

If you're shopping for a USB MIDI interface, the following devices have been confirmed as working properly by our testers. Feel free to contribute test results with your own MIDI interfaces and we can list known working ones!

Manufacturer	Device	Comments
M-Audio	Uno	1 in, 1 out; male DIN plugs. Tested by @dwhinham.
M-Audio	MIDISport 1x1	1 in, 1 out; female DIN sockets. Tested by @nswaldman.
Roland	UM-ONE mk2	1 in, 1 out; male DIN plugs. Tested by @nswaldman.

🎹 GPIO MIDI interface

You can build a simple circuit based on an opto-isolator, a diode, and a few resistors. If mt32-pi does not detect any USB MIDI devices present on startup, it will expect to receive input on the UART RX pin (pin 10).

Tip: You can disable detection of USB MIDI interfaces by setting usb = off in the config file. This can shave off a couple of seconds of boot time as USB initialization is then skipped on startup.

Schematic

Breadboard example

🔊 I2S DAC support

The Raspberry Pi's headphone jack is a simple PWM device, and not designed for high-fidelity audio. This becomes very obvious when you use mt32-pi - distortion in the sound is apparent when quieter sounds are playing.

Luckily, a plethora of inexpensive DAC (digital-to-analog converter) hardware is available for the Raspberry Pi, giving it true hi-fi quality audio output. These often take the form of an easy-to-install "HAT" board that you place onto the Raspberry Pi's GPIO pins. They make use of the Raspberry Pi's I2S bus for interfacing.

Note: We do not support any kind of USB DAC audio output device, due to the lack of drivers in the Circle baremetal framework that we depend on. Adding USB audio support to Circle would be a huge undertaking, although if that changes in the future and Circle gains USB audio support, we could certainly make use of it.

Setup

• mt32-pi defaults to PWM (headphone) output. Edit mt32-pi.cfg and change output_device to i2s to enable the I2S DAC driver.
• If your DAC requires software configuration, you may need to edit the i2c_dac_address and i2c_dac_init options to suit your particular DAC. Continue reading for further details.

Compatibility

Currently, we have been targeting DACs based on the Texas Instruments PCM5xxx series of chips due to their popularity, but other DACs could be supported quite easily.

Some more advanced DACs are configured by software (usually a Linux driver), whereas others need no configuration as they are preconfigured in hardware. This will vary between manufacturers, and so some editing of mt32-pi.cfg may be required.

Note: If a DAC requires software configuration, they will not produce any sound until they have been properly initialized. This initialization is done by sending it a special sequence of commands over the I2C (not I2S) bus. For the PCM5xxx family, you can set i2c_dac_init = pcm51xx to enable this.

Feel free to open an issue if you'd like to help us support your DAC, or even just to report success or failure so that we can build a list of supported DACs.

The following models of DAC have been confirmed as working by our testers. Please note the necessary configuration file options.

Manufacturer	Device	DAC chip	Config file options	Comments
Generic	GY-PCM5102	PCM5102A	None required	Stereo 3.5mm output. Found very cheaply on AliExpress and other sites. Tested by @dwhinham.
Generic	Pi-Fi DAC+ v2.0	PCM5122	i2c_dac_init = pcm51xx, i2c_dac_address = 4d	Stereo RCA and 3.5mm output. Tested by @rc55.
IQaudIO	Pi-DAC Pro	PCM5242	i2c_dac_init = pcm51xx, i2c_dac_address = 4c	Stereo RCA and 3.5mm output. Tested by @dwhinham.

Finding the I2C address of your DAC

The i2c_dac_address configuration file option determines what address on the I2C bus that mt32-pi will send initialization commands to, if i2c_dac_init is not set to none.

If your DAC does not appear in the compatibility table above, you can help by carrying out the following:

• Connect the DAC to your Raspberry Pi.
• Insert an SD card containing the latest version of Raspberry Pi OS (aka. Raspbian) and boot the Pi.
• Run the command sudo raspi-config.
• Select "Interfacing Options", followed by "I2C" and "Yes" to enable the I2C kernel modules.
• Exit raspi-config, and run the command sudo apt-get install i2c-tools to install some I2C utilities.
• Run the command i2cdetect -y 1. The output should be like the following:

       0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
  00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
  10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
  20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
  30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
  40: -- -- -- -- -- -- -- -- -- -- -- -- -- UU -- -- 
  50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
  60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
  70: -- -- -- -- -- -- -- --          
  

• In this example, the address is 4d. Make a note of this and set i2c_dac_address in mt32-pi.cfg.
• If your DAC now works, open an issue to let us know, and we can add it to the table! Otherwise, open an issue anyway, and we can try to work out how to support it.

💬 Custom System Exclusive messages

mt32-pi responds to MT-32 SysEx messages as you would expect, but it can also respond to its own commands. mt32-pi listens for manufacturer ID 0x7D, or the "non-commercial/educational use" special ID. Therefore, a complete mt32-pi SysEx message looks like:

F0 7D { command } F7

Currently there is only one implemented command, although more will probably be added in the future. Please note that these commands are subject to change until the project reaches a mature state.

Command	Description
00	Reboot the Raspberry Pi.

❓ FAQ

• Q: What happened to the old mt32-pi project that was based on a minimal Linux distro built with Buildroot?
  A: That's been archived in the old-buildroot branch.

⚖️ Disclaimer

This project, just like Munt, has no affiliation with Roland Corporation. Use of "Roland" or other registered trademarks is purely for informational purposes only, and implies no endorsement by or affiliation with their respective owners.

🙌 Acknowledgments

• Many thanks go out to @rc55 and @nswaldman for their encouragement and testing! ❤️
• The Munt team for their incredible work reverse-engineering the Roland MT-32 and producing an excellent emulation and well-structured project.
• The Circle and circle-stdlib projects for providing the best C++ baremetal framework for the Raspberry Pi.
• The inih project for a nice, lightweight config file parser.