MidiJoy - Using your Atari 800, C64 or VCS 2600 as a Chiptune-instrument



Both the MidiJoy software as well as the interface are a hobby of mine. While great care has been taken to ensure that this project allows you to enjoy making music with your home computer or console, there is no guarantee that the software or the hardware design is free of flaws and errors. Therefore, using the software and/or the device is at your own risk and I shall not be liable to any damage that might happen in conjunction with the software and/or the device.

Having said that, I now hope you'll enjoy MidiJoy as much as I do :-)!

What is MidiJoy?

MidiJoy is a software/interface combination that allows you to use your Atari or Commodore homecomputer as a musical instrument. The idea is based on the Atari 2600-PC-Interface created by Sebastian Tomczak and was expanded to suit the extended capabilities of the 8-Bit homecomputers.
The interface part emulates a USB-Midi (serial Midi is optional) device that can be accessed by any kind of instrument as well as sequencer software on a PC or Mac that can output Midi data (e.g. Ableton Live or Aria Maestosa). The MidiJoy software receives these data from the interface via the joystick ports and plays them on the POKEY (or SID or TIA) sound-chip. In contrast to most SIO-based Midi interfaces, a MidiJoy-driven Atari can bei used as a live instrument in real time with up to four sound channels simultaneously. At the same time, all POKEY parameters (AUDCTL, AUDC1-4) can be changed on-the-fly fly as well as activation of ADSR envelopes. Music input can be recorded and saved to disk for later usage - even in your own programs/games.
For limitations of the Commodore and VCS 2600 version please see the info at the bottom of this page.

The source code for the Teensy microcontroller (an Arduino offspring) on the interface is available as open source as the basic idea is based on Sebastian Tomczak's interface. The adapted code is very simple and just converts incoming Midi data into bit combinations that are sent to the Atari or C64 via its joystick ports. The MidiJoy software on the Atari/C64 end then plays the incoming notes live.
On the Atari, MidiJoy makes full use of the capabilities of the POKEY sound chip and thus partly extends the features of the Atari 2600-interface: Instead of just two voices with a 32-pitch range of the TIA, MidiJoy enables you to make full use of four voices spanning four octaves. Two 16-bit channels are also possible, and with corresponding POKEY frequencies, a much larger range of sounds can be created. The playback of samples - such as with the original interface - is on the development roadmap of MidiJoy.

What is MidiJoy NOT?

The device has some limitations due to the fact that I want it to run on several 8-Bit systems and thus have to make some compromises:

Getting and supporting MidiJoy

The MidiJoy software is available free of charge. Just send me e-mail with your system (Atari or C64) to me ("midijoy (ät) phobotron dot de") in order to receive your personal copy in ATR/PRG format. It will be sent to you via e-mail in and you need to be able to transfer it to the computer you want to use it with, for example with a SD2IEC (C64) or SIO2SD (Atari) interface.
Details for assembling the hardware interface can be found below. Currently, I still have a few prototypes left from developing which I would give away for material costs. Contact me if you are interested.

If you like MidiJoy and would like to support my work, how about buying me a drink? Or a meal? Or a drink and a meal :)?
Doing so is easy via PayPal, just click on the link below and send 4,99 € if you want to buy me a drink or 9,99 € if you want to buy me a meal. Those who treat me both will receive a personalized copy of MidiJoy with a name or text of their choice shown in MidiJoy's user interface :).

Atari Demo / Examples
(for a C64 demo video please scroll to the bottom of this page)

This disk image contains three Atari recordings (FSUITE.EXE, FUGUE.EXE and ESTHER.EXE) which demonstrate the playback capabilities in the background using VBI. In addition, there is ESTHENV.EXE which is the same as ESTHER.EXE but using custom ADSR envelopes (see below).
For those who do not have a real Atari or an emulator at hand there are recordings available in MP3 format (FSuite.mp3, Fugue.mp3, Esther.mp3 and EsthEnv.mp3).
Imagine creating complex tracks like these on your Atari within just a few minutes and no sound programming experience!

These two videos demonstrate the use of MidiJoy on an Atari 800XL (the first one was recorded at an early stage of development with not all currently implemented features visible):

Setting up the interface - non soldering

CAUTION! Joystick ports carry a relatively small but potentially damaging 5V current on one of the pins. Connecting the pins in a wrong way may potentially damage your home computer, the MidiJoy device and/or any other connected equipment. It is also generally safest to cold start the computer before attaching the device.

Due to the more powerful soundchips in the Atari (POKEY) and Commodore (SID) compared to the 2600's TIA, the setup of the orignal interface - for which all credits go to Sebastian Tomczak - as well as the Teensy code need to be adjusted in a few ways in order to use them on these home computers. The following images show the design and wiring used by MidiJoy:

(Click images to enlarge in a new window)

This setup also has the advantage that the interface can be assembled without any soldering or using old joystick cables.
To do so, one needs:

You might also use DB-9 ports which require the cables to be soldered (as in the video above). These are usually cheaper than the plug-in ones for breadboards and might offer more flexibility (at the cost of less stability) when plugging the connectors into you Atari or Commodore.

When all parts are obtained, simply plug the Teensy board as well as the DB-9 breakout boards into the breadboard. If you use a full-size breadboard, make sure that the DB-9 ports are in the right distance so they fit into the computer's joystick ports. In that case you would also have to remove the shoulder screws for the ports to fit in.
Once this is done you only need to connect the pins of the Teensy board to the DB-9-connectors. The images above give you a good indication, but make sure you double-check the exact pins as the perspective in the images can be misleading. You can find the mapping in the description at the beginning of the source code.

Pins 7 and 8 (D2 and D3) on the Teensy board are optional and can be used to connect a "classic" serial Midi board, the schematics can be found here. When installed (see third image above) you could use both USB- as well as serial Midi devices at the same time.

Setting up the interface - Printed Circuit Board (PCB)

The interface can also be set up as a standalone circuit. This has the advantage that the design already includes the necessary wirings for a "classical" serial MIDI input connector. Also, if you remove the metal screws from the joystick port connectors on the device, the board fits directly into the original C64 as well as the C64C - the Atari connectors are unfortunately too far apart for such a small design.

CAUTION! Joystick ports carry a relatively small but potentially damaging 5V current on one of the pins. Connecting the pins in a wrong way may potentially damage your home computer, the MidiJoy device and/or any other connected equipment. It is also generally safest to cold start the computer before attaching the device.

You can find the Gerber files for manufacturing the printed circuit board (PCB) here - they are for private, non-commercial use only. The board layout contains descriptions where each part should be placed.
Currently, I still have a few prototypes left from developing which I would give away for material costs. Contact me if you are interested.

For populating the PCB, you need the following items if you just want to use USB-Midi:

If you want to add serial Midi functionality as well, you need to add the following: For German customers, a list of all necessary parts can be found here at reichelt.de.

CAUTION! On the PCB you can find a location for a Schottky diode (1N 5817) right below the first joystick port connector. This is for powering the device from the current that the joystick port supplies.
DO NOT use this diode if you connect the device via USB as this could seriously damage your PC. This diode is only useful if you use serial ("classic") Midi input only, for example if you want to connect a Midi-keyboard directly to the Atari/C64.
If you want to use both USB- and serial Midi, you have to either prepare the Teensy or the USB-cable you use to connect the Teensy. Instructions how to do this can be found here.

Flashing the Teensy microcontroller

After the interface is fully assembled, perform the following steps:

Once this is done, the interface should be detected by the Midi sequencer software on the Mac/PC. For simply playing via your keyboard or Midi playback, Aria Maestosa is a great free program for Windows, Mac and Linux. For more complex tasks, Ableton Live is a powerful alternative. See here for a sample configuration of Ableton Live.

Connecting the interface

Depending on the way you assembled the interface and what homecomputer you are using it with, there are several ways to connect each component:

Using MidiJoy on the Atari

When you have connected the interface to both the Mac/PC and the Atari has booted into MidiJoy, you are ready to play music on your Atari by sending Midi notes from the Mac/PC to the interface, a simple equalizer will give you visible feedback as well.
All incoming data will be adjusted correctly to the Atari note table and then played immediately. As far as possible, proper scales are used also in distortion modes 2 and 12. Pressing the SPACE key switches between two different scales in mode 12.

Incoming Midi data is expected on Midi channel 1 to 4. These are then played through the corresponding sound channels on the Atari. Midi channel 1 is special: here, up to four voices are being split up automatically to unused sound channels on the Atari. This enables playing multi-chords with up to four voices in real time, for example via a Midi keyboard connected to the Mac/PC. Five or more voices will overwrite the last used channel.

You may use your own ADSR envelopes (see details below) during live playback by activating one of them by pressing keys 1 to 8. Envelope data needs to be loaded prior to starting MidiJoy (e.g. via DOS).

Keys O to I control the eight bits of the AUDCTL register. This enables making live use of all POKEY features such as filters, 16-bit voices or frequency changes. Like with sound programming in general, only certain combinations make sense. For example, it is advisable to set POKEY's frequency to 1.77 MHz (bits 5 and 6) when using 16-bit voices (bits 3 and 4 respectively).

Individual channels can use different distortions (AUDC1-4). These can be set using keys A to D, F to H, Z to C and V to N. The status of each bit is displayed on the screen.

The tabulator key starts and stops the recording mode. Pressing RETURN resets all recordings, for example to start anew.
While recording, all incoming note data is saved to RAM and can then be played back using the P key (after recording is stopped). A counter displays the memory area used by the recording. This area can afterwards be saved for later use by jumping into DOS (J key) and use the "Save Binary File" function there. This of course requires that MidiJoy has been started with DOS in the background - simply booting the executable from a game DOS or SD-card will not allow you to save your recordings!

Take note that when you enter recording more, also the silence until the first note value comes in is recorded. So if you press TAB and only then start to boot your Mac/PC, you'll have a long(er) silence at the beginning when you later start to play the recording.

ADSR/Distortion Envelopes

MidiJoy supports ADSR (Attack/Decay/Sustain/Release) and distortion envelopes both for live performance as well as for playback. In order to allow for maximum flexibility, envelope data can be loaded on a case-by-case basis and can also be exchanged with other users of MidiJoy.

An envelope data block consists of 256 bytes ($00 to $FF) which can be used in variying proportions for the ADS- as well as the Release-phases of each voice and also modify the distortion. Each byte modifies the volume for 1/50 second (1/60 on NTSC machines) and consists of two components: Bits 0 to 3 modify the volume for that time-frame and Bits 5-7 set the distortion. Any distiortion other than zero will override the general distortion set for this channel as long as the envelope is active. The other way round this means that if the distortion bits 5 to 7 are not set (only volume bits 0 to 3 are used), the standard distortion for this channel is used.

ADSR notation differs between ADS and Release phase: ADS values range from 0 to 15 and reduce the note's volume, i.e. a value of zero leaves the volume unchanged whereas a value of 15 will always silence it. In the Release phase it is the other way round: values here will be added to zero (although not exceeding the last volume value), i.e. a value of zero is silence and anything above will result in the voice still being played / faded out.

An envelope file (see here for a sample structure) has to start at $4F00 and begins with four index/offset bytes pointing to the start of the ADSR/distortion-data for each voice. These four bytes are followed by a fifth byte indicating the position after the end of the fourth voice's Release-phase.
Actual envelope-data begins with a status byte containing the length of the subsequent ADS-phase, followed by the ADS-envelope data. The last byte here is special: it indicates whether the sustain phase should be repeated from an earlier position onwards which is useful if you want to create a vibrato and keep it going indefinitely as long as the key is being pressed.
To do that, this byte contains the number of bytes the envelope counter should 'rewind'. A zero means the volume will remain at its last level, otherwise the counter will be set back by the number specified here. Take note to count this 'rewind' byte into the status byte as well.
Following the 'rewind' byte is the Release-phase data up to the beginning of the next voice's ADSR-data.

In order to use ADSR/distortion envelopes you simply need to load a file containing the envelope data in the prescribed format to address $4F00 prior to launching MidiJoy or one of the playback programs. The separation between note data and envelope data allows you to record music and add or change the ADSR envelope at a later stage, giving you all the flexibility you need.
If you want to use multiple envelopes during live performances, there are eight slots available from $3400...$34FF up to $4B00...$4BFF where envelope data can be stored and selected during playback with keys 1 to 8.

The MidiJoy disk contains DEMO.MJE that demonstrates the use of four different envelopes and the demo disk above contains ESTHENV.EXE which is the same track as ESTHER.EXE but with four different ADSR envelopes added.

Commodore users do not need this functionality as the SID chip can be set up for ADSR envelopes with just a few pokes prior to launching MidiJoy.

Recording format

The recording format is kept very simple and consists of five bytes per incoming Midi data:

Tracks with many simultaneous notes which are played very quickly therefore create a much larger memory footprint than fewer, longer notes. If no note (or pause) is longer than five seconds, then the high-byte can be omitted when parsing the recorded file. Similarly, when distortion is always the same, the sound channel and the volume could be written into four bits each. Both changes would require changes in the playback routine, but would also almost reduce the amount of data by half.

ADSR envelope data is not part of the recording, but ADSR envelope files can be loadad prior to playback.


Two small playback routines in assembler will be available for free, one for playback using VBI (as in the demo disk above) and one without using interrupts. Both can be accessed from BASIC as well. They are less than 256 bytes in size and thus fit nicely into page six for example.
You can play the songs from the demo disk in BASIC as well, simply boot into DOS with BASIC enabled, load the .EXE file and jump back into BASIC. In BASIC you can start playback with

(in your own songs, the second and third parameter must match the note data RAM area as displayed in MidiJoy). To stop/pause simply enter
POKE 207,0:FOR X=0 TO 3:SOUND X,0,0,0:NEXT X
and continue playing with
POKE 207,1

A third playback routine has ADSR envelope playback enabled. As it is too large for storing it in Page 6, it is generally advisable to use it in machine language programs only.

Using MidiJoy on a C64

Due to the similar design of the joystick ports in the Atari and the C64, the interface can be used without any modifications in hard- or firmware on the arch rival as well (and probably on any other computer that supports Atari-style joysticks). If you use the PCB version, the device even fits directly into the C64 if you remove the metal screws from the DB9-connectors!
Having never written any program for the Commodore before, my small proof-of-concept conversion only covers the product's main feature: live playback. And this works quite well, as the following demo video shows (first version shown here, for current user interface see newer demo video on the right):

On the Commodore, incoming Midi data is expected on Midi channels 1 to 3. These are then played through the corresponding sound channels on the C64. Midi channel 1 again is special: here, up to three voices are being split up automatically to unused sound channels on the Commodore. This enables playing multi-chords in real time, as with the Atari, albeit only up to three voices. Four or more voices will overwrite the last used channel.

Despite the limited user interface of the program itself, one can of course initialize the SID sound chip prior to launching MidiJoy (by entering or loading a program that POKEs the desired values into the corresponding registers). This makes it still possible to use almost all the features SID has to offer - one just cannot change them while MidiJoy is running. If you make use of that approach, make sure you choose "No" when asked at startup whether SID's registers should be cleared.
Since the wave form is reset within MidiJoy, use addresses 251 to 253 for POKEing the desired waveform to be used during playback. Otherwise (or if MidiJoy is told to clear the SID registers prior to start) a default value of 17 (triangle) is set for all voices.
You may press "Q" to quit the program and restart it with "RUN", for example if you want to POKE different SID values for different playback.

Using MidiJoy on the Atari VCS 2600

Using the same firmware above, the VCS can be used to play its internal sounds and distortions. The source code can be found here as well as a pre-compiled binary for use with a Harmony cartridge. However, this approach is limited to the (heavily reduced) scale of the VCS.
Due to the irregular tonal system of the VCS which does not correspond to our common chromatic scales, MidiJoy for the VCS consists of only one octave that is mapped to all the 10+ octaves of the Midi standard. Even within this one octave there are notes which are "off" and this cannot be prevented.
However, using the VCS with MidiJoy might still be interesting for musicians who would like to make use of the various sound effects / distortions built into the VCS. Just change the distortion value in the source code to something else and enable the alternative note table, compile and you are good to go.

For playing back pre-recorded digi-samples on the VCS, see another option below.

Using MidiJoy on other systems / sample code

So far, the MidiJoy software is available for the Atari and C64 homecomputer families. However, due to the design of the interface, it is relatively easy to adapt the software for other systems as well - as long as they have two standard 9-pin joystick connectors. So how about using a VIC-20, an Atari ST or Amiga in your setup?

Providing basic functionality for use with the interface is relatively easy and consists of just a few steps:

  1. Initialize the sound chip for playback
  2. Read trigger buttons from joystick ports 1 and 2 (port numbers refer to connectors on the MidiJoy interface and may not correspond with the port numbers on the computer)
    • Button values contain the sound channel to be used for the subsequent command
    • Button 1 is bit 1 of the channel value, button 2 is bit 0, i.e. 4 sound channels can be addressed.
  3. Read direction pins from joystick ports 1 and 2
    • If pin 1 ('Right') is active on joystick port 2 of the MidiJoy interface, the other direction pins ('Right', 'Left', 'Down', 'Up' each, i.e. pins 2 to 4 on port 2 as well as pins 1 to 4 on port 1) are volume data, otherwise (if pin 1 is not active), the other direction pins contain pitch data.
    • On the Atari, for example, you would get these data bits in the right order by reading the PORTA register.
    • On the C64, you would read $dc00 first, move the lower four bits to the upper four bits and then add $dc01's lower four bits.
  4. Proceed depending on whether the data just read was pitch or volume data:
    • In case of volume data: Set the sound channel's volume based on the data just read (sound data will only be four bits, i.e. 0 to 15).
    • In case of pitch data: Values range from 0 to 127 and correspond to Midi note data. Therefore, a table lookup needs to be made to read the corresponding value(s) the soundchip requires in order to play that note.
  5. Repeat from step 2.

The source code for a minimal example for the Atari 8-Bit can be found here.
It should be easy to adapt to other systems, even non-6502 compatible processors. If feasible, sound chip specific functions as well as other functions (such as saving data etc.) can then be added, too.
Due to the fast transmission of data through the interface, such a program would have to be written in assemlby language in order to provide satisfying results. Should you have written a port for a different system, I'd be happy to link to your website or host the file here on mine.

Playing digi-samples

In order to use the MidiJoy interface to play of digi-samples on your old computers/consoles via Midi instruments or sequencers, you can make use of the firmware for the Teensy developed by Sebastian Tomczak. His firmware can be used to play pre-recorded samples on the Atari VCS 2600, the Atari XL/XE as well as the C64 in the typical 4-bit sample sound. More information can be found on his project website.
Here's a little demo showing playback on an Atari 800 XL (works likewise on a C64 or VCS 2600) using the simple playback software that can be downloaded in source as well as binary below. Take note of the change of playback speed of the last sample, all these parameters can be controlled directly from your Midi keyboard or sequencer:

To make his firmware work with the MidiJoy device, first download his firmware. In the firmware code you need to just change the following four lines (which also make connecting the interface to the VCS more secure as no voltage runs from the interface to the VCS):

Then, download one of the following versions for your device and copy/install it on your homecomputer/console:

Details how to play and modify the samples from your Midi device or sequencer can be found on Sebastian's website.
Do not forget to re-flash the Teensy with the MidiJoy firmware above once you want to use it with MidiJoy again!

Troubleshooting / Frequently Asked Questions

  • I hear some notes playing, but they are out of tone or just varying in volume
  • Sometimes the MidiJoy software keeps playing the last note continuously although I stopped playback.
  • My device is no longer accessible / recognized as a Teensy Midi device
  • There seems to be a lag between pressing a note and actual playback on the computer?

    Questions? Comments? Suggestions?

    There are two forum threads where I look forward to questions, comments and suggestions - one thread in English on AtariAge and one in German in the ABBUC forum. Feel free to get in touch!

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