Introduction: Building a Retro Synthesizer With the Pocket Mini Computer

Got a MIDI keyboard with a MIDI-OUT jack?  

This Instructable will show you how to create a "retro sounding" synthesizer using your Pocket Mini Computer.

I'm a huge fan of the Commodore 64.  One of it's most memorable characteristics is the sound of it's 3 voice SID chip.   Most Xgeners will recognize it's unique sound from old school videos games like Commando or MULE.   The Pocket Mini Computer has access to an excellent emulation of the SID chip, called SIDcog, used extensively in this project.    

Ready to rock out with a SID Synth?   Read on!

  • The Pocket Mini Computer (see next page for details)
  • Breadboard or "perf" board.
  • 2 - 220ohm resistors
  • 1 - 4.7k resistors
  • 1 - 6N138 Optocoupler
  • 1 - 1N914 Signal Diode

I've included schematics and links to all sources so you can build and expand on this project to your hearts content.

Step 1: What Is the Pocket Mini Computer?

You might be scratching your head at the requirement of a Pocket Mini Computer.

What's a Pocket Mini Computer?    I'm glad you asked!

The Pocket Mini Computer is an open source "mini computer" design which uses the Parallax Propeller microcontroller as it's brain.   The kit from comes pre-loaded with a retro-style BASIC.  The hardware itself is extremely powerful & featured. 

The Pocket Mini Computer hardware specifications:
  • Parallax Propeller Quickstart Board (8 cogs, 32K RAM, 64K EEPROM)
  • Improved VGA Output
  • Stereo R/L Audio Output
  • PS/2 Keyboard/Mouse Input
  • Wii(tm) compatible controller connection
  • Optional microSD adapter connector
  • Optional iR connector
  • Experimenter's port

The Pocket Mini Computer is language compatible with Spin, Assembly (PASM), C, and BASIC.

COLOR BASIC features:
  • 4094 available bytes free
  • 64 colors
  • 1 "SIDlike" audio channel
  • 127 character re-programmable character set
  • plot, line, and box graphics commands
  • I/O access commands
  • 95+ BASIC Commands, operators, and functions
  • SD operating system
  • Full screen editor (requires a microSD card.)
  • Ability to execute non-basic binary programs
  • Extended memory features (requires a 23K256 SRAM chip)

More details about the Pocket Mini Computer can be found on this Wiki.
Schematics and Souce Code are avaible for anyone who wants to "roll their own".

Step 2: MIDI-IN Schematic

The MIDI-IN circuit is simple and surprising easy to assemble.  

The MIDI communications protocol is a type of slower, serial communication easily understood by the microcontroller. (The full "geeky" details of MIDI communication can be found here:  It's a recommended read.)

  • 2 - 220ohm resistors (source)
  • 1 - 4.7k resistors (source)
  • 1 - 6N138 Optocoupler (source)
  • 1 - 1N914 Signal Diode (source)
  • 1 - Midi 5pin DIN Jack (source)
I've attached a simple to follow breadboard as well as the schematic.

Step 3: Loading SIDSynth

Once you've attached the MIDI circuit to P12 of your Pocket Mini Computer, you are ready to play.

Download a copy of the SIDSYNTH.BIN binary and copy it to your microSD card.

Use the BASIC command:


You will hear a couple clicks from the speaker as the program initializes.   The screen will clear and you are ready to jam.

Waveform, Attack, Decay, and Sustain are controllable using the first four knobs or controls on your keyboard.   (varies from keyboard to keyboard)

You can also switch instruments to various great presets just as you would with a typical electric piano.

Step 4: About the SIDSynth Software

The magic behind this Instructable lies in two MIT licensed objects used by the Propeller chip:

SIDcog written by Johannes Ahlebrand
SIDcog is an emulation of the Commodore 64 SID chip for the Parallax Propeller. Details about this program can be found in this forum thread.  Complete source code is provided as well.

Polyphonic SIDSynth written by Alessandro De Luca.
Polyphonic SIDSynth takes advantage of the SIDcog software and StereoSpatializer written by Chip Gracey.  Details about this program can be found in this forum thread.   Complete source code is provided as well.

Without these two objects, this project simply wouldn't be possible.  

Thank you to Johannes and Alessandro!

Compiling the Polyphonic SIDSynth on your own requires the use of Brad's Spin Tool, as it uses some features not supported in the Parallax Spin Tool.   There are several options for various configurations including the use of a USB based midi keyboard, and SPDIG Audio output.

Step 5: Extra: SIDPLAYER

Something EXTRA!

If you watched the video in the first step of this Instructable, you saw me show off the capabilities of SIDcog with a SIDplayer program.

The SIDPLAYR.BIN file for the Pocket Mini Computer is available for download (source).  

I've also placed my entire SID "DMP" tune collection online, so grab a copy of Commando, Axelf, and MULE and enjoy the ride!

Step 6: Help and Links

There's a friendly community of folks who are using the Pocket Mini Computer.

Need some help with your SIDSynth project or just want to show yours off?   Come join us!


Getting started with the  Pocket Mini Computer (Instructable)

The Pocket Mini Computer is available as a kit from Propellerpowered:

Forum support and discussion for the Pocket Mini Computer:

DIY Audio

Finalist in the
DIY Audio