The Commodordion Turns Two C64s Into A Single Instrument

A man playing an accordion-like instrument made from two Commodore 64s

One of the main reasons the Commodore 64 became an icon of the 1980s was its MOS 6581 “SID” sound chip that gave it audio capabilities well beyond those of other microcomputers of the 8-bit era. The SID became something of a legend by itself among chiptune enthusiasts, and several electronic instruments have been designed that generate their sound through a SID chip. Not many of those look anything like traditional musical instruments however, so we’re delighted to see [Linus Åkesson]’s new project: two Commodore 64s joined back-to-back using a bellows to form a wonderful new instrument called the Commodordion. It can be played in a similar way one plays a traditional accordion: melodies are played with the right hand, chords with the left, and volume is adjusted by varying the pressure in the bellows.

An accordion-like instrument made from two Commodore 64sThe two computers are basically unmodified, and boot Commodore BASIC like they normally would. A custom circuit board emulates a cassette player and provides the software to be loaded into memory. Both computers run the same program and can be switched between the right-hand and left-hand role by pressing a specific key combination. The software in question is called Qwertuoso, and basically maps notes and various features of the SID chip to keys on the Commodore’s keyboard.

Of course, it’s the bellows that makes this instrument a true member of the accordion family. Made from 5.25″ floppy disks and sticky tape, it forms a more-or-less air-tight system linking the two computers. The airflow in the bellows is measured through a microphone placed next to the air intake: the amount of noise generated is roughly proportional to the amount of air being expelled or inhaled. This information is then used to modulate the volume generated by the two SID chips.

By [Linus]’s own admission it’s not the most ergonomic of instruments, so we’re doubly impressed by the amount of skill he demonstrates while playing it in the video embedded below. It’s not the first time either that he has turned a Commodore 64 into a musical instrument: he previously built a church organ and a theremin. While the Commodordion may look complicated, it’s actually much simpler in construction than a mechanical accordion.

16 thoughts on “The Commodordion Turns Two C64s Into A Single Instrument

      1. I remember not so long time ago people were selling them for 10$ but didn’t know that C64 prices went up so much – maybe it’s time to sell mine? But still you can find them fairly cheap in Poland on Allegro (now I found one for 50$ – just the computer no power supply etc).

        1. A major driver in price is the MOS6581 / 8580 SID chip. They have been dropping like flies over the decades. Making confirmed as working replacement SID chips worth their weight in gold even if they are used. Seen older chips go for 50$ and some of the “youngest” 6581 with production codes indicating ’86 that are more likely to have seen less use go for over a hundred!

          It has gotten so silly you actually have an entire market of substitute “chips” like SwinSid, fpgaSID and ArmSID that have modern programmable devices mimicking the original’s functionality.

          So if considering to get into selling your C64 It might not hurt to open your C64 and take a peek on what it says on the chips first and when one is buying a “new” C64 to see whether or not they specify anything about the SID’s health.

  1. The last article sharing this build – I commented within about 5 minutes of this video going live on that article about how long until HaD featured this as an article. Just a little over 2 days – not bad!

    1. Cartridge port use was my first thought as well.
      It is a wonderful project, by a guy with astounding keyboard skills.
      The tape emulator loader should also be worth a HaD.

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