Cynthcart is a cartridge I developed for the Commodore 64 computer that turns it
into a standalone analog synthesizer with MIDI support.
The Commodore 64's SID is one
of the most interesting 8-bit sound chips of all time, and I was unsatisfied with the available SID synthesizer software.
Cynthcart supports the original Commodore 64 piano keyboard overlay (shown below),
and MIDI interfaces by Datel, Passport, Sequential, and Kerberos.
Cynthcart's key-command user interface allows
it to be played and controlled without a monitor, making it especially suitable for live
● 30 preset sound patches
● Arpeggiator, portamento, vibrato, and tremolo effects
● Analog filter control and effects
● Control of attack, release, and other sound parameters
● MIDI support including pitch bend, patch changes, and controllers
● On-screen help
● Color visualizer display
● Mono stack, 3-voice mode, and 6-voice mode (with second SID)
● No latency
● SID hex editor for advanced users
● Ability to turn off video chip to reduce noise
● PAL and NTSC pitch tables with automatic selection
● Copies itself to RAM (cartridge can be removed after booting)
● Commodore 64 piano keyboard overlay
● Datel, Passport, Sequential, and Kerberos MIDI interface cartridges
● Paddle controllers in port 2 for filter, pulse width, vibrato, and pitch
● 2nd SID chip for stereo and 6-voice sound
● Works with Autodafe's Cynthcart MIDI editor VST
cynthcart2.0.prg for most configurations running on a real C64
(MIDI autodetect, 2nd SID at $DF00)
[Files and source are for personal use only, please do not sell cartridges online]
SID hex editor screenshot
Cynthcart running with a Datel Midi adapter and SIDcart
A multi-cartridge adapter allows the SIDcart, MIDI adapter, and Cynthcart to all be used at the same time
Here's Cynthcart installed as a replacement for the C64 BASIC ROM (this is no longer supported)
Where do I find the Commodore 64 piano keyboard overlay?
The overlay isn't required, but they show up on Ebay reasonably often.
It's often packaged as "The Incredible Musical Keyboard"
so try searching for something like "commodore musical keyboard".
Also they sometimes can be found at thift stores and online retro-gaming vendors.
How do I get the audio signal from the C64 to my amp/mixer?
You can buy an A/V cable (on ebay all the time).
It has a DIN connector on one end that goes into the C64, and RCA connectors on the other.
It's also possible to make one out of a MIDI cable.
Cut the cable in half and figure out which two wires are ground and audio and solder on a 1/4" plug.
Commodore 64 video cables typically have separate RCA connectors for chroma and luma video signals,
so if you don't have separate chroma and luma video inputs
on your monitor, then you will need a "Y" adapter to join the chroma and luma into a composite video signal.
What do I need to use the Cynthcart?
The minimum requirements are a Commodore 64 or 128, a modern power supply, and an A/V cable. Optional
accessories are the piano keyboard overlay, paddle (rotary) controllers, a MIDI adapter, and
a stereo SID cartridge or board.
Original Commodore 64 power supplies should not be used anymore. Instead, use a modern power supply replacement like a Nubrick
or Ray Carlsen power supply.
What's the difference between the Atari paddles and the Commodore paddles?
Both will work, but I prefer the Commodore paddles because the Atari controllers have a smaller range of motion
and are very sensitive. Atari paddles use 1 mega ohm potentiometers while Commodore paddles use 470K ohm.
Which model of Commodore 64 sounds best?
I have a slight preference for the 64C model (8580 SID) compared to the original model (6581 SID), but I use both.
How do I add another SID for stereo?
A secondary SID sound chip can be added either by installing an internal board like the
SID2SID or by using a cartridge
like the SID Symphony or SIDCart which is available through Shareware Plus.
There is no standard for where to put the second SID in the Commodore 64's memory, so the various solutions
for installing a secondary SID chip are inconsistent about this. Version 1.2.4 and earlier versions
look for a second SID at address $DE00 which is compatible with the SID Symphony cartridge
and the SIDCart. Version 1.5 and later has the second SID address changed to $DF00 by default to avoid conflicting
with midi adapters.
Note that the Kerberos cartridge is not compatible with SID cartridges because it has MIDI at $DE00
and a memory bank at $DF00. To use a second SID with Kerberos, the secondary SID must be installed
internally and set to address $D420 using something like a SID2SID board.
More info about installing secondary SID chips here.
changes in version 2.0.2 (released 2018)
- reorganized source and project files and put project on Github
changes in version 2.0.1
- disabled Restore key, which causes a lock up on a few 64C models
- reset paddle2 pitch value when pitch control is turned off
changes in version 2.0 (released 2016)
- now supports and autodetects Passport, Datel, Sequential, and Kerberos MIDI adapters
(note: autodetect is incompatible with VICE)
- 8 new preset sounds
- added mono stack portamento modes
- new instruction manual
- moved secondary SID to $DF00 to work with SID cartridges again
- presets can have different waveforms and sustain/release for each voice
- added key to cancel out of the SID editor
- various UI improvements
- various bugfixes
changes in version 1.5.1
- fixed clock and sysex bytes causing crashes/stuck notes (0xF0-0xFF)
- fixed bad pitch bend startup value
- added non-omni midi modes for channel 1 and 5
changes in version 1.5.0 (major update for Kerberos in 2014)
- added MIDI support for Kerberos cartridge/Datel MIDI interface
- mono stack mode and 6-voice mode
- new filter and pulse width effects modes
- additional presets
- improved clarity of help screen text
- moved SID location for MIDI version to $D420 to avoid midi and Kerberos interfaces
- smarter note buffering system
- reworked patch parameter display
- made LFO and all pitch modulations use proper tuning/scaling
- various bugfixes
changes in version 1.2.4 (released 2008)
- PAL and NTSC note tables and autodetection
- full-screen video mode
- second paddle controls pulse width, LFO depth, or pitch
- automatic activation for filter paddle
- improved keyboard layout
- onscreen help
- SID editor help and status display
- dummy SID register to edit 3 oscillators at once
- save an editor patch in memory
- fixed patch/sound change glitch when playing
- fixed bad notes in top octave
changes in version 1.1 (released 2006)
- added SID HEX editor
- changed tuning keys to avoid accidentally changing the tuning
- added ability to turn the SID filter on and off