Crumar GDS MIDI interface

Published on Jul 24, 2015 Michael Kukat

"The Crumar General Development System is a very rare late 1970s digital synthesizer based on the Bell Labs Digital Synthesizer, also known as the Alles Machine.

It might better be known by it's little brother, the Digital Keyboards Synergy. I'm fascinated by this machine since 7 or 8 years meanwhile, since i read about it in Mark Vail's Vintage Synthesizers book. One day, i want to have it replicated on a firmware basis running on a suitable micro.

A bit more than a year ago, i sort of accidentally bought this keyboard from eBay, missing the point that it might be hard to impossible to get the main unit for it. As the offer appeared several times from the seller before, i might not have been the only one.

But if i botch it, i'll fix the situation and not cowardly step back from the purchase. I mean. How hard can it be to make this a MIDI controller?

Really - not that hard. The "communications protocol" consists of several signals controlling some counters that control some multiplexers and a ramp ADC, not so difficult. It's a bit like a serial datastream for the keyboard and PWM for the potentiometers. But why not do it completely right and also support the other direction, adding MIDI to a Crumar GDS? So i took some more care analyzing the host-side interface circuitry to understand it and implement exactly the same signal behavior here.

So technically, this interface intercepts the keyboard and potentiometer lines using SPI DMA for the keyboard bits and timer PWM/capture for the potentiometer data. If the interface runs in master mode, there is another timer-controlled DMA generating the control signal stream, otherwise, this comes from the GDS and is just passed through.

The front panel LEDs, based on a ICM7218, are also intercepted, so i use externally-triggered timer controlling GPIO DMA to read the host writes into a MCU buffer, extract the LED data and forward this to the output code, which writes them to the keyboard. I can switch to internal LED control easily this way without losing host LED changes.

All this is based on a STM32F103RCT6 running at 72MHz. There is not even interface circuitry as the signal drivers in the STM32 are strong enough for the TTL receivers, the inputs already have schmitt-triggers, it just fits between the cable.

So far, everything basically works, basic MIDI support is there, now it's time to complete the configuration by SysEx and by the keyboard controller itself, implement firmware update via SysEx, add the USB MIDI support, make the 4 LEDs show some useful status information, but all this is just software.

The hardware is now completed and tested - which was the main purpose of this video, while showing you another not so common DIY hack."

Crumar GDS MIDI interface testing

Published on Jul 30, 2015 Michael Kukat

"Another video about the Crumar GDS MIDI interface.

This time, both interfaces are in use. One is running in master mode, sort of emulating a real GDS (just the scan processor, not the synth), the other one in slave mode, working as a GDS MIDI interface.

The purpose was to test if everything works as intended, as it already did on the breadboad.

Controlling the Korg Gadget on the iPad with a Crumar GDS keyboard - fun :)

In the second part, i try to describe a bit how the interface between the GDS main unit and the keyboard works and how i hook into this communication to implement the interface. The DK Synergy works exactly the same way, it even has nearly the same pinout, just number of keys and pots differ. My interface board could also be used in a DK Synergy."