Commodore 64 Music in the real world & other related SID stories

Saturday, January 21, 2006

The MonoVoice Synth

If there is one SID based synth we didn't know about yet is the MonoVoice Synth, a 3 oscillator monophonic synth with a register editor software and a simple 8-step sequencer. It seems to have been around since 2001... according to this google search result. Hmm...

"...After my efforts of connecting a Commodore SID chip to the PC parallel port, i had several people asking how to interface that thing to a sequencer or keyboard. So i had to write some kind of MIDI loopback driver for it; but i did not like that idea and built a standalone box instead: the MonoVoice. My first idea was to make it 3-voice polyphonic beacuse it has 3 independent oscillators, but then i changed to the idea of a monophonic synth with a three-oscillator voice.

The MonoVoice has many things you can control over MIDI: Oscillator detune, Oscillator waveshapes (separate and global), Filter parameters, 5 ADSR´s (one for each voice´s VCA, one for the global VCA, one more (software generated) for the filter cutoff), and some more. Because i do not have a decent MIDI controller knob box, i wrote a Windows program that can send those MIDI messages to the MonoVoice.

It has the additional plus of 21 preset memories where you can store and recall your favourite settings. It also has a Randomizer for all those settings. The sequencer is a simple 8-step one that learns from keyboard input not unlike an SH101. For that purpose the Keyboard does have the additional "0" key to create pauses.

The SID chip is connected to an AVR AT90S8515 Microcontroller running at 8 MHz. The clock signal for the SID comes from a separate 1 MHz crystal oscillator. 16 output lines of the AVR are enough to create a complete 6502 state machine: 8 data lines come from one port, and another port carries the control signals: Reset, Read/Write, CS and 5 address lines. (Further explanation is done by studying the WRITE_SID and RESET_SID functions in the source code.

Warning to all people building stuff with the SID chip: the SID´s output is very delicate. I managed to zap 3 SID chips during development by simply inserting the cable from the MonoVoice into my amp´s jack - the extremely short short circuit duting insertion can do that. So use an opamp buffer to save your SID ! ..."


Monovoice Homepage
MOS6581 Datasheet


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