[sdiy] Roland Juno 106

[brianw]

On Feb 11, 2024, at 3:41 AM, Tom Wiltshire wrote:
> Most of the details are in the service manual. It includes the envelope times, LFO range, DAC resolution (12-bit) and update rate (238Hz). We'd still need to know whether envelopes are curved or linear, for example, and any other quirks they have.

These are great details. It shouldn't be too hard to replicate these aspects. There are still 12-bit DAC chips in production today. Granted, they're not delta-sigma 1-bit/24-bit audio CODEC, but it's actually an advantage to have DC output rather than noise-shaped AC.

> The interaction between the LFO and the DCOs is one quirk in the 106. Since the timers only update when they finish a cycle, pitch modulation is only applied at the end of each waveform. This means it gets more and more steppy as the frequency goes lower. For a 1KHz note, the waveform cycle is finishing much more rapidly than the LFO is being updated, although it's highly unlikely that the two cycles coincide, so there's still an effect there. For a 100Hz note, that effect is going to be much more notable, since the LFO's 238Hz update rate is essentially being "down sampled" at the DCO's 100Hz pitch.
> 
> Tom

It seems possible that this could even be 'emulated' with actual timer peripherals, rather than software. Many modern embedded processors have several timer peripherals - some even have as many as eight timer/counters.

What is the master clock input rate for the Juno 106 timers? Most of the modern MPUs have pre-divide on the clock sources, so it might be possible to runs the timers at the vintage rate.

Brian