[sdiy] Best AVAILABLE OTA for building new designs?

[Mike Bryant]

The overload problem is one reason to go software.  Lots of algorithms available for setting the starting phases of each tone so that you are unlikely to overload the output.  Though avoid Schroeder-phase harmonic tone complexes which have an absolutely level envelope as the sound suddenly changes to a rising chirp.
________________________________
From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Mattias Rickardsson <mr at analogue.org>
Sent: 13 August 2024 10:06
To: Gordonjcp <gordonjcp at gjcp.net>
Cc: synth-diy at synth-diy.org <synth-diy at synth-diy.org>
Subject: Re: [sdiy] Best AVAILABLE OTA for building new designs?

Sounds like we are starting to overdesign things a bit here... :-D
Btw the "counting notes and keeping total volume constant" idea reminds me of one of the Unison modes in Roland Jupiter-4, where you depending on the number of depressed keys get 4-voice unison, 2x 2-voice unison, 3-voice poly or 4-voice poly. The level loss in this mode is remarkable and not very nice, especially when playing a 3-note chord and getting only 3 voices playing compared to playing 1 note and getting 4 voces playing.

Anyway, I don't think the all-polyphonic voice summing issue should be too over-thought, but just try to combine
- a summing where everything fits without clipping
- a level control that can gain even more so that clipping could occur but a melody player would have enough output level.

One of the trickiest bits is that resonant filters make peak levels vary a LOT more than you first might think, so you'll have to count with a peak voice level that is much higher than many typical sounds.

Hopefully this is less of a problem in a true PS3100 voice where resonances are very moderate. In the gainstaging of polysynth designs in general, there is a huge advantage if you manage to keep filter resonances limited - everything level related gets easier, but on the other hand it's hard to make a firm resonance limiter without adding unwanted distortion or side effects to the filter sound.

/mr

On Tue, 13 Aug 2024 at 08:49, Gordonjcp <gordonjcp at gjcp.net<mailto:gordonjcp at gjcp.net>> wrote:
On Tue, Aug 13, 2024 at 07:54:24AM +1000, Adam (synthDIY) wrote:
>
>
> > On 12 Aug 2024, at 11:38 PM, Mike Bryant <mbryant at futurehorizons.com<mailto:mbryant at futurehorizons.com>> wrote:
> >
> > I think you need to watch some videos of the late Keith Emerson.  He'd happily have twenty or more keys pressed on a Hammond, some of course held down with knives :-)
>
> David, you might be able to use this as a "feature"...  have a front panel control labelled "Knife Factor" that switches in a compressor with auto-make-up gain...

Given that compressors work by controlling their gain based on output level - a negative feedback loop - I have this feeling that you could probably sum all the VCA CVs to give a "total output demand" voltage, which could be fed back to the DAC Vref (taking us back around to the multiplying DAC question) to depress the output voltage and keep the total level approximately constant.

"Approximately" because you'd want it to have a bit of dynamics, you wouldn't want one note to be as loud as ten, but maybe five should be as loud as ten.

--
Gordonjcp

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