[sdiy] Digital modular backplane - update

[cheater00 .]

Hi John,
thanks for your email.

On Mon, Mar 31, 2014 at 5:59 AM, John Slee <indigoid at gmail.com> wrote:
> I am wondering what the actual problem is that you're trying to solve.

What have you taken away from the first few posts on the thread I have
forwarded to you? I ask because if I didn't explain myself clearly I
have to fix that in the future.

> If it is patches, then you need to save/restore control positions as
> well as signal routing, or the whole exercise is largely a waste of
> time IMHO. To do that, you need to address the _substantial_ increase
> in complexity and according cost that would be introduced by doing so.
>
> John

I disagree in two ways. First that it's not good enough just routing
the signals without storage of knob positions. I believe that this is
the most complex work done when setting up a patch and the most
non-obvious one. It's also the step that makes your interface the most
cluttered. A lot of ergonomy can be gained by removing this issue.

Second that a huge amount of complexity needs to be added. In a modern
modular synthesizer all control parameters have CV inputs already,
which means they would be connected to the digital backplane. So you
can have presets already. The question is just how to read off the
knobs and make sure they cooperate with the patching mechanism.
However, that is a generic circuit which can be mass-produced. It
contains an ADC and a connection to the local digital bus (rather than
the backplane), which then connects it to the module's CPU, that takes
care of all DSP needs such as presets.

The situation is even more biased in a polyphonic modular, where you
have, say, 16x more circuits behind a single interface. At this point
you absolutely have to use either analog or digital buffering and
digital is simpler. It also means that for the same amount of voices
you have 16x more budget for the user interface. However, even if
digital is simpler, the ultimate way to do this would be with the
capability of having direct analog access from the knob and
front-panel connectors to the module. This could be reconciled with
presets by using a balancing topology - i.e. if the knob it putting
out 1V, but the preset is 0.5V, then the DAC should put out -0.5V and
those voltages should be added. This can be done at the distribution
amplifier which you need to have anyways if you want to feed a single
voltage to 16 modules. In addition this amplifier can take care of
patchability, i.e. a single amplifier mixes the knob value, the DAC,
and the front-panel analog patch input. The DAC itself takes care of
the preset, and it takes care of digital patching.

Cheers,
D.