[sdiy] Digital modular backplane - update

[cheater00 .]

On Mon, Mar 31, 2014 at 10:59 AM, John Slee <indigoid at gmail.com> wrote:
> On 31 March 2014 19:18, cheater00 . <cheater00 at gmail.com> wrote:
>> 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.
>
> Pots have end-of-travel, unlike typical rotary encoders. So if your pot
> is at 90% position (you turned it to ~9.0 on a scale of 1.0-10.0)  and
> you want to increase the preset value by another 20%... you're stuck.
>
> So it's either
>
> (a) motorised pots/faders (do these introduce a bunch of noise?), or
>
> (b) encoder + digital pot + some kind of position indicator (LEDs?)
>
> or
>
> (c) ordinary pots and no "balancing" as you describe it, a la Roland
> Jupiter and probably countless other analog synths
>
> Of these, (a) and (b) are going to be expensive (relative to decent
> quality ordinary pots), (b) will require a more complicated design, and
> (c) is just plain awful.

You can just use parallax mode:

http://www.ucapps.de/midibox/midibox_plus_parallax.gif

This can be realized using the balancing mode. The issue of having
display isn't lost on me, but even without that, the most basic thing
you want to do with a rotary control is to pull it up or down... and
that lets you do it in all situations. So instead of looking at your
modular you have to listen to it.

> And then you have other inputs, such as toggle/rotary switches. I
> suppose you could replace those with momentary buttons and some
> kind of indicator. I'd prefer that most of the time actually.

With logic (i.e. all discrete-value controls) it can be a bit more
difficult to figure out what setting you're on, but LED indicators
don't cost so much given that you suddenly have 16x more budget for a
front panel.

I do believe if you're lost it would be a good idea to have a computer
display in the modular somewhere for the purpose of telling you what
preset settings are currently being used.

> Look, I get the desire for a better modular. I really do. But I think that
> to do it a better path might be to ditch the analog parts almost entirely
> and use DSP everywhere. Like Creamware/SonicCore's SHARC
> systems, which are brilliant but lack the touchy-feely aspect of a real
> modular.
>
> It'll almost certainly be cheaper, and with the savings you can invest
> in some spectacularly good DACs for the output stage, and for ADCs
> if you wanted a module to process external signals.

This isn't the point of the conversation. I don't think destructive
criticism has a place here. We can talk about your points in another
thread, though, if you'd like. I have got really good arguments that
go against what you said, but let's keep this thread strictly about
how to realize the idea proposed, rather than whether it's
fundamentally flawed due to the invalidity of an axiom that was
assumed. We can sit around and pontificate what the best of the best
is but unless we start working out the details on some approach none
of them will get realized.

> I'm not convinced that repeatedly munging signals with ADC/DAC stages is going to
> result in a great sound. Got prototype?

As far as repeatedly digitizing and recreating a signal, any ADC/DAC
chain is going to be higher quality than an analog backplane. We've
done the numbers (crosstalk, noise) on this list, maybe in the thread
you got forwarded, maybe another one before that where I was talking
about polyphonic analog patch cables. People have used analog
backplane modulars, mixers, etc for a long time with great success.
This means any hit record made until the early 90s.

Regarding an example of repeatedly digitizing a signal processed for
analog: a lot of people do it in their records nowadays, and nothing
bad has happened. Most professional studios use a something like a
protools or motu rig as a multitrack and do many, many stages of
analog processing. Finally, let me again bring up the example of the
album Thriller. You wouldn't say that sounds bad, would you? And
that's one of the very first generations of digital multitrack
recording.

Cheers,
D.