Keyboard/Midi Frontend (RE: PolyModular Standards)

[Ingo Debus]

jh wrote:
> Polyphonic
> Aftertouch - that's a *dream* of mine, too. I guess I have to get
> an Ensoniq keyboard then (?).

I have a Roland A-80 masterkeyboard. The A-90 has polyphonic aftertouch
too. The keys on the Roland have to be pressed really hard to get
aftertouch. I already replaced the sensors (before I did that it was
nearly impossible to get aftertouch). In the schemo there's a resistor
with different values for the A-80 and the A-50 (the
61-nonweighted-key-version). I changed that resistor to the A-50 value
too, which made it a bit easier to get aftertouch, but it's still hard.
I'll have to look into that again.

> I'm a little sceptical about the
> Midi slowing down with polyphonic AT, however. Is there a solution
> for that ?

Never tried to reach the limits, with normal playing it's ok, I think.


> A few points from my personal wish list for a Midi Interface (I don't know
> if that could be adapted easily):
> 
> (1) Hardware switches for every important functions, i.e. no menues.
> I want to change from unison to poly with a dedicated electrical contact,
> so I can even connect a footswitch if I want  to. Same for voice assignment
> modes. (see below.)

My user interface is indeed not very user-friendly. You enter the
parameter number (in hexadecimal) with a 16-button-keyboard, two digits
of the display show that number then, two other digits show the value.
Then you can change the value with up/down buttons. There are two
buttons for up and down each, one for small and one for large steps (for
instance, for pitch offsets its octave and semitone steps). But it is
very flexible this way. I can add new parameters and change value ranges
without having to change the UI hardware.

Which functions do you consider important? 
Of course split point, number of voices stacked, poly/mono scanning mode
(the latter for both keyboard zones).
The offsets (VCO pitch and VCF cutoff) are not important I think,
because there'll be switches for that on the VCO/VCF modules itself.
What about the VCF sensitivities (keyboard
tracking/velocity/controller/aftertouch)? These are a lot of parameters,
because, when voices are stacked they can be set separately for each
VCF.
So there are at least four important parameters.
What I'm thinking about: I cannot implement addtional UI features if I
don't have the hardware for them. OTOH, the box my synth lives in is
rather small, there's not much front panel space left. There are enough
buttons I think, but the 6-digit display is limiting.

Re footswitch: you can always choose another complete preset. But every
time a parameter value is changed the voice assignment logic is reset
(all voices turned off). This was necessary because when certain
parameters (e.g. number of voices stacked) are changed the logic would
be screwed. So it isn't possible to change assignment mode while voices
are playing.

> (2) Since I have played with an Obie / Emu keyboard scanner, I won't
> go back behind that anymore. I'd like to see it upgraded for a modern
> Midi / CV converter, velocity, aftertouch and all the things you mentioned.
> But I absolutely *need* all these various keyboard assignment modes.
> 
> (a) Reassign on/off. This determines if an additional voice is allocated
> if you hit the same key twice. In case it is, you get the most wonderful
> chorus effect between these voices (like unison), but you run out of
> voices early. In case no new voice is allocated, you just restart
> the envelope of the previously assigned voice, and by varying the
> time between hitting the same key you can model the contour of
> this voice way beyond a "boring" normal ADSR envelope (remember
> that thread?), just as you can with a monophonic synth.
> My Ob-8 does one thing (chorusing), and my Prophet 5 does the
> other thing only, and my "dream interface" must have both options.

At the moment only the latter mode is implemented. If a note is played
and a voice plays this note already in release phase, this voice is used
again. But I think the other possibility is cool. I'd like to add that
feature.

> (b) Continuous / Reset , and a mix of both.
> When I have released all notes, and asuming the last note was assigned to
> voice #3, I have a choice which voice will be assigned when I hit the next
> note on the keyboard. "Reset" will start again with voice #1, and "Continuous"
> will start with the next in line, #4 in our example. Both options will
> result in other voices (that might still sound in their release phase)
> being cut off and used for a new pitch. Both options are valuable,
> and again I want to choose between them "on the fly" with a hardware switch.

If there are some "free" voices (i.e. in release phase) and another key
is pressed, the voice is used whose key was released the longest time
ago. No matter if there are all voices "free" or only some of them.

> In general, I see two possible design goals for polyphonic synths:
> Either put in so many voices that you don't notice that it's not
> one voice for every key. Or use the limited number of notes as a creative
> tool of it's own. A PS-3200, and many modern digital synth, follow the
> first design goal. No way to compete with that with a PolyModular! I mean
> whether there are 4 voices or 6 voices, you don't come close to *that*.
> So I want to go for the second design goal. Limited number of voices
> can lead to a very special playing style. Especially when you have
> these parameters from (a) and (b) to choose from.
> 
> If you have a polysynth that allows to disable a certain number of voices
> (like the Oberheim's and the Prophets), I invite you to make the following
> experiment: Switch the synth to unison and set the VCA-ADSR to fairly
> long Attack and Release times. Same for VCF-ADSR, and set up an
> interesting sound to play sustained lead lines, according to your taste.
> Just the fairly long A and R times are important. You can play monoponic
> lines, if you remove your fingers the sound will sustain, and if you
> hit a new note, the old sound will be replaced by a new one, or the
> pitch will jump / glide to a new value - that's how we perceive what's
> going on. Even with long attack times, the "new" sound will be there
> rather soon, as the "old sound" was still in its release phase.
> *How* fast this wil happen depends on the duration of the "gap"
> between two notes you played. You can shape the envelope in realtime
> even without velocity or aftertouch.
> Now switch to polyphonic mode (5, 6, 8 notes depending
> on the synth you're using), and the effect is a completely different
> one. You will have a slow rise / slow fade out pad sound, with each
> note fading into the next. This is useful, too, but it's a completely
> different sound. Same parameters, completely different result.
> Now do the 3rd part of the experiment and switch off some of the
> voices in polyphonic mode. Make your synth a 2-voice or 3-voice
> machine deliberately. And suddenly you have these wonderful
> sustained lead lines again, just not monophonic anymore, but 2
> or 3 independent voices with all these realtime contour shaping
> capabillities as before.
> The same thing does happen with more voices, too, of course.
> But speaking for myself, I cannot control this on purpose anymore
> with 6 or 8 voices. It still happens that a note is notably cut off every
> now and then with 8 voices, but then it's more perceived as an
> "accident" than a means of creative expression.
> 
> So the bottom line for me is that a limited number of voices is not
> just a restriction, but a means to prase notes individually. And to make
> the best of it, we need various assignment modes, and to make use
> of these, we need hardware switches, not menues.

Hm, I hope I got this example right. You want to play, say 3-note
chords, one after the other, and have always the same three voices used,
even if there are even more voices available? So "reset" the voice
assignment logic every time no key is pressed? This shouldn't be too
hard to implement.

BTW, what is a PS-3200?

> Ingo, what would be the approx. component costs of your design
> (asuming that you'd donate your code to the community (;->) ) ?

Pfff, have to dig out the schematics and see what's still necessary
(this was my "computer" back then, not only my synth). Prices are in DM,
from Conrad.
6502 CPU            ?
Quartz oscillator   9.95
62 32k*8 SRAM       7.95
27C64 8k*8 EPROM    5.95
6520                ?    keyboard interface, possibly obsolete
6532                ?    front panel interface
6850                6.-- MIDI interface
21*pushbutton       depends on quality ;-)
6*7segment display  6*3.-- to 5.-- depends on size etc.
For the analog interface:
Some small SRAM, say 8k*8 6.95
(in the original circuit there are SRAMs with separate data ins and
outs, might be hard to get; with standard SRAMS some redesign is
necessary)
D/A converter, depends on resolution.
For eight analog outputs each:
1*4051   1,50 
2*TL064  2*2,--
1*cap

Add 50,-- to 100,-- for a handful of TTL, battery and other stuff...

Ingo