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Softpot Ribbon Controller
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Scott Stites
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PostPosted: Wed Aug 06, 2008 7:38 am    Post subject: Softpot Ribbon Controller
Subject description: Split from Stribe Thread
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Quote:
Sneaking suspiscion it aint as cool as whatever you are working on...


Naw, it's just the latest chapter in my never ending quest for a ribbon controller with gate, trigger and positional memory....done with caveman implements (as compared to this marvelous project).

Posted Image, might have been reduced in size. Click Image to view fullscreen.

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Luka



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PostPosted: Wed Aug 06, 2008 7:54 am    Post subject: Reply with quote  Mark this post and the followings unread

i was going to get a stribe kit a while back but the price knocked me out. i built the klee instead and had a couple of hundred left ontop for other things.

it would be great if they can make it so you dont need maxmsp/reaktor to run it.

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Scott Stites
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PostPosted: Wed Aug 06, 2008 8:17 am    Post subject: Reply with quote  Mark this post and the followings unread

Well, if my ribbon controller idea pans out, it wouldn't be such a leap to put a bunch of them on a unit and have something that could control a number of different voices, though that leads back to "how many modules you got?". In fact, Germaniac mentioned something exactly along those lines a few days ago. My cart is before the horse, though.
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numbernone



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PostPosted: Wed Aug 06, 2008 9:46 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh man this is good news!!!

I have dicked around with ribbon controllers in my mind and less so in plastic reality for the entire 2 years DIYing that I have so far indulged in. The position memory being key,and probably more complicated than the sample and hold simplicity I have always imagined.
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Scott Stites
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PostPosted: Wed Aug 06, 2008 10:17 am    Post subject: Reply with quote  Mark this post and the followings unread

Well, it probably is exactly the same S&H simplicity. Laughing The Moog ribbon controller was spec'ed on the schematic to have a droop of better than 10 mV/minute, which worked well enough for Keith Emerson, I guess. In any event, it's going to have to be a very good S&H.

There are three approaches to try, but they all depend on the physical properties of the Softpot material - if there's any signficant contact/decontact time and/or hysterisis.

Also I'd like to avoid using an extra plate to generate the Gates/Triggers (such as on the Moog ribbon controller). I'd prefer the contact on the ribbon itself to generate those signals. Two of the approaches would work in that regard. The third (Dr. Moog's approach) I haven't figured on yet in that regard.

If the transistion from "touched" resistance to infinite resistance is fast enough, then a constantly clocked Sample that locks to hold when infinite resistance is sensed should work. If the clock is fast enough (and infinite resistance comes in quickly enough), there would be slim to no chance of a sample being taken in the transition.

The third method would be to create an imperceptible lag on the CV (IE, fast enough to be non-apparent to the performer in the form of slew from one extreme of the ribbon to the other) that is yet significantly longer than the transition to infinite resistance when the pressure is removed, and is the point at which the S&H would hold the lagged CV, which (hopefully for all intents and purposes) hasn't had a chance to drop any noticeable amount.

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Scott Stites
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PostPosted: Wed Aug 06, 2008 10:20 am    Post subject: Reply with quote  Mark this post and the followings unread

And, of course, the ignition system for the flamethrower on the end of the chassis must be considered.
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PostPosted: Wed Aug 06, 2008 10:56 am    Post subject: Reply with quote  Mark this post and the followings unread

awhile back i was debating getting the PCB for the MFOS keyboard and using the soft pot for the voltage divider instead of the keys... i didnt test it out but i didnt see any problems with it other than the PCB was way too big for that project

http://musicfromouterspace.com/analogsynth/MFOSMINICONTROLLER/MFOSMINICONTROLLER.php



one thing im sure youve run into is that you need a pull down (or up) resistor if you do want that "infinite" resistence you mentioned

i set something up for an arduino project months ago where i had a switch that would switch in a pull up, a pull down, or neither

the neither is great because you get a fun randomly fluctuating voltage
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Scott Stites
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PostPosted: Wed Aug 06, 2008 11:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Right, the "wiper" of the softpot disconnects with no pressure applied, going to (for all practical purposes) infinite resistance. I figure a pull down on the wiper would work for detecting that condition.

I believe the Moog circuit lets the input to a sample cap, buffered by a FET, float when no pressure is applied, providing the memory.

I've got two smaller strips that I plan to configure as the modulation controllers typical of a MiniMoog. Center zero, with no positional memory, and 0 to + with positional memory.

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Coriolis



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PostPosted: Wed Aug 06, 2008 12:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott, the DIY community (well, me at least) waits with baited breath for a device like you're talking about...
I've already got one of those Spectrasymbol things, that I've been pondering what to do with for a while...

C

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State Machine
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PostPosted: Wed Aug 06, 2008 12:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Right, the "wiper" of the softpot disconnects with no pressure applied, going to (for all practical purposes) infinite resistance. I figure a pull down on the wiper would work for detecting that condition.


I second like the approach Scott. In addition, you can use the wiper as one input to a comparator with the other input referenced to some very low threshold voltage so that any touch of the softpot will create a usable logic level output to create a GATE and TRIGGER signal.

Quote:
I believe the Moog circuit lets the input to a sample cap, buffered by a FET, float when no pressure is applied, providing the memory.


Probably a very low leakage variety I am sure and a GOOD low leakage FET that is carefully biased to make sure it's off when not sampling . Wink

Anyhow, I like the sounds of your project. You can make it modular so that one can add soft wipers as their system grows.

Well, I am sure we will hear more as things progress.

Bill
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Scott Stites
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PostPosted: Wed Aug 06, 2008 12:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yep, that's exactly it, Bill - the comparator signal would control S&H and the generation of gates and triggers.

I was thinking of an LM398 for S&H - they're extremely good and easy to work with as well.

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PostPosted: Wed Aug 06, 2008 5:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
I was thinking of an LM398 for S&H - they're extremely good and easy to work with as well.


Yes, thats a nice chip, cheap, and readily available. By the looks of things, you will probably have to use a 0.1 uF capacitor to achieve something close to the 10 mv / Min droop rate that the Moog is specified at. Actually you will get about 11 mv/min droop rate using this value. The trade off is increased charge time using larger holding capacitors which may not be bad at all for this application. In fact, if we plod through the numbers, the time it will take for a capacitor to charge to say 10V will be expressed as :

T=(CV)/I where C= capacitance, V is the voltage between the plates of the capacitor, and I is the charging current. So, T=(.1uF*10)/5*10E-3=200 uS. That should be plenty good you think? This time will of course dictate your sample gate as you know. Of course there will be a small amount of overhead for switch delay, etc ...

Bill
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PostPosted: Wed Aug 06, 2008 7:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:
I was thinking of an LM398 for S&H - they're extremely good and easy to work with as well.


The technique here:
http://www.national.com/an/AN/AN-294.pdf
could help you to get fast charging and low droop rates.

Very Happy

Ian
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PostPosted: Wed Aug 06, 2008 9:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

would it be possible to sit it ontop of led bars as well using the lm3914 to power the bars?
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PostPosted: Wed Aug 06, 2008 9:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks, guys!

That app note is killer, Ian Very Happy

Seems to me (I haven't looked at it in years) the datasheet had a low droop rate app, too(?). IIRC, it employed two 398's. Seems to me someone mentioned years ago it was the technique used by MOTM for their S&H.

I applied voltage to the SoftPot tonight. Oh, yes, this is going to be a very fun project, indeed. Very Happy

The action is very smooth, and quite linear (I think it's spec'ed for 3% linearity). Quite sensitive, but not too sensitive - just about perfect, actually. With higher voltage range, I felt like Morton Subotnick on a Silver Apples flashback rolling my finger around on it Laughing . I hooked the output up to my EG (which uses a comparator to gate) and had a lot of fun controlling a VCO and filter cutoff with the control voltage and filter cutoff and VCA amplitude with the EG.

It's a bit awkward to use still because I don't have it glued to anything. I notice there's a bit of bounce on it, which surprised me a little. That'll have to be dealt with, but it doesn't worry me. Near as I can tell, the fall time is around 200 uS, though I could be wrong about that.

Luka - I s'pose one could use 3914's to monitor the voltage if one wanted.

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State Machine
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PostPosted: Thu Aug 07, 2008 7:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
could help you to get fast charging and low droop rates.


Great application note. I guess the question would now arise, can the stock stand alone circuit be enough performance or will the higher performance circuit be needed. Persoanlly I would think the droop rate would be of most concern at this point but more experimentation by Scott would need to take place.

I am wondering, if there are S/H's in a single chip that have the better specification if higher perform were needed as I have not dealt with these chips for a while?

Good conversation guys ...........
Bill
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State Machine
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PostPosted: Thu Aug 07, 2008 7:30 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
would it be possible to sit it ontop of led bars as well using the lm3914 to power the bars?


Wow, I was thinking about that on the way into work today .... Cool The chip works in DOT or BAR mode right?

I don't recall either, will it respond lineraly or LOG wise?

Bill
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PostPosted: Thu Aug 07, 2008 9:14 am    Post subject: Reply with quote  Mark this post and the followings unread

State Machine wrote:
Quote:
would it be possible to sit it ontop of led bars as well using the lm3914 to power the bars?

Wow, I was thinking about that on the way into work today .... Cool The chip works in DOT or BAR mode right?

I don't recall either, will it respond lineraly or LOG wise?

Yes, bar or dot mode. The 3914 is linear. Its brother is log.

Very Happy

Ian
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PostPosted: Thu Aug 07, 2008 10:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
I am wondering, if there are S/H's in a single chip that have the better specification if higher perform were needed as I have not dealt with these chips for a while?


IIRC, AD has an S&H IC. I think Tom Bugs was using those a lot. I'm not sure if it's any better or better enough to justify going to the expense of getting the part. Rene Schmitz's YASH uses the LF398, as does the Buchla 266 SOU, both of which excercises got me used to using it. For this app, I think it's fine.

A super-low droop rate, of course, would be great. 10 mV/S actually, however, is certainly acceptable for the most part I think. The voltage is going to hold as long as the ribbon is pressed - I'm just concerned with what happens after pressure is removed from the ribbon. IOW, the release portion of a gated envelope. Unless one is after a very long sustain after the pressure is released, droop isn't going to figure into it much, as long as it's a reasonable amount. Using one for setting modulation would actually fairly critical, if you want to "set and forget" for a fairly long length of time. I certainly am going to try for the best droop rate possible with parts that are reasonably easy to get. My main concern right now is just making sure I have a good way of acquiring the signal using just the ribbon voltage as the signal......

Man, Ian - that app has a droop of 30 uV/second? Wow - what's that, 40 odd minutes for a drop of a semitone? That analog switch used in it is near unobtainium, however - Arcadechips has them for $11.00 apiece. I would bet there is a switch that would work as a replacement out there somewhere.

I've attached the 398 spec sheet. The app I was thinking about is the two 398 configuration labeled "Fast Acquisition, Low Droop Sample and Hold". A quick sip/long swig sort of thing. It doesn't mention droop rate, but, as I mentioned before, I seem to recall someone mentioning this was the basis for the MOTM S&H.


Lf398.pdf
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LF398 Datasheet

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PostPosted: Thu Aug 07, 2008 12:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:
A super-low droop rate, of course, would be great. 10 mV/S actually, however, is certainly acceptable for the most part I think. The voltage is going to hold as long as the ribbon is pressed - I'm just concerned with what happens after pressure is removed from the ribbon. IOW, the release portion of a gated envelope. Unless one is after a very long sustain after the pressure is released, droop isn't going to figure into it much, as long as it's a reasonable amount.

I agree. If I listen to a VCO with a droop rate of 5 mV/sec my ear does not detect the drift. Like the old problem of a capello choirs drifting. Of course if you have perfect pitch you might be sensitive to it.

My TGTSH circuit has a droop rate of 1.5 mV/sec and an acquisition time of 33 usec. This could easily be scaled to lower droop and longer acquisition times. The only challenge would be finding a large-value, low leakage cap. It currently uses a 4.7 nF polystyrene unit. You could parallel a couple of 10 nF units to get a factor of four lower droop rate.

I'm currently working with Elby on a single channel version of this circuit. The first half of a 13700 is used for the S/H circuit and the second half provides the current switching pulse for the first! If you have a well conditioned trigger pulse of the proper length, you can drive the the second half directly from that pulse. So the whole circuit takes a TL071 input buffer, the dual LM13700 and a CA3140 output buffer.

Just an idea to keep in mind. Very Happy

Ian
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PostPosted: Thu Aug 07, 2008 1:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Just an idea to keep in mind.


I think I just will! Thanks!

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PostPosted: Thu Aug 07, 2008 1:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
I'm just concerned with what happens after pressure is removed from the ribbon. IOW, the release portion of a gated envelope.


Yes, this is critical as you dont want to sample the bounce.

Quote:
Using one for setting modulation would actually fairly critical, if you want to "set and forget" for a fairly long length of time. I certainly am going to try for the best droop rate possible with parts that are reasonably easy to get. My main concern right now is just making sure I have a good way of acquiring the signal using just the ribbon voltage as the signal......


The best choice in this situation is more like an infinate memory type circuit topology.

Also, using the signal off the wiper may complicate the issue of sampling the release unless maybe you delay the wiper voltage to the LF398. You don't want to get too ocmplicated Very Happy

Quote:
I've attached the 398 spec sheet. The app I was thinking about is the two 398 configuration labeled "Fast Acquisition, Low Droop Sample and Hold". A quick sip/long swig sort of thing. It doesn't mention droop rate, but, as I mentioned before, I seem to recall someone mentioning this was the basis for the MOTM S&H.


Yes, but that droop rate must be really small Wink

Bill
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State Machine
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PostPosted: Thu Aug 07, 2008 1:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Yes, bar or dot mode. The 3914 is linear. Its brother is log.


Thank you sir for settling that Wink I always confused those chips even after all these years .. Shocked

Bill
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PostPosted: Thu Aug 07, 2008 3:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Also, using the signal off the wiper may complicate the issue of sampling the release unless maybe you delay the wiper voltage to the LF398. You don't want to get too ocmplicated


Yep - lag is a foregone conclusion. I experimented last night with how much I could get away with before it became noticeable. Using a simple exponential lag, I found 10 mS fell in that area (imperceptible when using a VCO). Much more than that would build in a natural portamento that should be avoided. I'm aiming towards (for the first experiments) constantly clocking the S&H and holding when the wiper voltage triggers the comparator. Ideally the last sample will have hit before the voltage dropped. If it should hit after the voltage begins to drop (or during any bounce that might occur before the comparator level is reached) the lag's responsibility is to keep the effect at an un-noticeable minimum.

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PostPosted: Thu Aug 07, 2008 3:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
You don't want to get too ocmplicated


Well, if things just don't work out, the Moog touchplate will work. Germaniac had a great idea of using aluminum or copper tape. I just would prefer the ribbon to be the sole required contact point.

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