Follow that trigger, trigger that envelope, mute that tron

[Mark Hammer]

This is one of those "Why didn't I think of it before?" things.

I was responding to a posting over at AMPAGE from someone who wanted to sync volume ramp ups with a Mutron-type swept filter, and something useful dawned on me that I thought meritted posting here.

Background:  Ultra-slow rise/attack times are difficult to achieve with the normal envelope-follower signal.  In the case of "Slow Gear" type volume ramp ups, you also expect to have a fixed ramp-up time independent of picking intensity, which is going to be hard to do with JUST an envelope follower.

HOWEVER....such ultra long times are possible via use of simple envelope generators such as the many 555-based AD and ADSR units posted around.  The bridge from the one system to the other is the production of a trigger or gate pulse.

In many instances, trigger extractors which output a brief pulse at the start of any detected note/chord use an envelope follower, plus a comparator.  When the output of the envelope follower is above some threshold set by the comparator, the comparator outputs a pulse/level shift of a predetermined voltage which serves as the  signal to kickstart the envelope generator into action.  A perfect example can be seen here in the PAiA 2720 follower/trigger module (http://www.synthdiy.com/files/1/2004220/PAIA_2720-11_Env_Follower.gif) .

If you already HAVE the envelope follower, though, all you need to do to produce a trigger is tack on a comparator.  In the PAiA design, it consists of IC4.  In principle, you should be able to tack everything around IC4 (from R9 onwards) onto a stock Mutron/Neutron/Q-Tron.  The beautiful part is that it runs off +/-9vdc, which the Mutron will already have.  Armed with a trigger extractor, you can simply plug your guitar into your Mutron, and direct the trigger output to other modules for additional processing.

The threshold at which a trigger/gate pulse is produced is determined by the voltage available at the junction of R12/R13.  These two divide down the supply voltage to obtain a much lower reference" voltage.  When the output of the envelope follower goes above that reference, IC4 pumps out the pulse.  Since the PAiA module is based around some assumptions about likely envelope signal, you'll probably want a way to adjust the threshold for triggering.  I suspect a wide enough range of sensitivities can be gotten by replacing the 68k and 33k resistors with 39k and 12k resistors, and sticking a 47k/40k linear pot between them, with the wiper going to the 22k resistor.  This will allow you to adjust the ratio of resistance on each leg to achieve different reference voltages.

So where do you tack it on to the Mutron?  Best spot is likely the junction of the 1N914 diode and 330R attack resistor on the output of the envelope follower. My guess is that just about any op-amp would work, and since it carries no real audio signal, go for the lowest current op-amp you have and damn the noise specs.

Understand as well, that trigger pulses can be easily *delayed* which opens up possibilties for things like aftersweep.  I.E., the envelope follower sweeps the filter, and shortly after the initial sweep settles back down, a fixed sweep of some sort is imposed.  The possibilities are endless.  

For the moment, interested parties could simply stick in a mini phone jack in their existing Mutron/clone, and run a line from the designated point in the circuit to the jack (with ground), placing the trigger extractor in a separate module.  Ideally, though, it would be great to have the trigger output and threshold control integrated into the pedal itself.  This would permit you to build an envelope generator and other processing module in a separate box, and just run a patch cord from the trigger out on the Mutron to the trigger in on the outboard module.  It also means that if you have any analog modular synth modules "just hanging around", you can use them as is.

[puretube]

just can`t imagine You really haven`t thought about that before...   ;

another route can be followed by using a trigger circuit plus AD or ADSR generator, and:

start an LFO with the trigger;

or modulate the amplitude of an LFO with the AD generator...

[gez]

another route can be followed by using a trigger circuit plus AD or ADSR generator, and:

start an LFO with the trigger.

Penfold uses a similar approach in a few of his circuits.  I recently used an envelope-driven comparitor and 555 chip to trigger/synch a sine LFO.  It works and the LFO fires up straight away but the sensitivity needs tweaking (LFO stops before the note decays).  Just  waiting for the right parts to come through (that'll teach me to reach in to the box for 'subs').   :oops:

[Mark Hammer]

Just back from a meeting on the other side of town.  On the way back it occurred to me that if a person wanted, they could have multiple comparators tapped off the same point (with appropriate isolation if needed) of the envelope follower, and use different trigger thresholds.  In other words, you could have one envelope generator triggered by "normal" notes, and other envelope generators that would not come into play unless you really whacked the strings - in a sense, your audio trump card, saved for special occasions.

[StephenGiles]

........which brings me back to my old chestnut of making the start voltage of a sweep generator proportional to the strength of the note struck,and the decay independent thereof!
Stephen

[puretube]

(Mark:) well - to be honest - that never occurred to me   :oops: ...

then again: one could [/i]count, how many trigger events (one shots)
happen over a certain time...

[didn`t C.A. have something similar? - pick-follower - ]

[puretube]

........which brings me back to my old chestnut of making the start voltage of a sweep generator proportional to the strength of the note struck,and the decay independent thereof!
Stephen

...which brings me back to the post in the other thread...
(http://diystompboxes.com/sboxforum/viewtopic.php?t=27023)

[StephenGiles]

Help me here Ton
Stephen

[Mark Hammer]

(Mark:) well - to be honest - that never occurred to me   :oops: ...

then again: one could [/i]count, how many trigger events (one shots)
happen over a certain time...

[didn`t C.A. have something similar? - pick-follower - ]

I don't think you can have THAT many events/transitions at once, or else it starts to turn to mush, but certainly three (envelope follower + 2*EG) is reasonable.  For instance, EF drives filter frequency, EG1 drives amplitude/VCA, EG2 drives filter resonance.

Wanna go super-crazy?  Have the trigger output drive a flip-flop and switch back and forth between filter outputs of a Mutron (note1 = bandpass, note2 = highpass).  If the filter-mode switching is yoked to the higher threshold trigger, then you can wail away and switch modes on-line just by slamming a chord.

Personally, I love the way Jeff Beck can just decide he wants to capture the soul of another instrument and produce it with his fingers.  I'm not THAT good, so envelope controlled switching would be fine for me.

The C.A. thing (pluck follower) was not a strictly envelope-controlled thing, but a trigger-pulse summer, packaged as a sort of lag module.  So, the more trigger pulses were emitted in a given unit of time (by playing a lot more notes), the higher the control-voltage produced.  The pulses are averaged out in a pretty typical cap to ground at the input of a BiFet op-amp.  That could ALSO be implemented by addition of a simple trigger extractor, and could produce another control voltage without requiring envelope generators.  Thanks for bringing it up.

[puretube]

Mark: I was more thinking of a relatively slow thing: 1-2-3-wham!
like activating a reverb or echo or whatever, with every 4th chord (or every x-th "pluck"...

Stephen: I`d LOVE to get in deep into those topics, but momentarily there are some personal priorities I have to deal with, that don`t allow me to spend the neccessary time with "fun"...
(a.o.: http://diystompboxes.com/sboxforum/viewtopic.php?t=25591&start=30, last post...)

[Mark Hammer]

Mark: I was more thinking of a relatively slow thing: 1-2-3-wham!
like activating a reverb or echo or whatever, with every 4th chord (or every x-th "pluck"...

Stephen: I`d LOVE to get in deep into those topics, but momentarily there are some personal priorities I have to deal with, that don`t allow me to spend the neccessary time with "fun"...
(a.o.: http://diystompboxes.com/sboxforum/viewtopic.php?t=25591&start=30, last post...)

Ah, then I suspect you're cogitating about a counter arrangement that counts incoming trigger pulses, and produces one when a given count is reached.  There was an a POLYPHONY article about a "quad sequential counter" that I think I scanned and posted on my site.

Bets of luck with the personal stuff and hope you get back to fun soon.

[StephenGiles]

HeyTon, I hope the personal stuff is not too serious, best of luck.
Stephen

[Torchy]

crp

[puretube]

was it this one? :
http://diystompboxes.com/sboxforum/viewtopic.php?p=180892#180892
:wink: