Noise Gate to trigger digital ASDR

[jonokt12]

Hi everyone,

I'm trying to design a circuit where a digital ASDR envelope generator controlling a VCA will be triggered when an input signal passes a certain threshold. I've been reading up on envelope trackers and edge detection but it's all quite complicated, and I'm wondering if I could simply modify a noise gate to gate a 5V control input into my digital ASDR (the electric druid ENVGEN 8 https://electricdruid.net/product/envgen8/)?

The schematic below is based off of No Noise Gate schematic from PCB guitar mania (https://pcbguitarmania.com/wp-content/uploads/2018/07/No-Noise-Gate-1.2v-Building-Docs.pdf). Am I correct in thinking that the op amp in top right gates the input based on the threshold? If so, if I supply a 5V signal to the positive terminal, could I use the op amp as a switch, allowing the signal through to the digital ASDR only when the input signal crosses the threshold?

This is my first time trying to modify / design a circuit, any advice is appreciated!

[ElectricDruid]

Let me describe how I *think* that noise gate circuit works. Someone else will be along soon, and they can correct whatever I've misunderstood.

Q1 is a simple input buffer. After that, the signal splits two ways, one signal path along the top of the schematic, and one sidechain path along the bottom. Let's look at the sidechain. IC1A is a non-inverting amp with plenty of gain. The threshold control acts as a volume control in front of it, allowing you to set the overall gain through the stage. When there is sufficient signal at the output of the op-amp, Q2 will turn on, which shorts C5 to ground. When there's no signal, C5 will charge up to Vbias via R11. This rising voltage is our control signal: high for "off", low for "on".

Looking at the signal path now, the key part is the FET Q3. When the control signal is high, this shorts the input to Vbias, cutting off any audio to the op-amp and muting the output. The op-amp itself isn't switching anything really. It's just a buffer.

Ok, so how would you get this to control the EnvGen8? It's not simple. For starters, the control signal is the wrong way around - you need a signal which is high for "on" and low for "off", not the other way. Also the control signal here doesn't look to me like it would be suitable for driving a logic input, since I think it's going to rise slowly. You need something that's going to switch on and off smartly, not rise and fall gradually.

Honestly, I think I'd have a look for some other gate designs and see if another one might be more suitable. This one is going to need so much redesign it's virtually going to be redone from scratch. Sorry.

[Mark Hammer]

The boundaries between a noise gate and envelope follower are quick fluid.  Both will track the amplitude of an input signal.  For a gate, the gate is turned "on" when a critical threshold is exceeded, and stay on until the threshold is no longer met, often with a slowish turn-off/decay time, so that the changeover isn't so jarring.  Many envelope follower circuits will also provide for a similar sort of high-enough/not-high-enough distinction, and generate a brief trigger, once the threshold is met, and/or a gate that lasts as long as the threshold is exceeded.  There is an abundance of such circuits posted around.

Depending on what gate circuit you are using, it may be amenable to producing a 5V trigger at "turn-on".

[StephenGiles]

The boundaries between a noise gate and envelope follower are quick fluid.  Both will track the amplitude of an input signal.  For a gate, the gate is turned "on" when a critical threshold is exceeded, and stay on until the threshold is no longer met, often with a slowish turn-off/decay time, so that the changeover isn't so jarring.  Many envelope follower circuits will also provide for a similar sort of high-enough/not-high-enough distinction, and generate a brief trigger, once the threshold is met, and/or a gate that lasts as long as the threshold is exceeded.  There is an abundance of such circuits posted around.

Depending on what gate circuit you are using, it may be amenable to producing a 5V trigger at "turn-on".

An interesting development might be a circuit which held the CV until a new input was detected, like in say the Aphex Compellor 320.

[Mark Hammer]

So basically a sample and hold, in which the "clock" pulse that enables the storing of the CV in an isolated cap is essentially the rising edge of the gate?

[StephenGiles]

So basically a sample and hold, in which the "clock" pulse that enables the storing of the CV in an isolated cap is essentially the rising edge of the gate?

That sounds about right.

[jonokt12]

Thanks for the advice everyone, it prompted me to do some useful research. I've thought further about what I'm actually hoping to achieve and realised that I'm actually trying to design a threshold-triggered AD envelope pedal - I think the ENVGEN might be overkill, I don't really need a sustain and release.

The E&MM string damper seems to create AD envelopes, albeit under the control of a foot switch. I'm unsure about the schematic posted here though - https://www.freestompboxes.org/viewtopic.php?t=8616 - would depressing the foot switch in the bottom left trigger the envelope or prevent the audio signal passing through the op amp? I'm confused by what allowing the 9v supply into the negative input of the op amp would do.

In any case, maybe the best thing to do would be to replace the foot switch in the string damper with a simple envelope follower circuit feeding into a comparator.

[ElectricDruid]

You're talking about this schematic, right?:



That op-amp stage with the "Foot trigger" switch is a non-inverting comparator, with hysteresis. Compare it with the circuits on this page: https://www.homemade-circuits.com/opamp-hysteresis-explained/
The way it works is that when the input voltage on pin 2 is higher than the reference voltage on pin 3 set by 5K6 and 100K, the output goes high, and when it's lower, the output goes low. Pressing the foot trigger grounds the comparator's pin 2 input, so the output will go low.
What I don't fully understand is *why that's desirable*. Perhaps when you release the trigger, the comparator output goes high again, and it starts a new AD envelope?

[Mark Hammer]

The circuit Tom shows is not the E&MM String Damper, but is oriented to the same goal,  Looking at the drawing my sense is that it does not require use of the footswitch to operate.  Rather, the footswitch allows one to generate a "sweep start" in order to set the controls without having to feed a signal.  The LED labelled Gain looks like it illuminates and dims with the settings of the attack and decay controls.

[ElectricDruid]

Looking at the drawing my sense is that it does not require use of the footswitch to operate.

+1 agree. The foot trigger looks like an optional extra. It's designed to trigger from incoming notes played, but you have a manual trigger if you need it too.

[Mark Hammer]

Here's a circuit for converting a gate to a trigger: https://www.haraldswerk.de/Gate_and_Trigger/Gate_2_Trigger/Gate_2_Trigger.html

[jonokt12]

Excellent, that's pretty much exactly what I'm looking for. I'll try to breadboard up this schematic and see how I go! Thank you for all the help.

Mark that looks like a very simple setup which would be very useful if I end up using the ENVGEN.

[Ben N]

The circuit Tom shows is not the E&MM String Damper, but is oriented to the same goal,  Looking at the drawing my sense is that it does not require use of the footswitch to operate.  Rather, the footswitch allows one to generate a "sweep start" in order to set the controls without having to feed a signal.  The LED labelled Gain looks like it illuminates and dims with the settings of the attack and decay controls.

https://www.muzines.co.uk/articles/the-string-damper/7881

"
Now is the time to decide whether you require on-off or momentary operation." So, no.