Please explain the function of a Low Pass Gate?

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Naenyn
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Please explain the function of a Low Pass Gate?

Post by Naenyn » Mon May 21, 2018 12:19 pm

So, I've been reading all I can about how LPGs work, what the point is, etc. Watched some videos, read some articles.. yadda yadda.

I still don't quite get it. Apparently they were (are?) a popular secret sauce. I get what a gate is.. and what a low pass filter is.. but .. I just can't seem to wrap my head around LPGs. :despair:

Could someone take a stab at explaining it? Maybe then I can ask questions related to the explanation and can figure out whether I should add one to my synth rack or not.

Thanks! :tu:
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Post by hermbot » Mon May 21, 2018 12:26 pm

So a VCA is just a volume control, right? If you plug in the original source and turn the knob all the way down, the input signal is fully attenuated and goes away.

A filter (VCF) can do the same thing, if you turn it down far enough. If you lower the cutoff frequency eventually you'll attenuate all of the original audio, so the signal will be gone.

A low pass gate does both at the same time. It lowers the volume and a filter cutoff frequency simultaneously, so as the signal gets quieter it also loses high frequency content.

Why does that matter? It's a very natural sounding way of attenuating something. When you impact something in the real world, the high frequencies are usually attenuated first while the low frequencies ring a little bit longer. Low pass gates are often called "organic sounding".

Second, if you hit a low pass gate with a really short envelope, it will "ring" a little bit due to the nature of the vactrol, or photoresistor. This decay sounds really neat. Different vactrols have different response time, so differences from unit to unit will be apparent.

West Coast synthesis is pretty popular these days, and a big part of that sound is the low pass gate. Look up "buchla bongos" for more reading on that.

I don't know if they're necessarily a secret sauce or will change your life, but hey have a distinctive sound that some people really like. (Myself included.)
Last edited by hermbot on Mon May 21, 2018 12:28 pm, edited 1 time in total.

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Post by ersatzplanet » Mon May 21, 2018 12:27 pm

It is basically a LP filter being used as an audio gate (not the DC gate from a clock or keyboard etc). Tuning the LP all the way down, or below the signal of the audio going through it, gates it off. Basically it is a Low Pass Filter configured to act like a VCA.
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Post by Naenyn » Mon May 21, 2018 12:45 pm

Thank you both for explaining! I think the key thing I was missing was this:
hermbot wrote:When you impact something in the real world, the high frequencies are usually attenuated first while the low frequencies ring a little bit longer. Low pass gates are often called "organic sounding".
It makes sense now!

So, the common use of the gate is to gradually attenuate the higher frequencies towards the end of a note? Would this be controlled by an envelope? Could someone describe a very basic patch that would include a LPG?
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Post by pre55ure » Mon May 21, 2018 1:52 pm

Naenyn wrote:Thank you both for explaining! I think the key thing I was missing was this:
hermbot wrote:When you impact something in the real world, the high frequencies are usually attenuated first while the low frequencies ring a little bit longer. Low pass gates are often called "organic sounding".
It makes sense now!

So, the common use of the gate is to gradually attenuate the higher frequencies towards the end of a note? Would this be controlled by an envelope? Could someone describe a very basic patch that would include a LPG?
In general, most basic LPG patches usually use the "pinging" technique mentioned above. There's not usually a need to use an envelope as the LPG essentially has its own decay envelope built in (the natural decay of the vactrol).

Basic patch can be as simple as OSC -> LPG (signal input) -> Mixer
And then Trigger -> LPG (CV input)

The trigger signal is usually just a really short (5ms or so) transient. You can use an envelope set to it's shortest decay and release or a pulse from something else as long as it is very short without much sustain.

In theory you can recreate the same sound with independent elements (filters / envelopes / vca's etc...). But in practice, a good LPG just has all of those elements blended together in a way that is very pleasing to the ear.

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Post by Naenyn » Mon May 21, 2018 3:12 pm

Awesome, thanks for the explanations!
-n :ninja:

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Post by starthief » Mon May 21, 2018 3:33 pm

I usually do use envelopes with LPGs.

The traditional lowpass gate uses vactrols -- (a light emitter and detector packaged together as an "opto-isolator") -- which have a tendency to decay more slowly than the signal that controls them. That's where the "pinging" comes from, rather than resonance as in filters.

However, vactrols vary a lot in response, and a lot of them are too short to be musically ideal. My Doepfer A-101-2 is just a quick "chuck" or "chirp" if I just give it a trigger, and Dynamic Impulse Filter isn't much longer (despite "faking" the vactrol electronically). And then there's Make Noise Dynamix, which is sort of an LPG in that it rolls off high frequencies as it closes, but it responds as fast as a VCA.

Using an envelope gives you a lot more control (letting you vary the decay times for accents or expressive purposes for instance, or use an ADSR with a low sustain to sort of drag out the decay a different way.)

Streams, Natural Gate and the upcoming Single Andore (and possibly others) have built-in envelopes :)
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Post by cptnal » Mon May 21, 2018 3:40 pm

Agree with ST on the envelope thing. You can also get a lot of tonal variation by attenuating the envelope.

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Post by leftbracket » Mon May 21, 2018 6:58 pm

Another advantage of using an envelope is that on LPGs with dedicated ping inputs (e.g. Optomix) you can use both that and an envelope for accents or varied rhythmic patterns.

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Post by mskala » Mon May 21, 2018 8:03 pm

I think "pinging" a filter usually refers to feeding a short signal (such as a sharp envelope itself, or a burst of noise gated with a sharp envelope) into the audio (not control) input of a resonant filter without changing the filter cutoff. Normally you adjust the feedback so that the filter almost oscillates, but not quite. When pinged, the filter generates a signal with an extended decay after the input is gone. The patch and the sound are both quite a lot different from using the filter as a low-pass gate, even if some filters sold for use as low-pass gates can also be pinged; and it's caused by the filter resonance, not by vactrols, even if the filter also has vactrols in it.

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Post by wackelpeter » Tue May 22, 2018 10:48 am

i regulary ping my LPG's with triggers... mostly from the CGS gate to trigger converters... high Amplitude, not as normal triggers but the decay of the LED add's a smooth edge to it... and well you can also attentuate the trigger pulse via the CV control Input to get different respnoses... mixing trigger pulses with other CV voltages work but mostly the trigger looses it's sharp characteristics with most of my mixer modules... perhaps a few faster OP amps in the mixer might be worth a try...

This works best in VCF and a little bit less spectacular in VCA/VCF mode combinded... in VCA mode i prefer "real" envelopes.

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Post by captjrab » Tue May 22, 2018 2:36 pm

Not to derail but Ive always wondered:
VCF in front of the LPG or after? Prolly it depends, so both, right?
I usually do VCO/filter first then LPG.

Been digging Optomix lately. I attenuate the strike trigger for variable tone and a nice clock divided eg to cv in to fade up and down the filter. I never used the damp in until last weekend. Thanks to another thread on Muffs for reminding me about it. Good for adding another syncopation to the mix.

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Post by wackelpeter » Tue May 22, 2018 3:13 pm

often use my VCO'sor other Sound source straight into LPG and then into the mix when i use them in VCF mode with trigger pings... sometimes i then feed them through an VCF most likely the Mutant Vactrol VCF using both outputs (one channel to the left and the other to right in the Stereo mix) so i have the filtered voice then panning form left to right or vice versa... sounds really cool

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Post by starthief » Tue May 22, 2018 3:18 pm

captjrab wrote:Not to derail but Ive always wondered:
VCF in front of the LPG or after? Prolly it depends, so both, right?
I usually do VCO/filter first then LPG.
I don't use a filter with an LPG, unless it's for filter FM, or the filter is the sound source, or the filter is being pushed enough to create instead of remove harmonics.
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Post by mskala » Tue May 22, 2018 4:48 pm

starthief wrote:
captjrab wrote:Not to derail but Ive always wondered:
VCF in front of the LPG or after? Prolly it depends, so both, right?
I usually do VCO/filter first then LPG.
I don't use a filter with an LPG, unless it's for filter FM, or the filter is the sound source, or the filter is being pushed enough to create instead of remove harmonics.
Yes - remember that the LPG is a filter, and you don't usually need two. But if you can make a sound that you like by using two filters, then, obviously, do it!

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Post by BenA718 » Tue May 22, 2018 5:42 pm

Well, this makes me want to get one and try it out now. Thanks, Muffs! lol :doh:

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Post by captjrab » Tue May 22, 2018 6:12 pm

mskala wrote:
starthief wrote:
captjrab wrote:Not to derail but Ive always wondered:
VCF in front of the LPG or after? Prolly it depends, so both, right?
I usually do VCO/filter first then LPG.
I don't use a filter with an LPG, unless it's for filter FM, or the filter is the sound source, or the filter is being pushed enough to create instead of remove harmonics.
Yes - remember that the LPG is a filter, and you don't usually need two. But if you can make a sound that you like by using two filters, then, obviously, do it!
Thanks guys, been learning alot about LPG’s here. Ive used them for a few years and gotten all sorts of great sounds with them. Its good to catch some tips and get a fresh take.
I picked up a Metasonix RK 5 a few months ago and have had PEG plugged into it ever since for all kinds of syncopation, so more of a gate with natural tone. Next time I’ll run some slower envelopes and test out the filter range.

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Post by jkjelec » Tue May 22, 2018 11:37 pm

Assuming your specific LPG doesn't have a resonance control, does "pinging" a LPG with a trigger require there to be an input source like an oscillator, or does it make a sound all on it's own?

If the latter, and the specific LPG doesn't have a dedicated "ping" input. is the ping signal fed into the control input or the audio input?

Thanks!

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Post by pre55ure » Wed May 23, 2018 12:34 am

jkjelec wrote:Assuming your specific LPG doesn't have a resonance control, does "pinging" a LPG with a trigger require there to be an input source like an oscillator, or does it make a sound all on it's own?

If the latter, and the specific LPG doesn't have a dedicated "ping" input. is the ping signal fed into the control input or the audio input?

Thanks!
I apologize if I have introduced “ping” into the lexicon of miss used words, but I couldn’t think of a better thing to call it.
Anyway, if you “ping” a non resonant lpg with no audio signal going into it, you wont hear anything. Its like a filter or VCA in that sense (and sort of both realistically). The trigger signal goes into the control input (or “strike” if your make noise) and is analagous to triggering a short decay envelope patched to a filters cutoff and to a VCA’s control input at the same time.

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Post by Dilibob » Wed May 23, 2018 9:26 pm

Excellent Info ! I attempting to follow these suggestions and sent a envelope into a VCF with a triangle wave from a VCO, made a real solid "piano" note. Was surprised how natural it sounded, so this isn't just for making synth type of sounds. I guess another comment is "gated" low pass gate is very doable in 5u, but just takes a few more modules in 5u (rather then a single module LPG in EU format).

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Post by Euro Trash Bazooka » Thu May 24, 2018 6:22 am

hermbot wrote:So a VCA is just a volume control, right? If you plug in the original source and turn the knob all the way down, the input signal is fully attenuated and goes away.

A filter (VCF) can do the same thing, if you turn it down far enough. If you lower the cutoff frequency eventually you'll attenuate all of the original audio, so the signal will be gone.

A low pass gate does both at the same time. It lowers the volume and a filter cutoff frequency simultaneously, so as the signal gets quieter it also loses high frequency content.

Why does that matter? It's a very natural sounding way of attenuating something. When you impact something in the real world, the high frequencies are usually attenuated first while the low frequencies ring a little bit longer. Low pass gates are often called "organic sounding".

Second, if you hit a low pass gate with a really short envelope, it will "ring" a little bit due to the nature of the vactrol, or photoresistor. This decay sounds really neat. Different vactrols have different response time, so differences from unit to unit will be apparent.

West Coast synthesis is pretty popular these days, and a big part of that sound is the low pass gate. Look up "buchla bongos" for more reading on that.

I don't know if they're necessarily a secret sauce or will change your life, but hey have a distinctive sound that some people really like. (Myself included.)
Thank you for this amazing, concise and thorough explanation, sir. You made things much clearer for me as well. :sb:

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Post by dthorn » Thu May 24, 2018 12:09 pm

starthief wrote:However, vactrols vary a lot in response, and a lot of them are too short to be musically ideal. My Doepfer A-101-2 is just a quick "chuck" or "chirp" if I just give it a trigger
My A-101-2 hardly rings at all if I feed it a short pulse, but responds very well to stepped CV from a sequencer, like it has a weird unpredictable slew limiter built in. The slew time depends on the setting of the F/A knob, the magnitude of change in the CV, and the amount of time since the last change in the CV. Another useful thing about the Doepfer is that it's DC coupled, so it can process CV too.

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Post by starthief » Thu May 24, 2018 12:28 pm

dthorn wrote:My A-101-2 hardly rings at all if I feed it a short pulse, but responds very well to stepped CV from a sequencer, like it has a weird unpredictable slew limiter built in.
Hmm, I'll have to try that :tu:
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Post by dthorn » Thu May 24, 2018 1:26 pm

So it turns out my A-101-2 actually does ring pretty well, but it just needs a fairly long pulse in order to get a decent amount of decay. I can definitely hear it swinging shut when I use an LFO square wave at the CV input (I only got the module yesterday morning, so I'm still learning what it can do). Also, what I originally thought was the LPG not closing completely is just a very long decay tail, like it decays in a hockey stick kind of curve that lasts about 10 seconds before reaching absolute silence.

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Post by jkjelec » Thu May 24, 2018 2:10 pm

pre55ure wrote:
I apologize if I have introduced “ping” into the lexicon of miss used words, but I couldn’t think of a better thing to call it.
Anyway, if you “ping” a non resonant lpg with no audio signal going into it, you wont hear anything. Its like a filter or VCA in that sense (and sort of both realistically). The trigger signal goes into the control input (or “strike” if your make noise) and is analagous to triggering a short decay envelope patched to a filters cutoff and to a VCA’s control input at the same time.
Thank you pre55ure; you really helped me to understand these. Also I think your use of the term"ping" is just right.

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