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WMD digital VCA style custom unit?

Just thinking out loud here but just occurred to me that perhaps it would be possible for the 301 to emulate this digital VCA in order to eliminate click and pops from sounds very fast envelopes?

Still new to the game and I think I’m going to give it a shot but that’s an unit perhaps one of you experienced user could whip up quickly? @NeilParfitt @Joe ? :wink:

wmd digital vca

Here is how to do it with the Sample & Hold unit:

  1. Insert a Sample & Hold unit in the level sub-chain of your Linear VCA.
  2. Set the source of the level sub-chain to your super-snappy envelope.
  3. Set the (thresh)old of the Sample & Hold trig parameter to zero (to detect zero-crossings).
  4. Set the source of the trig sub-chain to be the same audio input as the Linear VCA.

This will process your envelope with a S&H, so that it is only updating when there is a zero-crossing in the audio.

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Love that vca! I have 4 of those modules in my rig.

It works by only opening the vca exactly on a zero crossing, preventing the click noise most of the time.

I can’t think of a way to detect zero crossing using current sets of units.

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Here is an audio example on a sequenced TTA Z3000 sine wave with the snappiest envelope a 4ms PEG can generate:

Starts with no pop-suppression and then half-way through I enable the S&H.

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Oh and if it is an internal oscillator that you are shaping then just feed its sync input with the same gate signal that is triggering the envelope.

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This is really clever, I wouldn’t have thought to do this.

I would like to say that this is one of my favourite aspects of the ER-301, it really does make you think about how to patch up solutions to problems rather than reaching for the mouse and browsing for modules to solve them! I kinda knew what I was doing anyway, but I’ve definitely learned a lot over the last 10 months or so - thank you once again!!

And to repeat previous exclamations along similar lines, it’s really great having so many folk around who ask these questions and others who know how to solve these challenges even if I don’t :smile:

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Sorry, I have to ask:

If the S&H fires when a zero crossing happens, doesn’t that mean that the S&H just samples a single static voltage from the envelope every time there’s a zero crossing, meaning that the envelope is very choppy?

I’m sure that the case here is either:

The envelope doesn’t fire until there’s a zero crossing OR
The envelope doesn’t affect the VCA until there’s a zero crossing BUT

I just can’t figure out why things are happening. In the spirit of mutual learning, right? :slight_smile:

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It works because you’re trigging the S&H with audio i.e. it is firing very rapidly, but only ever at a zero crossing of which there are a lot!

If you need further explanation let me know :smiley:

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Thanks @anon83620728. So far I had only read Brian’s recipe and listening to his audio sample, it clearly works. I didn’t fully grasp why either, and was just going to build it later and figured it would make sense when I saw it in action.

Now that you describe it this way, I think I get why it works. You’re sampling your envelope at zero crossings but the sample rate is extremely high - undiscernable other than removing the click.

Looks like one for the recipes wiki to me. :slight_smile:

Yep … that’s right!!

The reason why the clicks occur because you’re going from no signal to a strong signal almost instantly, perhaps think about how the speaker would have to move to accommodate that.

I think you’re right about the wiki recipe :slight_smile:

Aha that’s just great glad I could contribute with an idea.

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Yeah always wanted one of these but oh wait I have a 301!

oh wow we must have been typing at the same time as i didn’t see your reply with the solution!! Awesome!

Thanks kel! It makes sense now, but it’s unintuitive it’s not very choppy :smiley: But if it’s just the volume level updating in very very quick steps, I guess it figures that there’s no audible difference.

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I think you’re not quite there yet, I may be wrong, but think about what’s happening step by step and remember it’s happening really quickly:

First without any envelope signal,

  1. Envelope signal is 0
  2. Incoming audio passes through zero-crossing
  3. S&H triggers
  4. Envelope output is sampled and ‘Hold’ at 0

Then with envelope signal at 1 i.e. not zero lets say the attack phase goes from 0 - 10 just to make it easy to understand,

  1. Envelope signal is 1

  2. Incoming audio passes through zero-crossing

  3. S&H triggers

  4. Envelope output is sampled and ‘Hold’ at 1

  5. Envelope signal is 2

  6. Incoming audio passes through zero-crossing

  7. S&H triggers

  8. Envelope output is sampled and ‘Hold’ at 2

Etc.

There will never be any choppiness because the value is held at the last known value!

Of course there are many many more sample and holds, so the held values follow the incoming envelope signal at a high enough resolution that you will never be able to hear anything other than the envelope working as it should.

Does that make it clearer?

As an aside, you actually only need the S&H to work right at the beginning of the envelope so that the moment the envelope opens the VCA and the volume goes from 0 to something it’s not at a point where the signal has to jump. After this initial transition it’s not necessary, it’s just easier to leave it running than try and work out a way to only activate it at the beginning of an envelope - ugh!

Also remember that if you have a slow attack this is much less of a problem because the first sound that is allowed through is quiet anyway. This technique is only necessary when you are using a very short attack and you want to avoid opening the envelope while the audio signal is high which is why clicks happen. All the does is make sure the envelope opens at the same time as the audio transitions a zero crossing.

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Maybe a better word than ‘choppiness’ would be ‘staircasing’ or similar :slight_smile: In a sense, a side effect of the S&H is that the envelope’s resolution is reduced.

I THINK I have it, but that’s never stopped me from being wrong… thanks for the explanations :slight_smile:

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I think it would definitely ‘staircase’ it. But if you put an external envelope from an analog module into the 301, the ADC will technically staircase/sample it too, right? But the steps are so small/fast, you don’t notice it. The sample rate is high enough that it’s indistinguishable.

When I thought about it that way, it helped me understand why this doesn’t cause a problem.

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Good point :slight_smile: every digital thing has steps, and I’m not too worried about it, at all – I’m actively trying to cut down on obsessing over tech details.

Thanks guys for the great discussion. So nice to talk when there’s such a lack of one-upmanship.

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Haha yes - actual civil discourse - it’s a winner for sure :smiley:

It’s not exactly relevant here, because it’s a different process, but staircasing is one of my favourite subjects and I don’t know anyone who debunks the myths surrounding this topic any better than Monty - what a dude!

I think this is worth watching several times, I know @Joe has seen me post it before at least once hehe, I may even have posted it here before too, but it is so good it is worth repeating!

Enjoy :smiley:

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The process effectively tries to reconstruct the enveloped version of the input using only sinusoids that are harmonically-related to the sinusoids already present in the original input. However, since it can only use harmonics, it fails to reproduce the pop but does a good job of reproducing everything else.

Thought experiment:

  • Compare the spectrum of a sine wave to its half-wave rectification. See that rectification introduces no inharmonic frequencies.
  • Now convince yourself that rectification is just a special-case of the ZCD VCA where you use the most drastic envelope possible, a gate.
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