AND Gate vs. VCA

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AND Gate vs. VCA

Post by hinterlands303 » Sun Dec 01, 2019 6:45 pm

I regularly use AND logic in my patches and since I don't have a dedicated module for AND logic I use a VCA to do this. I've considered picking up a Pulplogic 1u AND logic tile (it's around $25) but I realized I could get another VCA for not much more.

I was wondering, is there any advantage to using a dedicated AND logic module over using a VCA instead?

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Post by kpreid » Sun Dec 01, 2019 7:01 pm

Each digital logic input has a certain voltage threshold which it considers "high". So if you patch two CV sources (call them A and B) to an AND and its output to another digital input, you get the logical condition "if A > T_AND and B > T_AND" where T_AND is whatever threshold voltage the AND circuit uses. But if you use a linear VCA instead, you get "if A × B / 5V > T_the_destination_thing" (where "5V" is actually whatever control voltage puts the VCA at unity gain).

So, if you are starting with two analog CV sources, you will get different behavior. The AND provides a "rectangle" of input conditions where the output will be high (in a 2D plot of the two input voltages against each other), but the VCA has a curvier result because it's multiplying.

If your inputs are low-or-high only it could in principle still matter if one of them has a high level that's too low to get the VCA up to unity gain, in which case the VCA won't work at all. But that's unlikely since most gates are at least 5V and most VCAs will be unity at 5V (as far as I have noticed).

In summary: the difference matters a lot if you like to play with sending continuous CVs to digital inputs, and not very much if you don't.

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Post by kindredlost » Sun Dec 01, 2019 7:40 pm

A well outlined summary kpreid!

The only other advantage of a dedicated Logic module over a VCA is if the output from the logic module is different and more appropriate for your use than the VCA output is.

If you consider a logic module then I would look for a stacked or interactive logic module first. Like a PLOG from Intellijel. The amount of "stuff" you can get out of a module like that puts its value way over the top compared to the modest or lower price of a single logic module. My two bits FWIW.

You can probably find one used for a decent price. I bought one when they first came out and sold it. Mistake. I bought another as soon as I could and wish I had four of them.
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Post by hinterlands303 » Mon Dec 02, 2019 12:17 am

Thanks for the responses!

kpreid - I rarely use analog sources with logic although I do use XOR logic at audio rates quite a bit, but I tend to use square/pulse waves. I see what you mean though. If I were to use two CV sources and use the output to trigger a LPG the results would be quite different between a VCA and an AND gate. At one point I had CMOS based AND Logic in my system and I remember it sounded quite different at audio rates than amplitude modulation with a VCA so it makes sense that other continuous sources would behave differently as well.

kindredlost - PLOG has always looked awesome to me and I've come close to ordering it on several occasions. I do have a micro Ornament & Crime (Hemispheres) that I can use to emulate some of the functions of PLOG (using cv to switch logic states), but it's usually being used in other ways. Also I think there are some advantages to splitting the functions up into separate blocks and the 1u tile format is great for that.

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Post by hotstepper » Thu Dec 05, 2019 12:23 pm

I don't really know if there are any advantages to using a VCA vs AND logic, but recently I got the Abstract Data Logic Boss and I like it so far.

4 inputs, two different sets of logic outputs including pretty much every kind of logic I'm aware of, and the ability to create links between logic events.

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