Blinds. How does it work?

Hey, I’m a beginner in electronics and have been studying the Blinds schematics, and I am unable to figure out how it works! I’ve modelled parts of it using an online simulator but I am unable to see how a) an input to j1 comes out the other end and b) how the module can possibly act as a four quadrant multiplier.

I have studied Veils, and understand how the v2164 pair works there as a VCA, but I’m unable to see how a VCA can work to pass an audio signal when the audio is really working as the modulator of the VCA.

Anyone who would like to help me understand?

Hope this is in the right form (and subforum)!

Let’s say your signals are between -10V and +10V.

IN is your (bipolar) input signal (carrier).
CV is your (bipolar) cv (modulator).

Your toolkit consists of op-amps to add and subtract signals, and unipolar linear VCAs which compute V (IN, CV) = IN x CV / 10, but only for unipolar CVs (no restriction on IN though).

You want to use this to build a four-quadrant multiplier that computes IN x CV / 10 even if CV is bipolar. Let’s do a couple of algebraic manipulations…

IN x CV / 10
= IN x (CV / 10 + 1) - IN
= IN x (CV + 10) / 10 - IN
= V (IN, CV + 10) - IN

And that’s it… Shift the CV high enough to ensure that it is unipolar, and compensate by removing the input from the result.

Now let’s get to the tricky bit. Because four-quadrant-multiplication is commutative, you can swap the inputs IN and CV and get a circuit that should (in theory!) output the same thing. Indeed we have V (IN, CV + 10) - IN = V (CV, IN + 10) - CV. At least algebraically…

Blinds’ circuit computes V (CV, IN + 10) - CV, that’s why you see the audio signal going into what you’re believing is the control path of the linear VCA ; and the CV going through two branches.

So why this odd choice, of computing V (CV, IN + 10) - CV instead of V (IN, CV + 10) - IN?

Let’s have a look at V (IN, CV + 10) - IN… In the real world, the two terms in this equation will go through different paths, the first term will go through a 2164 cell and an op-amp before hitting the op-amp that does the subtraction, the other term won’t go through that, so the first term will have some tiny bit of distortion, noise, high frequency roll-off and slew-limiting. Which means that when CV is 0, V (IN, CV + 10) - IN won’t be zero, but will contain some faint garbage (high frequencies that are not completely nulled, higher harmonics from distortion, bonus noise). However, V (CV, IN + 10) - CV will be 0 as expected, first term is zero because the VCA has good offness, the second term is zero.

Since people expect the output to be silent when CV is null, I have chosen the V (CV, IN + 10) - CV variant. This swap is not magical - its downside is that if IN is null, we’ll still hear the high frequencies of the CV bleeding through. But in a typical modular applications, when using Blinds as a VCA, the signal going into IN is rarely silent; and the CV rarely has super hard edges. So it’s less of an issue…


Thanks, Olivier, this was super helpful! Not least understanding the design consideration for getting a better silence! Thanks for taking the time to write this and thank you so much for opening your schematics and source. It really is a fabulous resource for learning.

I now have a better understanding of op-amps and how you can just use them as algebraic operators.

(Hopefully a) quick question. How is the 2164 protected from the V- disconnection failure mode?


Thank you.

What’s the difference compared to Make Noise ModDemix (obviously it’s quad instead of double…)

As far as I know, the ModDemix uses a diode circuit which is much dirtier.