Adding Envelope to an Output?

I was wondering on this simple thing: get the output of a PWM signal and apply an Envelope to it by using yet another PWM output. But I have no idea on how to do this.

First I would need to convert the PWM into a Voltage, then use it in an Voltage Controlled Envelope, also know as ADSR->VCA (Atack Decay Sustain Release Envelope to Voltage Controlled Amplifier)

If anyone has any idea on a simple way to do this, as the idea is to do on multiple voices, not just one, so a complex system won't work.

I could do the envelope in the digital world, but since we are using 8 to 10 bits, it sounds terrible. Having it in the Analog side would make things much smoother.


Forgot to mention this great site, on how to convert PWM into Voltage:


Oh, multiplying DAC, great idea, I will check the options out. But from the price point, it will get expensive and hard to handle.

What I would love to do is, for instance, use an Arduino Mega multiple PWM outputs instead, as the external filter is much cheaper than a single DAC per channel.

Imagine this, right now, BeatVox uses 1x 16-bit timers to generate two outputs a 0 to near 800 value, that's over 9 bits and near 10 bits, and it sounds pretty nice.

Now, the Arduino Mega has 4x 16-bit timers that can drive up to 12 outputs like that. (at least in theory) Imagine now that you have 6 outputs and 6 envelope-outputs. The code will handle, as right now we are already getting 6 voices on the BeatVox on a single ATmega328 (Arduino Uno/Duemilinove) board at 16Mhz.

But I'm just a big dreamer!!! ;-)


Here’s an interesting DAC I found on eBay. Any hints on this one?


Interesting page on some VCA modules:


Why not create multiplying DAC inside of the arduino itself? Instead of PWM output, you need 8 digital pins with R2R leader. Tone's amplitude would be multiply by envelope and send to resistors network / filter. That shouldn't be a problem with Mega board to build 6 voices with unique envelope for each of them ( 6 x 8 = 48 ). Two tones for other smaller boards, that have at least 16 pins.

Edited: Just realized, probably it would sound awful with 8 original bits scaled down to even less... But idea would still be practical with 16 bit or 24 bit R2R. Only less outputs 'd be available

Magician: Edited: Just realized, probably it would sound awful with 8 original bits scaled down to even less... But idea would still be practical with 16 bit or 24 bit R2R. Only less outputs 'd be available

Yup, that's the whole problem. That's why I want to do something in analog-domain. ;-)

I did some testing with OP-Amps, but since I have on clue on what I'm doing, it did sound good, until the volume lowered, then it didn't sound that good anymore. So I guess its a bit more complex to handle this out. Those VCA schemes I posted above are nice, but still a bit complicated, if I want to add 6 or 8 times those circuits, to get multiple voices. :astonished:

Take, for instance, the Beat707 SY LE project I did:

If I could add at least 4 x 8 bit PWM outputs as the Envelope CV, that would be great. It would end up with at least 4 voices with 2 Square Osc each, not bad, all in a single chip design. Now add another ATmega328 or even run it at 20 Mhz, and improve even better the system. Then I could, later, find a way to add analog filters to it, also in simple terms.

I was hoping to get that with transistors or simple op-amps. But I don't know, it seems it would be more complicated than that. I will experiment some more another time.

The idea is to keep the whole thing as simple as possible. I know I could do all this in code, like the MidiVox did, but it only does one voice and I didn't like the sound...


Just apply the envelope to a voltage controlled amplifier. This is what the Moog was based on back in the day, it is a very simple circuit.

Dude, read above, I even posted a link to some VCA schematics above. ^_^

What I'm trying to figure out is the simplest circuit possible, as the ones I found are still a bit "complex". :grin:


How bout a 1 transistor VCA?

It was used in the 808 drummachine (heh, a subtle tribute to the line of drummachines that I’m assuming was the inspiration for the beat707 name) but it can only do 1 polarity voltage.

But hey, the arduino only does positive voltages! How convenient!

Here’s the page it’s from:
DIY 808 circuits

Here’s the actual VCA:

Not sure what the values should be.

I found direct schematics for the 808:
808 schematics

Attached is a screenshot of part of the circuit that uses the transistor VCA, in the open hihat generator.
The left is coming from the envelope generator. Below this part would be the signal source. And it would be outputted to something on the right.

Picture 1.png

Oh! Oh! Oh! Thank you so much! I'm impressed I missed that one out! :blush: Thanks again.

Tomorrow I will check which parts I need, I don't have many transistors here, so I have to check which would do the job. Here's the list of what I got already:


        12 x 2N5087 PNP
        20 x PN2222 NPN
        25 x 2N3904 
        25 x 2N3906


BTW: Beat707 was named after the TR707 I had for a while, it was actually my cousin's, had to return to him...


Some interesting posts here:


Glad I could help

It seems that one of the main thoughts on that forum is that the transistor VCA is not good for say, a mixer. Too lofi. but the lofi-ness shouldn't be a problem for the arduino, you're not looking for the best sound quality in a synth ever.

I’ve been looking for a while during the last month to find a cheap and simple way to do a VCA, spent a whole day trying to do some tweakings with a BF245 and playing with its variable resistor polarisation, and eventually happen into a 4 Fet schematic with a lot of resistors…

Digging into the schematics of my old Synths like the paia Fatman showed a good tradeoff, but still costly according to your needs :confused:
Try a look at this :

it’s a kind of tradeoff, I chose the simpler way to do it : in digital world, using an 8bit LUT, one multiplication and a 8 bit shift for scaling. It’s very efficient though.

Quote from: Magician on September 20, 2011, 02:01:39 PM
Edited: Just realized, probably it would sound awful with 8 original bits scaled down to even less… But idea would still be practical with 16 bit or 24 bit R2R. Only less outputs 'd be available

Yup, that’s the whole problem. That’s why I want to do something in analog-domain. :wink:

The problem only I can see, is not that design isn’t possible in digital domain, but only less outputs would be available in digital domain solution, than if we use PWM filtered to analog → to VCA, just because PWM use single pin. So basically problem narrowed down to shortage of digital pins, ( for 8 voices x 8-bit envelope for each, would require 16 x 8 = 128 outputs ).
It could be easily solved with shift register extender.
Analog multiplier (1 transistor VCA) posted above create more distortion than 4 bit sound, and should not be considered seriously. More sophisticated VCA require more components, and higher price IC for all design.

What I’m saying, that shift registers + R2R 16-bit (homemade DAC) would win for
quality to simplicity/price ratio.

If we set amplitude resolution to less than 256 levels, R2R could be simplified to 14 (64 levels) or 12 (16 levels ).

But all that would take too many cycles and sample-rate would lower too much. To be honest, I could just add Envelope at 8-bits, but it would require multiplication or division and it slows the whole thing down to a point that I get like 1 voice.

Now, imagine the current Beat707 SY LE, which does 8 voices, now, combine that with 4 PWM outputs, there's no extra cycles added, and the Envelope math is very simple and is not per-sample calculated, so it would still fit on the code. Doing the VCA outside in analog domain would make all this work like a charm. Its only 4 voices, but still, 4 dual OSC voices, not bad for a single ATmega328. ;-)

Another path would go nuts and use another chip for processing audio, like my XMOS Shield idea, but that's another talk, check the other thread. ;-)


Than just add digital pot, single chip: AD5204 / AD5206

Ah, I forgot about those, I did check some last time but they are a bit expensive, but them again, its a single chip solution, so I will check this out again, thanks. :blush:


How about this one?

I like that it has 4-channels for twice the price of a single channel chip.