pw8888

AC audio to DC converter ?
« on: May 19, 2020, 02:35:20 AM »
HI Everybody
thanks already for your help !
Sorry for such a basic question (hope I am posting in the right place)
I have a couple of VCA I need to listen and test .
I would like to control those audio DBX VCA directly from my DAW (digital audio workstation).
I was thinking of generating a frequency (like 1khz) and find a converter AC-DC

Basically looking to convert my audio signal , or steady generated frequency I should say , and then be able to generate a 0-5vdc voltage from it
The more I increase the LEVEL from my DAW output (specific output of course) … the more DC voltage I get
Is there an easy way to do that ?
just looking for the converter itself , I already have the vca etc etc working

Thanks for your help   ;D ;D ;D

P

that was the plug in I was planning to use
« Last Edit: May 21, 2020, 04:36:26 AM by pw8888 »


squarewave

Re: AC audio to DC converter ?
« Reply #1 on: May 19, 2020, 03:14:59 AM »
Well that sounds like a dubious thing to do but I like it if you can create the control signals reasonably. I don't have that much experience with VCAs but I do know that the control voltage has to be completely free of any modulation in the audio range or it will feed through. So your AC-DC converter needs to be pretty much perfect.If your DAW and converter can put out frequencies above 20kHz you should start there. Go as high as you reliably can. Then run it through a bridge rectifier and low pass filter. Maybe make an LCRC filter where the LC takes out the control frequency. That way you can limit the size of your inductor. Use those Bourns high-Q shielded inductors. Then make the RC can go as low as you want the VCAs to react. If you want them to be able to "waveshape" symbol crashes, then the RC would have to go all the way up to 20kHz. But if you're just using them like faders you could go all the way down to 1Hz-ish and pretty much eliminate the possibility of feed-through. Design the whole thing to a characteristic impedance of 1K or so. It could be completely passive. Unfortunately I don't think you can have series capacitors. If you could, you could generate negative or positive control signals. Maybe with small pulse transformers you could. Maybe even make it differential and use both positive and negative.

pw8888

Re: AC audio to DC converter ?
« Reply #2 on: May 19, 2020, 03:46:01 AM »
ok I see
I would like to pilot some THAT DBX202XT .. that are already running on their own inside an SSL console (external card I have of course)
the DAW could  provide the 20khz (max 22...KHZ) if running at 44.1K of course
now....I understand what you propose.... but is there any "board" of "kit" available , or even "active" component that could do it ?
I don't think I am not able to actually create it from scratch . put together would be easier :-)
I post a schematics as well regarding the circuit I have .
then instead of the pot ... doing what I am talking about .
maybe there is another way WAY easier to do it ...and I missed it  ?
maybe a midi to cv converter would do ? but then it need to be controller by a fader inside the daw....
thanks for your help !



« Last Edit: May 19, 2020, 03:59:08 AM by pw8888 »

abbey road d enfer

Re: AC audio to DC converter ?
« Reply #3 on: May 19, 2020, 04:56:48 AM »
HI Everybody
thanks already for your help !
Sorry for such a basic question (hope I am posting in the right place)
I have a couple of VCA I need to listen and test .
I would like to control those audio DBX VCA directly from my DAW (digital audio workstation).
I was thinking of generating a frequency (like 1khz) and find a converter AC-DC

Basically looking to convert my audio signal , or steady generated frequency I should say , and then be able to generate a 0-5vdc voltage from it
It's not terribly hard to do, but there are some caveats.
You could use a full-wave precision rectifier
https://electronics.stackexchange.com/questions/412542/how-does-this-precision-full-wave-rectifier-work
The value of C2/R5 will determine how well the output signal is smoothed, but also how fast it can react to changes in input signal amplitude. this is something you would need to experiment with.
Now, some details: the control law is in V per dB. Let's say gain at 0V  is XdB, gain at 5V will be X-100dB. That means that no audio signal from DAW -> max gain, 5Vpeak from DAW -> 100dB attenuation. If that works for you, it's ok.
Now I don't think there are any kits available for that circuit.
You may make it on perf board if you need just a few of them, but if you want to fit that into all channels, you need to have PCB's made.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: AC audio to DC converter ?
« Reply #4 on: May 19, 2020, 05:08:49 AM »
Hello

What is the purpose ? fader level or compressor CV ?

long time ago, before motu offer volta plug-in (CV pilot via soundcard) I drive 8 VCA (automation) directly via my old 828
You have to generate a DC sound file (I draw it in a wave form editor) and have a DC coupled converter, nothing more is requested
The issue was more the CV scaling, as I played DC as audio via DAW track and fader (log)

For fader function with your VCA, It will be way simple to use a midi to CV board.
Have a look at midibox,  AOUT_NG board (with 12bit TVL5630) It work.
I have (working) prototype with this, driving 12 VCA (4mono +4 stereo) but don't go further as it was a parallel development to my fader automation, and no longer need for VCA.
You'll be able to drive CV via DAW fader (HUI/MCU) and scale the CV as requested (both soft and hardware side)

Best
Zam

JohnRoberts

Re: AC audio to DC converter ?
« Reply #5 on: May 19, 2020, 09:49:59 AM »
What Abbey said...

the typical circuit is called a "precision rectifier" and there are many variants.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

pw8888

Re: AC audio to DC converter ?
« Reply #6 on: May 19, 2020, 02:53:54 PM »
thanks for all those precious infos !
the goal is not to pilot some faders...nearly :-)

I am building a mix between a SSL4K and 9K , basically a summing system.
I would like to be able to control those VCA from protools , to be able to do "analog" automation (from my DAW).
and of course be then recallable inside each mix session.
I will have a very simple LCD  "reader" in the front plate  ... and a some pot of course when I want to use it without the DAW.
summing will be  16:2
I need to think again and see what is best and more easy to do !
thanks again !
P


squarewave

Re: AC audio to DC converter ?
« Reply #7 on: May 19, 2020, 03:50:08 PM »
Not sure it's really necessary to go active. You could use a tiny $3 pulse transformer with a center tap to make +- control voltages and drive the VCA control ports deferentially. The schematic would be effectively just like that of a dual linear power supply with much smaller surface mount parts. At low attenuation/gain levels the rectifier diodes would give a little bit of a reverse log characteristic. Less so if you can find a pulse transformer with a decent step up/down.
« Last Edit: May 19, 2020, 06:57:18 PM by squarewave »

Re: AC audio to DC converter ?
« Reply #8 on: May 20, 2020, 05:11:41 AM »
Hello

the goal is not to pilot some faders...nearly :-)

I am building a mix between a SSL4K and 9K , basically a summing system.
I would like to be able to control those VCA from protools , to be able to do "analog" automation (from my DAW).
and of course be then recallable inside each mix session.

What I say, for "fader" volume automation a midi to CV like midibox is cheap and work great.
Using NG system (on the stm32F4) you'll be able to scale CV from daw fader value to VCA value and talk to hardware with MCU (midi) protocol allowing automation and recall.

Best
Zam

pw8888

Re: AC audio to DC converter ?
« Reply #9 on: May 21, 2020, 04:32:09 AM »
Hello

What I say, for "fader" volume automation a midi to CV like midibox is cheap and work great.
Using NG system (on the stm32F4) you'll be able to scale CV from daw fader value to VCA value and talk to hardware with MCU (midi) protocol allowing automation and recall.

Best
Zam


HI Zam
Thank you so much for the tips and infos
Because of my lack of knowledge ... I have some question regarding the NG system in relation with fader.(hope it is fine to ask here)
what I miss here is what would be the "connection" between  the fader and the vca.
what I mean by this ... fader in protools are carrying audio . I would "of course" create some virtual fader with a tone generator (that was my idea) but with the NG I would use midi right ?
Midi would be converter to DC (that I get) ... but still
sorry for this basic question ... but I think I don't get the "global " picture here
thanks for your help !

P


that was the plug in I was planning to use
« Last Edit: May 21, 2020, 04:35:44 AM by pw8888 »


Re: AC audio to DC converter ?
« Reply #10 on: May 21, 2020, 05:57:09 AM »
but I think I don't get the "global " picture here

Hello

The STM32 core connect to your computer (USB) and emulate a mackie controle
Up to four 8 channel 12 bit DAC can connect to the core, and generate CV (that you can scale and offset electronically (0/5V -5/+5 etc...)
In your DAW setting a controller will send and receive 14bit data (RPN 0 /Pitchbend) which will translate directly at the CV output controlling your VCA.
At DAW some options depending of your work flow and way of thinking it.
-If you don't care audio and automation lane not in the same track. Create blank audio or bus track, dedicated to analog VCA control via the faders where you can write and send automation/volume
-If you like automation lane at the same track as audio, send audio prefader to your summing box assign a blank bus/output to track output, and use the fade for analog VCA automation only
Both of those option eliminate a redundant pass in the DAW fader then VCA "fader" (which will produce cumulative and wrong attenuation)
The MB_NG software offer map in the config files, where you can scale the curve from 14bit pseudo log DAW fader and the math log of your VCA, by this VCA attenuation reflect exact value of your DAW fader

BUT if you are in protool which use HUI (tweaked midi), I suspect you will be restricted to 9bit value (512 steps at your VCA)

Hope it's clear

Best
Zam

abbey road d enfer

Re: AC audio to DC converter ?
« Reply #11 on: May 21, 2020, 06:21:48 AM »
what I mean by this ... fader in protools are carrying audio .
Actually, faders carry a digital word that controls gain in the DSP.
Basically you have two solutions:
either, you do what you suggested, converting an audio voltage to a control voltage, that's what has been discussed before.
or, you're asking this digital word from the fader to be broadcast to a box that converts it to an actual voltage. MIDI is the protocol of choice to do it, since it allows one MIDI track to control many VCA's.

I surmise part of your question is where to connect the voltage?
You should connect it to the point labelled CV, that connects to R578, and which receives the fader voltage taken from its wiper.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: AC audio to DC converter ?
« Reply #12 on: May 21, 2020, 06:48:08 AM »
since it allows one MIDI track to control many VCA's.

Yes that's the third  solution, using only one midi track to control the VCA, which can be a good option (depending of accuracy requested)
you can use RPN/NRPN like DAW fader in with MCU protocole, but you have to be sure one track can handle all of them or even that your DAW can send NRPN (not so obvious unfortunately)
The only drawback of this solution (which can also be done with a midibox setup) is that midi value are not so friendly for feedback of the VCA value, DAW fader show dB and with proper scaling/mapping what your see at daw fader is what the VCA do

Best
Zam

pw8888

Re: AC audio to DC converter ?
« Reply #13 on: May 21, 2020, 07:39:54 AM »
Zam thanks for the very clear explanation... and all of you guys thank you ! ;D
this is clearly the path !
now.... I have to do my homework :-))
put everything together and see what is the best solution  (specially let's see what protools can do or generate !)
I am sure I will be back with some question ... but first lets learn it
speak soon
p


 

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