Hi everyone,

I own a pulse generator and a function generator with voltage controlled functions (trigger/gate in, vco, vcf).
Operating them through the soundcard kind of works but not really, for the DACs send too low a voltage and have those capacitor / dc offset limitations you may know about, caps of wich i don't want to mess with (the soundcard is a cheap one but is the only one I own and works well as a soundcard as is).
I read here and there that one could use the DACs in the VGA outlet to drive analog synths, but I could never really find a thorough DIY report on such a hack.

I wish I could find a way to get Pure Data to operate my analog gear through VGA.
Has anyone here ever tried this?

Any help would be appreciated.
Cheers.

Sebastien.

@Sebastien I think you will have to get your soldering iron out.
As you say, audio dacs will probably not do 0Hz and will not supply enough voltage.
PWM will be bad news (noise on analog buss).
A usb data acquisition box would be expensive-ish and not report through the os as an audio device.... so useless.

Even a very expensive audio dac that can go to DC would be channel restricted.
You would still need an op-amp

There are chips that can do frequency to voltage conversion..... that would take your existing dac output.
You would need to scale and trim to the output voltage to match the VCA voltages of your synth, or maybe just use the synth voltage rail if the chip can cope with that voltage.
And you would need to build a patch sending appropriate frequencies and scaling..... that is.... do the opposite conversion in your patch...... voltage to frequency.
http://www.ti.com/lit/ds/symlink/vfc32.pdf

Otherwise....... midi to CV.....
https://www.muffwiggler.com/forum/viewtopic.php?t=68228&sid=9b7253b1ad45a1e75a4bb2f3795a56c7
David.

Thank you David for your reply.
I guess I get it.
One more thing, though. What do you mean by "PWM will be bad news (noise on analog buss)."?

@Sebastien Sending square wave pulses into an analog system would most likely generate some nasty high frequency harmonics, if not simply noise.... which you might be able to screen or smooth out, or you might not. Hard to tell without trying....... but?........
P.S. motorised potentiometers with digital control are another option...... but you would have "steps".

For PWM and motorised pots you might need to go with a RPI / arduino.
Which makes me think..... there might be some cheap "robotics" usb stuff that you could talk to through comport.
Motorised pots keeps the pulses away from the analog circuit with any luck. I don't mean motorising the pots on the synth. I mean using a motorised pot to send a voltage to the synth.
David.

Okay, thank you David. I am still processing your replies.
I think I'll go for a expensive-ish dedicated chip.
Is your guess that this kind of VGA hack never even existed ?
Have you ever heard of such a hack ?
Sebastien

@Sebastien I have seen discussion of the dac >> VCA.
I think most have gone the midi >> CV route, with proven success.
I don't know whether the systems they have built could translate NRPN for 14-bit resolution, or if they were stuck with 0-127 (8-bit signed).

This looks like a very (VERY!) good place to start...... http://wiki.midibox.org/
In fact this looks like a real solution with good resolution.....
http://wiki.midibox.org/doku.php?id=aout_module

I am asking myself if I read your OP correctly.
I read VCA (voltage controlled amplifier) instead of VGA.
Did you mean the analog video VGA output from your computer?
VGA sends data in discrete steps, so PWM-ish data..... RGB + H+V sync (0-0.7Volts).
You might be able to use capacitors (pF probably) to smooth the RGB channels and then op-amps to get the desired control voltages.
If you could "window" the data first you could also get a large number of channels.
Every screen pixel could be set with [pix_set] giving 3x (screen resolution) channels.
An interesting project?
David.

@whale-av said:
I am asking myself if I read your OP correctly.

I read VCA (voltage controlled amplifier) instead of VGA.
Did you mean the analog video VGA output from your computer?
David.

Yes, I did actually mean the common built-in analog VGA screen outlet that every computer would feature back in the days, prior to DVI . Wich both voltage range and lack of DC limitations would allow voltage control purposes (allegedly, i mean this is something I would come across a couple of times on analog synth DIYers forums, not knowing at all wether it could be an actual thing they are able to do.

@Sebastien said:

Wich (...) voltage range.
I mean when amplified.

@Sebastien Very sorry then...... to have taken you on a wild goose chase.
Yes, for 3 channels you could use the R G and B pins.
The colour bit depth of VGA will be good for a smooth waveform output.
You would need to boost the voltage enough for your synth controls...... simple op-amps will do.
The only problem will probably be the "black burst" in the analog output.
The "horizontal blanking interval" here,.......... http://www.eecg.toronto.edu/~tm4/rgbout.html
A small capacitor would smooth that (and help with noise), as it occurs at 50-75Hz usually for VGA.

If you need more channels you would need to "window"....... use the HV sync and a clock to mask the data...... capturing individual pixels or blocks of pixels, and then a larger capacitor for smoothing.

Splitting the signal to a monitor you could have a very nice colourful display of your output as well!
I am very rusty on the electronics.
The Pd patch would be easy (I think).
David.

@whale-av said:

Yes, for 3 channels you could use the R G and B pins. The colour bit depth of VGA will be good for a smooth waveform output.

Yes that's what I was thinking too

You would need to boost the voltage enough for your synth controls...... simple op-amps will do.

I do agree

The only problem will probably be the "black burst" in the analog output. A small capacitor would smooth that (and help with noise), as it occurs at 50-75Hz usually for VGA.

Yes. It would just be great if I could tell the DAC not to dial those bursts in the first place. I don't know how those do work. Whether they get information from somewhere or rely on built-in circuits locked on a particular sync signal rate, I don't know, I am way out of my field

Splitting the signal to a monitor you could have a very nice colourful display of your output as well!

Crazy cool