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Old 13th January 2010, 10:16 AM   #1
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Default Scaling voltage for VCA

Hi everybody,

I have a rather basic question that unfortunately exceeds my knowledge! (or at least exceeds my logic!)

I'm currently designing the side chain for a compressor project at university. I have my full wave rectifier designed, built and working. I understand that the ratio is simply a pot to adjust the amount of voltage through and the threshold is some DC voltage added to the side chains input. The problem comes with the chosen VCA.

I've decided to use an Analogue Devices SSM2018 VCA. The data sheet states it has a control voltage gain of -30mV/dB, with 0V being a 0dB change, and 3V being maximum attenuation (-100dB). Negative voltage also applies gain, up to -1.4V for +40dB.

I'm struggling to work out how to scale my side chain voltage to work with the VCA. I'm assuming I need an exponential amplifier referenced to 0.775V (0dBu) to convert the logarithmic dB input to a linear dB output, then an amplifier to adjust the gain to be a 30mV difference between each dB?

I'm also struggling to understand how the VCA applies 0dB compression at 0V. Surely this means that ANY signal being put through would be compressed? How do I raise the threshold so that the pure audio signal will not trigger the VCA until it is summed with the DC voltage for the threshold setting?

Sorry about the big post, it turned out longer than I intended! To sum it up, my main areas of confusion are getting the input voltage scaled to -30mV/dB, and the 0V threshold of the VCA. Thanks in advance!
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Old 13th January 2010, 11:43 AM   #2
dhaen is offline dhaen  Europe
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Quote:
Originally Posted by dan_thompson87 View Post
Hi everybody,

... struggling to understand how the VCA applies 0dB compression at 0V. Surely this means that ANY signal being put through would be compressed? ...
Not sure if it's a typo, or a failure of logic, but the VCA chip does not compress, it only attenuates. So multiplying by 0dB is easy. I presume your side chain produces the control voltage to feed the VCA.
I've used these chips, but only as VCA volume controls, and they do exactly what it says on the box.
At this moment I can't get my head round why you would need an exponential amp, but that might be because I've no experience of compressors....
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Old 13th January 2010, 12:08 PM   #3
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Sorry, it is a typo.

Basically, I have my audio signal splitting off to the side chain at the input of the VCA. For ease of explanation lets say the audio is coming in at 0dBu/0.775V, so I have a 0.775 control voltage (without any ratio or threshold settings). This would attenuate my audio signal on the input of the VCA by 0.775/30mV = 25dB.

Let me to try to explain the way I see it so that you can tell me if I'm correct or not! If the VCA attenuates by 0dB at 0V, this is the threshold right? That would mean that any +ve voltage input is above the threshold and attenuation would occur? This is no use for my compressor as the audio needs to pass through unattenuated below my desired threshold.

As for the exponential amp, I assumed that to work on the -30mV/dB scale of the VCA control voltage, I must convert the logarithmic dB input into a linear output with -30mV between each decibel?

Again, these are all assumptions based on my limited knowledge!
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