Mixer / Pre-Amp Levels

screenscribe wrote: »

I'm working on a module for a TOA 9000 series Mixer / Pre-Amp. The device has 8 input channels and 8 output channels, each of which has a gain setting. I'm looking for advice on how you guys might normally control the gain settings on a device like this? Would you normally preset one side of the equation (say the output channels) to a 50% level and then just adjust the input side up and down...or vice versa?

Cheers,
Matthew

Audio Engineer to the rescue!!!

You adjust the output for room volume control and leave the input gain alone. The easy way to think of the input gain is it is used to make sure that when you switch from one source to another the percieved volume stays the same. So, on devices that are softer than other sources you can adjust the input gain.

I find that iPods and Cable boxes tend to be lower output than other things. So, I end up trimming the other stuff down or add gain to the lower stuff.

Another piece of advice for you...

It is important in audio to maintain good "gain structure." In digital audio processors, the closer you are to the peak level, the greater the bit depth in which your analog signal will be sampled.

That being said, I like to run my DSPs as close to digital overload (dBFS) as possible without hitting the red line. It's best to get your output volume control by means of analog attenuation at the amplifier.

That's the basic idea, as has been mentioned use your input gain to set up a nice gain structure then attenuate on the output.

The best way to think of it is say compare it to a photograph with a really low depth of field. The part of the audio that is in sharpest focus is infinitely close to 0dBFS, as you drift below this everything starts to get a bit more fuzzy as you loose bit depth. Now in the analog world you're got a bit of leeway (headroom) above 0dB before things start getting fuzzy on that side and as you drive it harder the wave form clips in a nice rounded sort of way, however in the world of digital, if the input level happens to go over 0dBFS it can't and is just clipped, full stop. To use the photography analogy its as though your lens just got smashed.