Re: [AH] "calibrating" my audio mixer?

From Brian Willoughby
Sent Fri, Jan 4th 2019, 05:47

On Dec 28, 2018, at 11:01 AM, Mark Spano <xxxxxxxxxxx@xxxxx.xxx> wrote:
> I think somewhat simply, you should look through the documentation for =
your gear and parse for the word "unity". If you can find it in regards =
to signal flow and gain staging, it should illuminate the way. The ideal =
is to have all gear connected to each other, passing signal along =
between each other at "unity" gain. That is, no added or removed =
amplification other than maintaining its nominal level. A lot of gear =
won't have this information, and then you kind of have to gauge it =
yourself. In those cases, I would play a pure (sine, triangle, etc.) =
tone, and adjust for 0 VU / -20 dBFs on output, and see if you can land =
that same level on input to the incoming piece of gear. That gives you =
decent level going out, accounting for enough headroom for additive =
tones and manipulation without clipping.

This is good advice, but it might be more difficult than you think.

* That pure sine or pure triangle will be quieter than other patches =
with more going on. Different wave shapes might produce high =
peak-to-peak voltages than other wave shapes. If you have a two =
oscillator or three oscillator synth, then you really should crank up =
all of the oscillators in the synth=E2=80=99s pre-filter mixer to =
determine the maximum patch output level. If you have a polyphonic =
synth, then you should endeavor to play full chords that engage all =
voices. If you have velocity assigned to level, then you=E2=80=99ll want =
to set levels against maximum velocity. If you want your studio setup to =
be =E2=80=9Cset and forget=E2=80=9D then you have a lot of work to do up =
front.

* Even with all oscillators cranked up to full volume, there might be =
still more headroom beyond that if you set the VCF to extreme resonance =
and find a particular frequency that aligns oscillator and filter. Since =
this sort of signal path might occur in some of your synth patches, you =
probably want to make sure that your output volume knob and mixer input =
trim are set to avoid distortion for maximum resonance.

* Some filters reduce gain with resonance, others do not. This makes it =
more difficult to find the maximum output.

* If there are any effects on your synth, that adds another dimension of =
level variations to deal with. Some effects add a lot of noise, too.

* Finally, the concept of unity and nominal levels only makes sense for =
mixers and devices that have a standard output level (like a CD player =
or a tape deck). A synthesizer does not fall into this category. There =
is no unity level on a synth, because there is no standard level. Every =
synth has a different power supply voltage, and thus the waveforms have =
different levels. I=E2=80=99ve looked at synth schematics where some =
waveforms use the full bipolar supply for peak-to-peak levels, while =
other waveforms only use the positive supply, and thus are 6 dB quieter. =
Even the CD player broke from the standard for audio electronics that =
came before, because they wanted to increase the signal-to-noise ratio. =
But synths have no standard output level, so you can=E2=80=99t really =
say what is nominal, what is low, and what is high. Adjusting for -20 =
dBFS might not be the best choice. Targeting -18 dBFS or even -12 dBFS =
might make more sense. If you actually can reliably determine the =
maximum level that your synth can output, then you might as well set the =
channel for -d dBFS or even -3 dBFS.

* The concept of unity might be useful, though, if you can find out =
about the final circuits in your synth. Some synths might have a lot of =
gain in the final output stage, and so you will probably get better =
results if you leave that final stage at unity instead of boosting the =
volume drastically. Other synths have nothing but attenuation at the =
final stage, so you might do best there by leaving the master volume =
knob at maximum. It really depends upon the synth.

* My experience with older MOTU interfaces is that they have gain, but =
not attenuation. If that=E2=80=99s true of the 828 mk3 and/or 8pre, then =
the synth master volume will have to be turned down to avoid clipping.

Brian Willoughby