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[Tim McCulloch]

https://www.youtube.com/watch?v=MMMmR1u0CFk

Knew i'd seen something on this somewhere! Dave Rat's blog from ages back. At about 6 to 7 minutes onwards he speaks about using compression while routing signals through a group and using a DCA. Uses both to change the compression of the mix and therefore varying the energy of the mix without affecting overall loudness. Just reinforces to me that DCA's and groups can be a musical tool as well as a techncial thing and they can work together in harmony.

I use Dave's bus/dca trick with compression to vary the perceived density of the mix bus content.  Some relatively small fader moves can make plainly audible differences.

[Nathan Riddle]

true but would the choice of the fader being log or linear become part of the equation.  So with digital boards do we have log or linear fader and is there a way to determine this by the manufacturer.

I think this was the big question/thought of mine.

If fader 1 is at -20dB and fader 2 is at -10dB and DCA is at 0dB

If a DCA fader controls the FADER (relative to its current position) rather than absolute dB then when moving the DCA -5dB results in fader 1 at -30dB and fader 2 is at -15dB.

I think we've sufficiently debunked/proved that most consoles' DCA/VCA change the dB for each controlled fader by the same difference in the DCA.
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I am curious mathematically if there are any differences between lowering summed inputs or lowering a summed output.

Fader A,B,C
D = reduction

DCA} A*D + B*D + C*D =
or
Group}  (A+B+C)*D =

Mathematically there isn't a difference?
Are my formulas even correct?

---

I think I'll try this out by adding signals on a sound board and see what the output differences are.

[David Winners]

I use Dave's bus/dca trick with compression to vary the perceived density of the mix bus content.  Some relatively small fader moves can make plainly audible differences.

This is why I purchased my first X32. I really like running bands this way. You can change the sound of the band with just a couple small adjustments. It's also nice for bands with lots of vocalists. It's easier to keep things in line when one becomes 5.

[Michael Lawrence]

I think this was the big question/thought of mine.

If fader 1 is at -20dB and fader 2 is at -10dB and DCA is at 0dB

If a DCA fader controls the FADER (relative to its current position) rather than absolute dB then when moving the DCA -5dB results in fader 1 at -30dB and fader 2 is at -15dB.

I think we've sufficiently debunked/proved that most consoles' DCA/VCA change the dB for each controlled fader by the same difference in the DCA.
-----

I am curious mathematically if there are any differences between lowering summed inputs or lowering a summed output.

Fader A,B,C
D = reduction

DCA} A*D + B*D + C*D =
or
Group}  (A+B+C)*D =

Mathematically there isn't a difference?
Are my formulas even correct?

---

I think I'll try this out by adding signals on a sound board and see what the output differences are.

VCA / DCA is simply a gain stage, or the mathematical equivalent. Electronically it is done per channel (no summing) but the dB change is the same for each channel. If you want to visualize it, check out the way Avid does it on the VENUE series desks, showing the ghost red faders for equivalent positions.

In your example, with Channel 1 @ -10 dB and Channel 2 @ -20 dB, moving the VCA to -5dB would change those levels to -15 dB and -25 dB respectively.

If you draw it as a block diagram it might be easier to visualize: a -5dB gain stage in series with each channel.  A group could be visualized as a summing stage with variable gain.

You're using the distributive property, which is mathematically valid and also a pretty good way of thinking about this topic.

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