32C Channel Plugin

[madmaxmiller]

All right, day 3:

Subject:
Can I replicate a setting in Mixbus32C channel simply by setting the same dial to the same amount in AVA32C channel strip?

Possible outcomes:
1) If yes and yes (same dial+same amount)--> the two are most probably identical.
2) If yes and no --> this could reveal a gain bug suspected by others, you can still achieve the same result with the same ease but slightly different dial parameters (e.g. x dB more or less in the frequency band gain)
3) no + yes don't exist, really (amount is meaningless)
4) If no and no that means they are not the same at all (but still unique each, proven at lease for the AVA32C in my former findings and empirically found for the Mixbus32C in thousands of mixes all over the world)

So we are ready to go, here are the three signals:
Patchbay:
Signal feeds into a control channel in Ardour ("SIGNAL"), into a Mixbus32C utility bus ("Bus1") and into the AVA32C plugin.
The outputs feed into the Ardour channels "MB32Cv7" and "AVA32Cv1" which contain RTAs for signal check after the respective MB/AVA channels (or input check into the Ardour channels respectively).
The two channels feed into Ardour Master bus which does the summing and contains the RTA for the result.
All RTAs are derived from one instance ("SIGNAL") via drag & drop to ensure the exactly same settings for an optically correct result.

   

Left: original signal from TAP Pink Noise/Fractal generator
Middle: direct output from the Mixbus channel
Right: direct output from the AVA channel

   

Again, I isolate the EQ here only to eliminate complexity and stick to the original question: are the EQs the same and are they the same as any stock EQ? The results for AVA versus the Reaper stock EQ are in my other two postings, this is between Mixbus and AVA. Let's begin:

1) as always, a dry test first: both EQs OFF, one signal flipped, signals should be exactly the same, therefore it should "null"

SURPRISE!
- both signals come in at -6 dB, the straight sum shows logically -3 dB, all good
- BUT: suddenly there's a dip at 63 Hz! Where does that come from? Can only be a phase shift, right? And since this didn't happen when comparing the AVA with the ReaEQ I suspect the Mixbus32C channel being the culprit

Whatever I did, activating/deactivating one or both EQs, no matter which, the result stayed the same. This ends my experiment for today, because there's no point flipping phase, as we see.

The same area around 63 Hz seems overrepresented when 1 channel was flipped (sounds logical) while the rest appears unchanged - completely as if I would use pink noise from two different generators.

This again seems to indicate a statistical but random phase shift which accidentally comes close to 180 degrees around 63 Hz. Would it be a fixed amount phase shift we would see comb filtering in the sum. And this must happen *before* it hits any channel DSP.

sum straight

   

sum with 1 channel flipped

   

This of course gave me an idea: if this phase shift sits somewhere in the input stage, the the outputs of two channels in Mixbus32C should be exactly the same and the "damage" is done already and evenly. Let's check this:

Top: sum straight with 2 MB32C channels
Bottom: sum with 1 channel flipped IN ARDOUR, NOT in Mixbus! I also flipped around (haha) in Mixbus and both Mixbus and Ardour at the same time - as long as the sum was 180 degrees it nulled.

   

CONCLUSION:
Mixbus 32C and AVA 32C EQs cannot be compared directly and mathematically via null test as Mixbus changes the signal in the input stage. This somehow contradicts Ben's mentioning of a bug in the AVA plugin which's output in fact seems to be more phase coherent to the input. Also, as shown in my previous experiments, I couldn't find a gain error.

This phase shift seems to be statistical. Next step would be testing the Mixbus32C channel with sweeps and record the RTA to find out more.

I'll write Ben an email, Harrison may be interested, however that takes nothing away from Mixbus because if this is a systematic bug in the input stage it will occur the same way in all channels and if it still sounds good all is good. Or is this the

Secret Sauce ™ ? 

MMM