phase and timing alignment over bus structure?

tk2k · 12 November 2012 21:07

Hi all,

I’ve got a very technical question, but it’s also real-world based so I thought some of you might have an answer to this…

We’re getting a Nexo rig in a few days, and I came across this tidbit in the manual.

I mix LR+S currently, and that seems like it could cause a problem…

quote:
Precautions
It is common to use the AUX send of a mixing desk to drive the Sub section of a PA system. This gives

the mixing engineer more flexibility to set the level of the subbass relative to the main PA, apply special

effects, or to use a different EQ on the Sub. However, it also raises some serious issues for the

performance & safety of the system (mostly time alignment).

At NEXO, great care is taken to design optimum phase alignment from one octave above to one octave

below the crossover frequency point. By doing so, drivers are working perfectly together and providing

the best efficiency possible. It is then up to the user to adjust the delay on the NX TDControllers to

match the physical path difference of the different systems. It is thus possible to get a well adjusted

system, even without measuring instruments.

If RS15s are driven form an AUX output, NX TDController is fed with two signals coming from different

sources. If those two sources (MAIN output & AUX send) are not exactly in phase, delay is introduced

into the crossover between GEO S12 array and RS15s.
</blockquote id=“quote”>

They suggest

quote:
Before using different outputs of a mixing desk, ensure that MAIN and SUB outputs are
in phase;

Never add additional low pass filtering on the SUB output or high pass filtering on the

MAIN output;

Always apply identical processing (EQ etc…) on both outputs, so that the phase

relationship between MAIN and SUB is not altered;
</blockquote id=“quote”>

So, it sounds like I should gang basically everything except level, routing and mute on the sub feed to the mains? I read something in the 1.9 release notes about… well ill just quote it below, should mean I’ll have no trouble?

quote:

Coherent Mix buses

Mix buses of the same type are now time coherent to avoid comb filter effects when combined, for example when mixing Groups together
</blockquote id=“quote”>

iDR-48, T-112, Mixpad

College

steffenromeiss · 12 November 2012 21:40

That may be true that changes in any bus affect the coherency of the system. But the main problem for uncohereny is the distance between the systems. That is more harmful than any processing.

At least the room adds “processing” that affects the system tuning more than an additional EQ in a separate sub out.

But keep in mind " less is more"… [ ]

quote:
At NEXO, great care is taken to design optimum phase alignment from one octave above to one octave
below the crossover frequency point. By doing so, drivers are working perfectly together and providing

the best efficiency possible. It is then up to the user to adjust the delay on the NX TDControllers to

match the physical path difference of the different systems. It is thus possible to get a well adjusted

system, even without measuring instruments.
</blockquote id=“quote”>

That should be standard since the late 90ies… It’s called time alignment.

quote:
So, it sounds like I should gang basically everything except level, routing and mute on the sub feed to the mains? I read something in the 1.9 release notes about.. well ill just quote it below, should mean I'll have no trouble?
quote:

Coherent Mix buses

Mix buses of the same type are now time coherent to avoid comb filter effects when combined, for example when mixing Groups together
</blockquote id=“quote”>
</blockquote id=“quote”>

This quote has nothing to do with the separated outs for subs. It means that the summing of busses was not completly coherent. As in the analog world was always.

So don’t worry about the statement from Nexo. It’s save to use separated sub outs if you do a proper time alignment if your speakers spaced…

SRV-AVB

R-72, iDR-16, xDR-16, Dante

mdsaudio · 13 November 2012 01:34

Eq, no matter where it is applied, will always cause phase shift(unless using FIR filters).

Not just a GEQ, but channel eq, crossover filters etc.

Once the system is set up and aligned, any eq should be done across the whole system, not on individual passbands. Nexo is not doing anything new here, as stated above this has been common practice for a long time.

Darren

jens-droessler · 13 November 2012 21:29

You should also gang level. Having different levels on the main outs and the sub out from the inputs will vary the crossover frequency between main system and subwoofers on a per channel base. You will get overlap or gaps in the response, both undesirable. The only thing you can do with a dedicated sub out is turning channels on and off, but not change the level.

tk2k · 13 November 2012 22:08

quote:
Originally posted by Jens Droessler
You should also gang level. Having different levels on the main outs and the sub out from the inputs will vary the crossover frequency between main system and subwoofers on a per channel base. You will get overlap or gaps in the response, both undesirable. The only thing you can do with a dedicated sub out is turning channels on and off, but not change the level.

I understand that, but how is that different form changing the sub feed level on the channel though? Or are you saying that your sub feed level from the channel should be at 0, and the fader should be at level with the mains?

iDR-48, T-112, Mixpad

College

mervaka · 13 November 2012 22:43

Jens is bang on. From a practical point of view however, I think it’s overall more useful to use post-fader sends to a subwoofer bus per channel over using just in/out routing buttons per channel. I think of it as an EQ tool, but with a better phase response.

woutert · 14 November 2012 13:40

quote:
Originally posted by Jens Droessler
You should also gang level. Having different levels on the main outs and the sub out from the inputs will vary the crossover frequency between main system and subwoofers on a per channel base. You will get overlap or gaps in the response, both undesirable. The only thing you can do with a dedicated sub out is turning channels on and off, but not change the level.

If that is so, then why is there a sub level control per channel when using an LRsub set-up?

Wouter

IDR32, R72, Dante, Mixpad

laptop, TP-Link TL-WR1043ND

simonstpauli · 16 November 2012 11:03

Interesting topic, my 2 cents:

Time differences between mains and the sub aux don’t matter as long as they stay the same. Time align subs and mains and you’re done.
Changing fader levels are changing the acoustical crossover point, as are send levels. Time alignment should be done in a way that works over a wide frequency range to account for the crossover point shift.
Different EQ changes acoustical crossover point and phase.

Conclusion: Be very sure shat you do when doing subs on aux. When I’m doing system I do it for BEs that like to have the system this way, when I’m mixing I tend to just go for a complete system.

Christian Tepfer

Hamburg, Germany

jens-droessler · 18 November 2012 02:16

quote:
Originally posted by woutert
If that is so, then why is there a sub level control per channel when using an LRsub set-up?
</blockquote id=“quote”>

Because people going for “subs off aux” are used to it. Most of them never think about the implications and consequences. If A&H just put a button for “sub on/off” for this matter, a lot of people would ask “how can we set a level?”. They are a manufacturer and not responsible for you to use the equipment properly.

Also, we have a adjustable highpass on each input, and if that’s not enough, we can switch the lowest EQ band to a highpass too. You can work a lot more precise with those controls than with a “per channel highpass set by the PA manufacturer/the system operator” (cause that’s what “subs off aux” actually is). Subs off aux is completely unnecessary and a waste of busses on a full fledged digital mixer. It was an idea in the old days to overcome shortcomings of cheaper mixing desks, and some people took this “technique” along on their way to the top.

mervaka · 18 November 2012 11:17

While on the subject of bank switching…

I would like to see an option that allows mixpad to move up or down in fixed banks, rather than the scrolling method currently available. It would just be a faster way of seeking channels for me.