# Group Delay Plots



## crossrh (Jun 18, 2006)

In the REW Help file, chapter 14, "Minimum Phase", there is an "Excess Group Delay" graph.

Is what is shown in the "GD" button of my sweep actually the excess group delay, or is there something else I have to do to show this? I'm trying to see if there is a delay between my subs and my main speakers, and it looks like this might be a good way to show the delay.


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## jtalden (Mar 12, 2009)

Excess GD is not available on the overlay plots.

You can find Excess GD on the main GD panel as shown below. 'Generate Minimum Phase' must be activated first so that REW will make it available.


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## crossrh (Jun 18, 2006)

Here's a screen shot of my GD page. I'm not seeing the same thing you are.


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## crossrh (Jun 18, 2006)

OK, I just noticed the difference between our two screen shots. I had to select "Controls". Here's my Excess GD:

Is there anything here I should be concerned about? I don't really know how to interpret these.


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## crossrh (Jun 18, 2006)

I'm including my mdat file in case there's anything else you'd need to see. 

If you can, take a look at my SPL and Phase plots. Should the phase look like that?

These are measurements of my Left main, set to "Small", and two subs, crossed over at 80hz in a Yamaha RX-V765, using "Straight" input mode, with AISO and an HDMI cable from the computer to the receiver, with the Dayton UMM-6 mike in the maim listening position. I eq'd the subs independently with a miniDSP.

Thanks.


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## jtalden (Mar 12, 2009)

You chart looks pretty typical. I don't normally use the Excess GD plot, but others find it helpful for some tasks. The large peaks represent room modes and strong reflections. EQ of nulls at those frequencies is often not effective. 

What is your situation and objective? Will you be using an automated setup routine? Will it all be manually setup? What equipment and room configuration are in use? 

Your SWs appear to be timed about 36.6ms apart at your mic position. Is there a 35ft distance difference? Probably not.  Maybe it's a reflection. They don't have be timed alike but, I usually start there and only depart for good reason. That said, the SW SPL looks good as is, so maybe this is the reason you chose that timing. Did you compensate (set a delay) for any difference is distance and then sum the SWs signal and EQ both SW alike? Do you have different plan?

There is also a hint that the XO timing at 80Hz is not ideal, but it is impossible to tell for sure with these measurements. The dip at 66Hz is probably a room mode, but proper timing may help the XO range.


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## crossrh (Jun 18, 2006)

I ran YPAO and used the distances from that. I assume there's an increase in the SW delay due to the signal going through the miniDSP and then on to the plate amps. YPAO said the subs were at 16', when they're actually at 13' and 11'. That's only a 3-4ms difference. I wonder if I increased the distance for the subs drastically, like 35', that would time-align the subs and mains better. But why wouldn't YPAO figure this out? isn't that the reason the set-up procedure measures distance (time?)

As I said, I eq'd each sub separately. I have two sub pre-outs and each one goes to its own channel on the miniDSP. miniDSP will let me add a delay to the subs, but that's not what I want; I want all the other speakers delayed to match the subs. I'll play around with the timing (distance) some more tomorrow and see what happens.

Looking at this picture I realized how small my TV is. My wife wants a bigger one. TV, that is. I knew I married her for a reason.


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## jtalden (Mar 12, 2009)

I don't see any benefit to increasing the initial SW setting and then rerunning YPAO. I would think the distances set should come out the same based on its measurements. I have no experience with YPAO however so if that is a recommended strategy then it may be worth a try.

Regarding the current settings:
Since there is no delay set between the SW channels in the MiniDSP and assuming that the AVR is sending both SW outputs the same signal (no delay between them) then the timing offset between the 2 SWs is 2ft or ~2ms. This is not enough to create a significant XO problem. This can be left as is. If YPAO added a delay to one of the AVR SW outputs then that may be a problem. Most AVRs don't have the capability to apply different delays to the 2 SW outputs though. You may want to confirm this.

My recommendation is normally to sum the SWs and apply the same EQ to both so the mono bass output is always just as you measured above, pretty nice and smooth. If they are retained as stereo and the SPL of each SW individually is nearly as good as measured together it should perform well also.

The major question remaining is regarding the delay/distance of the handoff between the SW and mains. The GD and phase charts hints that the timing may not be ideal, but there is no way to know for sure with these measurements. If it is idealized and the SWs are set to 16ft in the AVR then the mains would be expected to be set to their measured distance plus about 4 feet [16-((13+11)/2]. So, if the mains are 10ft from the LP, then they should be set at about 14ft assuming the polarities are correct. It's very possible to get very good results with other settings depending on polarity and other factors so it's possible that the YPAO AVR settings are good even if they are very different from this. One simple check to see if the settings are reasonable is to reverse the polarity on the SWs and see if a broad SPL sag results centered at the 80Hz XO. If so, then all is probably okay.

Given REW loopback timing capability and lots of new measurements it is possible to confirm or manually setup the SWs and mains timing/distances to a conventional target alignment situation. The resulting sound quality may not be preferred to other reasonable alignments however. If you are interested in experimenting I can assist.


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## crossrh (Jun 18, 2006)

I found the Denon AVR-X4000 for a closeout price, as I'll need another receiver for when I go to Korea for a year. This model has Audessy XT32, so I'll give that a try. It should be here Wednesday PM.

Thanks for your insights, jtalden. I'll report back after Audessy.


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