Understanding sub phase/group delay for placement

[jtalden]

I checked Rotel manual and found it confusing, but don't think the XO points can be 'mixed' at different frequencies. The low and high pass filters in the XO will always be at the same frequency. There is the option to run the FL and FR speakers as large however. that may or may not be something we already looked at. My understanding of the manual is that the SW XO is a global setting that is applied to all XOs set as small speakers unless that setting is overridden in the advanced speaker setup menu. In that case that new XO frequency will applied to that particular pair of small speakers. I suggest you leave the SW XO setting at 120 as I think that it also applies the LFE filter to the SWs. I would then use the advance menu as you listed above to set the actual XOs to be applied to the small speakers. It's possible I may not be understanding the Rotel manual correctly, if you want to confirm how it actually works you could take measurements with different settings in these two menus to confirm the actual operation.

I don't really recall the the detail of the actions we took but a quick look suggests it was just to establish favorable XO timing. What is it you are trying to achieve - a smoother SPL response? The PEQ settings should be helpful, but some problems cannot be effectively addressed without moving the speaker and LP positions or with acoustic treatment in ways we are not able or willing to do.

 

[FargateOne]

My goal here is again to check if I understand phases and delays to find a favorable XO timing in regards of the possibilities to mix SW XO and main XO with Rotel. So forget the PEQ for the moment. With your help in this thread, I chose delays for one SW XO settings of 80Hz for all my satellites and the sub. All speakers are set to small and we didn't try to set fronts to large in this exercise.

A long time ago...(in a galaxie far far away) I took a lot of measurements to understand the way Rotel applies the xo and how an Xo for the sub and a different xo for the satellite work. I summarize those in 3 files here. I tried to to be clear with the name of each measures. Tell me if it is OK to proove that when I set a global SW xo and a different xo in the advance speaker setup menu for the fronts, for instance, the LPF for the sub remains at 100Hz but the HPF for the main changed from 100Hz to 80Hz. If you agree (after seeing the mdat) I would like to measure and see what happens to phase and delay in relation with a favorable timing between the sub and the mains.

 

[jtalden]

I would like to measure and see what happens to phase and delay in relation with a favorable timing between the sub and the mains.

Well I am still not entirely clear on how the Rotel works, but it sounds like you have a particular combination of settings that you want to try out. I can help with that analysis. If there is a big difference between the SW XO and mains XO settings it will clarify what the Rotel is actually doing. If both XOs are set close to the same value we may not learn much about that, but just choose the settings you want and we can find the most favorable delay setting.

Measure full range:

1. FL (alone)
2. FR (alone)
3. SW (alone using channel 1 the FL with the FL main disconnected)

The SW needs to measured this way because we need the redirected bass. If we measure using channel 4 (the LFE channel) then the XO is normally not active and should not be used to determine favorable XO timing.

 

[FargateOne]

OK, here is the mdat .

AVR settings

• no PEQ
• all speakers to small
• Distances/Delay
  • Fronts Left and Front Right 270cm
  • Center 210cm (physical distance is 285cm) I chose this distance trying to found the best timing after that I push back the center speaker by 30cm to minimise sbir effects using the learnigs here and the alignment tool...
    
  • Surround Left 225cm
  • Surround right 240cm
  • Sub distance 240cm (physical distance 255cm) The distance chose with your help see #37.

XO SETTINGS FOR THIS EXERCICE

1. SW xo to 100Hz
2. FL and FR advanced mode XO set to 50Hz
3. CC advanced mode XO set to 80Hz
4. Surround Left (SL) and Surround Right (SR) advanced mode XO set to 100HzMeasurements

Measurements

• mic at MLP
• timing reference surround right
• sub measured with FL disconnected
• full sweep 20-20kHz

Edit: change the mdat because measurement 3 was labelled FL instead of FR. new mdat with measurement 3 identified correctly

 

[jtalden]

I completed the analysis of the XO timing for the FL, FR. The action of the Rotel XO settings tracked your description of them rather than the conventional method. As a result there is a little wider effective XO range of about 30-150 Hz.

The most favorable SW delay timing was found with a change of -3 ms (+1 m distance in the Rotel) from the current setting and with the SW polarity reversed (negative polarity):

Below is the SPL of the FL, FR and FL+FR using this SW setting:

 

[FargateOne]

Hi,
maybe it is a dumb question but I just want to check if I understand correctly. When alignment tool found a delay it is always in regard of the reference speaker. So as indicated in Rew help: " the delay will corresponds to the difference in their distance from the mic compared to the distance of the reference speaker. If the reference speaker is further away the value would be negative"

The most favorable SW delay timing was found with a change of -3 ms (+1 m distance in the Rotel) from the current setting and with the SW polarity reversed (negative polarity):

Here is the note in the note aera:
Aligned copy of SW xo 100-copy
Delayed by -3,0000 ms (-1,029 m, -(3 ft 4,5 in))

So I must add 1m to the sub because now the "reference speaker" 1m further away.

Another thought. In post#37-38 it was suggested to change the delay of the CC from 255cm (tape measure to mlp to 165) in order to better match the phases. It was good. Is it a good idea to do it now?

 

[jtalden]

Hi,
maybe it is a dumb question but I just want to check if I understand correctly. When alignment tool found a delay it is always in regard of the reference speaker. So as indicated in Rew help: " the delay will corresponds to the difference in their distance from the mic compared to the distance of the reference speaker. If the reference speaker is further away the value would be negative"

Here is the note in the note aera:
Aligned copy of SW xo 100-copy
Delayed by -3,0000 ms (-1,029 m, -(3 ft 4,5 in))

So I must add 1m to the sub because now the "reference speaker" 1m further away.

+1 m is the correct distance to change the SW distance in the Rotel. I just remember that the when changing distances in an AVR An increase in time delay is a reduction of distance.

Another thought. In post#37-38 it was suggested to change the delay of the CC from 255cm (tape measure to mlp to 165) in order to better match the phases. It was good. Is it a good idea to do it now?

The timing of the CC looks good, but the polarity needs to be reversed. See the chart below showing the changes needed. The timing adjustment is only +0.11 ms that is -3.3 cm distance reduction in the Rotel which may be within the resolution limit of the Rotel. The polarity needs to be changed though. Just reverse the wires going to the CC.

 

[FargateOne]

The timing of the CC looks good, but the polarity needs to be reversed. See the chart below showing the changes needed.

Thanks! After acouple hours of listening session, I can say that a better match of phase at MLP is very important. It sounds better when we try to get the best phase tracking between mains and sub. But also we must trust our ears. I my particular case, I must do some comprimeses. For example, I can't flip the polarity of my sub although I can change the phase from 0 degree to 180 degrees; also, having more than one sub is not an option, placement is best I can and no other options are availble.
So, compromise I said. Ther is one. If I change the sub phase to 90degree instead 180 (in order to achieve a facsimile of inversing polrity) it seems to my ears to better integrate all the satellites with the sub.
That's the fun with our hobby!