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Processing for Multiple Surrounds

post #1 of 10
Thread Starter 
I'm in the process of designing a 600 sq ft dedicated home theater that will have 2 rows of seating. I plan on having a 9.2 setup with 6 surrounds that consist of a side speakers for each row of seating and 2 rear surrounds. So, I'm not doing the conventional front wides or heights as others have done. So I'm curious to what others have used for processing the side surrounds to ensure the timing and levels can be matched independently. I know the Erskine Group recommends the QSC 322 which requires hiring someone who knows how to program one of these devices. I'm also researching using JRMC through a HTPC which has the ability to copy channels and set individual EQ, timing and gain for each channel However, that will only work for sources originated by the HTPC so I'm not sure how to integrate all my other sources into the home theater (mainly a DirecTV HD DVR or OTA box).

Looking for some suggestions on alternatives. I would also like to utilize a processor with balanced outputs and was originally looking at the Marantz 8801. Thanks.

David
post #2 of 10
Quote:
Originally Posted by dlbeck View Post

I'm in the process of designing a 600 sq ft dedicated home theater that will have 2 rows of seating. I plan on having a 9.2 setup with 6 surrounds that consist of a side speakers for each row of seating and 2 rear surrounds. So, I'm not doing the conventional front wides or heights as others have done. So I'm curious to what others have used for processing the side surrounds to ensure the timing and levels can be matched independently. I know the Erskine Group recommends the QSC 322 which requires hiring someone who knows how to program one of these devices. I'm also researching using JRMC through a HTPC which has the ability to copy channels and set individual EQ, timing and gain for each channel However, that will only work for sources originated by the HTPC so I'm not sure how to integrate all my other sources into the home theater (mainly a DirecTV HD DVR or OTA box).

Looking for some suggestions on alternatives. I would also like to utilize a processor with balanced outputs and was originally looking at the Marantz 8801. Thanks.

David

What about the QSC 922 variants? They can be tweaked via internet................................though are a little more coin than 322.

I'm looking at the Trinnov TEQ 12 channel...................by the time you figure two QSC units plus having calibrator fly/come to add their own special secret sauce.................both avenues become relatively equivalent in coin.
post #3 of 10
David this topic was discussed last week in a couple of threads, just scroll down. but this one has discussion of an alternative far cheaper solution than the QSC

http://www.avsforum.com/t/1488704/having-four-speakers-as-surround-for-two-row/40_40#post_23801950
post #4 of 10
There are several schools of thought about this - Dennis's approach seems to be the most thorough, but others (including those with credentials) claim that less is sufficient.

Best I can tell from reading about this, Dennis and Shawn would set up the QSC with both a delay and also an all-pass filter. (This is entirely inferred on my part, obviously I'm not speaking for them, or from experience.) The QSC unit, different from most others, has a fixed latency, which makes calibrating multiple channels very easy, compared to other less costly units where the processing delay adds up, making final alignment more challenging to determine. They may also use some other processing, but I don't know what it would be.

BIG posted just the other day that Dr. Floyd Toole - noted and sometimes controversial researcher - spoke at CEDIA and claimed that a 10ms delay was adequate.

Other forum members claim that they just split the signal and carry on, very happy with the results. Others split the signal and then adjust mounting position and/or level so that precedence keeps the image where they want it, again very happy.

If you want to emulate the performace of the QSC for less, there are other choices, like some Xilica procesors which get good reviews from professionals, but lack fixed latency. Then there are less costly choices - in particular miniDSP. I think an all pass filter, which changes phase based on frequency, requires FIR functionality. Only certain products from miniDSP are capable of FIR filters, so shop carefully, if that is your goal.

Sorry I don't have links for all these claims, but I bet someone will come along and either back me up and provide some links, or clarify where I'm wrong.
Edited by HopefulFred - 10/6/13 at 3:44pm
post #5 of 10
Thread Starter 
Thanks for the links and the replies. I searched around but couldn't find anything quickly and thought surely others have had this issue.
post #6 of 10
Quote:
Originally Posted by HopefulFred View Post

There are several schools of thought about this - Dennis's approach seems to be the most thorough, but others (including those with credentials) claim that less is sufficient.

Best I can tell from reading about this, Dennis and Shawn would set up the QSC with both a delay and also an all-pass filter. (This is entirely inferred on my part, obviously I'm not speaking for them, or from experience.) The QSC unit, different from most others, has a fixed latency, which makes calibrating multiple channels very easy, compared to other less costly units where the processing delay adds up, making final alignment more challenging to determine. They may also use some other processing, but I don't know what it would be.

BIG posted just the other day that Dr. Floyd Toole - noted and sometimes controversial researcher - spoke at CEDIA and claimed that a 10ms delay was adequate.

Other forum members claim that they just split the signal and carry on, very happy with the results. Others split the signal and then adjust mounting position and/or level so that precedence keeps the image where they want it, again very happy.

If you want to emulate the performace of the QSC for less, there are other choices, like some Xilica procesors which get good reviews from professionals, but lack fixed latency. Then there are less costly choices - in particular miniDSP. I think an all pass filter, which changes phase based on frequency, requires FIR functionality. Only certain products from miniDSP are capable of FIR filters, so shop carefully, if that is your goal.

Sorry I don't have links for all these claims, but I bet someone will come along and either back me up and provide some links, or clarify where I'm wrong.

Delay plus all pass filter for phase shift is basically how you do it. The exact settings will vary from theater to theater depending on how you have your seats and surround speakers.

For most people a Xilica XP-2040 will be all they need. This unit has XLR ins and outs and is under 1k. Take the surround channel output from pre-pro, wire to inputs of XP-2040. Set up a simple channel copy inside Xilica with delay applied to the second set of surround channels and all pass filters sufficient to result in minimal or no comb filtering at the listening seats when both surround channels are playing the same input signal. Not all that difficult to set up using an acoustic measurement rig such as Room EQ Wizard, just a little time consuming.

All pass filters do not require FIR functionality. Also fixed latency is not a feature worth paying more for in this context. Latency introduced by the Xilica is only ~2ms in the above application and can be easily adjusted for by setting pre-pro for surround channel 2ft further than actual measured distance.
post #7 of 10
How do you adjust the phase response of the IIR filter to reduce the comb filtering? For an FIR filter, it seems fairly straight forward to work in the frequency domain to design the desired phase response, and the convert back to the time domain to calculate the filter coefficients.

Is a nonlinear phase response of an IIR allpass filter enough to eliminate the comb filtering?
post #8 of 10
Quote:
Originally Posted by J_P_A View Post

How do you adjust the phase response of the IIR filter to reduce the comb filtering?

With practice, patience and lots of measurements! There are too many variables (distance between surrounds, distance between each surround and seat) to provide a simple 'forum' process that will work in all situations.

I am pretty sure (but who knows) Erskine Group and Co are not creating their own FIR filters for decorrelation of additional surround channels. It is precisely the phase shift characteristics of an all pass filter that assist in the decorrelation.
post #9 of 10
Thanks, Nyal!

I've been playing around with this lately, and I think my hangup is I've been operating under the assumption that near perfect decorrelation is the goal (i.e. correlation coefficient ~=0). It seems that with a standard IIR allpass filter, where we can adjust Q and center frequency, the amount of decorrelation is pretty limited. I've read papers that would indicate it requires a significant number of coefficients in the IIR filter to achieve near-zero correlation. A miniDSP for comparison is limited to two poles and two zeros. These can be cascaded of course, but still.

It sounds like perhaps the goal is to achieve a local decorrelation near the seating positions rather than decorrelation everywhere. I'm guessing that would relax the condition a bit with regard to how decorrelated the signals need to be.
post #10 of 10
Would it make sense to focus decorrelation efforts in the mid- and high-frequency ranges where the comb filter response is most pronounced?
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