Film-Tech Forum ARCHIVE: QSC DCP/DCM crossovers

This is topic QSC DCP/DCM crossovers in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Marco Giustini (Member # 4544) on 03-01-2013, 11:32 AM:

Greetings gentlemen,

I have recently set up some sound systems equipped with QSC DCP or DCM10/20/30. Before that I had proficiently used DCM1-2-3.
While time-aligning bi-amped systems, I've always been able to perfectly adjust the crossover point with almost all brands of speakers.

Since my first DCM 10 I noted that I wasn't able to properly align LF and HF. No matter what delay I used, polarity or crossover frequency, there was always a deep dip at the crossover point. I could't even hear the difference in sound when changing polarity and delay. This has been happening with several installations, so I am now wondering what I am doing wrong.
The only compromise I found is to change the crossover types from L-R 4th order to 2nd order. 2nd order crossovers give a less deep dip and I can see/hear a difference when I tweak the delay.

DCM1/2/3 specification table say that the crossovers used are Linkwitz-Riley 24dB/octave. Which should be 4th order if I'm not mistaken. The default crossover used by a DCP are 4th orders as well. Am I missing something here or maybe when QSC say 4th order on the DCM1/2/3 they mean two x two 2nd orders?

I appreciate any help on the matter.

Regards,
Marco

Posted by Steve Guttag (Member # 268) on 03-01-2013, 02:04 PM:

QSC DCMs are always 4th orders L-R. Note, this should normally have a dip at crossover that the time alignment should be able to get maximized. Some will overlap the crossover region to get rid of the dip.

Posted by Marco Giustini (Member # 4544) on 03-01-2013, 02:47 PM:

Thanks Steve,

Then the default DCP setting (L-R 4th order) is the same filter found in the DCM1-2-3?
No, the dip I've been seeing with DCP and DCM10-20-30 cannot be improved by time alignment. It's a massive dip. Different brands of speakers (JBL, Martin) have given the same result. I set the crossover frequency according to the manufacturer PDF. And I try to reverse the polarity as well of course.
I've never such an issue before: as I said with DCM 1-2-3 it's always been easy to get rid of the time error.

As you suggested, I have also tried overlapping the two ways a bit to improve the response. But I must admit that I'm puzzled here. In the past I've often ended up with perfectly-ish flat response when time aligning speakers with the earlier generation of DCM, with no EQ required.

Have you seen any of that or is it just a coincidence?

Marco

Posted by Steve Guttag (Member # 268) on 03-01-2013, 08:44 PM:

It is a mathematical fact that if you have 4th order L-R crossover where both are set to the same frequency, you will have a 3dB dip at crossover...I've seen it on all systems. If one wants a maximally flat response through crossover, a 3rd order Butterworth will do it.

The L-R crossover has other advantages though...including very step skirts and does not introduce the phase anomalies like the other variants.

I'm not a proponent of the DCM series of monitor/crossovers (don't particularly like the gain-structure) so I don't have tons of experience with all of their permutations but I have used them.

Posted by Marco Giustini (Member # 4544) on 03-02-2013, 03:55 AM:

If it was a 3dB dip I wouldn't complain.
What is your choice of filters when using other crossovers?

Posted by Steve Guttag (Member # 268) on 03-02-2013, 06:16 AM:

It depends on the speaker system, honestly. I've used both L-R 4th order and Butterworth 3rd order. Sometimes even a mix an match...but again, it depends on the speaker system. If the manufacturer has a specification, I always start with that. I don't try to reinvent what has already been done.

One thing you might try is to connect your analyzer right to the DCM output (or on the amplifier input, if that is easier and look at the actual output without the room/speaker getting involved. If you have a system that will allow you to combine A and B inputs to an analyzer (like the D2 analyzer)...you can really see what the speaker is being presented.

Posted by Marco Giustini (Member # 4544) on 03-02-2013, 08:10 AM:

Thanks Steve,

I was indeed thinking of checking the outputs to better understand what the speaker receives. My analyser allow to sum two inputs, so it's definitely doable. I'd like to compare a DCM1 with a DMC10 to see if I'm dreaming here or I'm facing something new.

I do love DCMs and DCPs, lots of options available. I just need to sort out this main issue.

When checking the raw response of a driver - paying attention not to break it if it's an HF one - what do you look to identify the crossover point? the -10dB point?

Marco

Posted by Marco Giustini (Member # 4544) on 07-18-2013, 01:08 PM:

I managed to make time for a test today.
The filters used by a DCM1 and a DCM10 are indeed different. I'll see if I can post a picture later.

Specs sheet do confirm LR 24dB/oct for both models though, I'm confused.

Posted by Samuel Hynds (Member # 2610) on 08-19-2013, 01:12 PM:

what software/firmware are you using with the DCM10/30?

there was an issue with the orignial software/firmware release dealing with crossovers curves

please visit our site qsccinema.com - look under resources for software releases.

Sam Hynds
Product Specialist, Cinema
QSC Audio Products, LLC

Posted by Marco Giustini (Member # 4544) on 08-20-2013, 12:29 PM:

Hello Sam,

The DCP was running the latest software/firmware 3.00.41. I can't remember which FW version the DCM10 was running. I Have DCM Manager 1.20.5 on my laptop, I'll check the FW version and let you know.

Thanks