DCX2496 XO phase response oddities - diyAudio
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Old 13th April 2013, 09:01 AM   #1
wxn is offline wxn  Lithuania
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Default DCX2496 XO phase response oddities

So everybody is saying that DCX has the same phase response as analog filter. Well I measured it and it is simply not true - take a look at the pictures.

HP filter does seem to act somewhat "textbooky" but the LP does not, it seems to be far more linear at the crossover frequency. I've tested this in various configurations and this tendency does not depend on frequency (100Hz and 1Hz illustrated) or filter type (LR48 shown for more clarity).

Anyone got an explanation for this and perhaps a way to overcome or compensate for this strange phase behavior?
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File Type: jpg dcx 100 lr48 spl.jpg (74.7 KB, 94 views)
File Type: jpg dcx 100 lr48 phase.jpg (133.5 KB, 91 views)
File Type: jpg dcx 1k lr48 spl.jpg (70.3 KB, 85 views)
File Type: jpg dcx 1k lr48 phase.jpg (99.6 KB, 85 views)
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Old 13th April 2013, 04:29 PM   #2
Davey is offline Davey  United States
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Your measurement scheme is incorrect. You need to discount the latency of the unit when evaluating phase response.

The easiest way to view phase response is to use one output channel of the DCX with zero programming applied as your reference channel and then measure relative to that with your other channel in a differential scheme.

I guarantee you'll then see phase response identical to an analog equivalent filter.

Cheers,

Dave.

Last edited by Davey; 13th April 2013 at 04:33 PM.
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Old 13th April 2013, 04:55 PM   #3
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WXN,I agree with the above. I've done DCX measurements using HOLMImpulse and the transfer functions were pretty much textbook. Take a look at latency. HOLMImpulse can manage that for you, if you don't have another way to do it.
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Old 13th April 2013, 05:20 PM   #4
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WXN-

Since you are measuring the DCX can you do me a favor and measure some EQ bands with a couple values of Q and equal amounts of boost and cut dB?

For instance, can you measure EQ centered on 1000Hz (any frequency is OK, but keep it constant) for:
Q=1 gain = +6dB
Q=1 gain = -6dB
Q=4 gain = +6dB
Q=4 gain = -6dB

Also, can you measure the output with two bands on the same channel that have the same Fo, Q, and gain magnitude but have +gain and -gain, like this:
EQ1: Fo=1kHz, Q=1, -6dB
EQ1: Fo=1kHz, Q=1, +6dB
measure the output = EQ1 + EQ2

I want to see if the bands cancel and the output is "flat" (or not).

Can you post the results here?

If anyone else can do it, or knows the behavior of the EQ bands for the DCX, please post. Thanks,

-Charlie
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Old 13th April 2013, 07:43 PM   #5
wxn is offline wxn  Lithuania
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Ok, thanks guys, that was enlightening! I'll post the measurements Charlie asked in a minute
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Old 13th April 2013, 08:01 PM   #6
wxn is offline wxn  Lithuania
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Here are the graphs, no smoothing. Please ignore the wiggly baseline, it's caused by the aging EQ in my mixer.

The EQ1+EQ2 curve does seem to be pretty much identical to the baseline.

There is a peak at 1kHz and its harmonics present in all of these curves, not sure about its origins.
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File Type: jpg charlie spl.jpg (120.5 KB, 70 views)
File Type: jpg charlie phase.jpg (118.8 KB, 25 views)
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Old 13th April 2013, 09:02 PM   #7
Davey is offline Davey  United States
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Charlie is asking for the "definition of bandwidth" for the Q settings of the DCX2496. Here's a measurement of Q=1 and Q=4 for a 1khz notch filter -6db. (The +6db measurements are exactly opposite of these as expected.)

The second measurement you asked for yields a perfectly flat response.......as expected.

Cheers,

Dave.
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Old 13th April 2013, 09:20 PM   #8
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Quote:
Originally Posted by Davey View Post
Charlie is asking for the "definition of bandwidth" for the Q settings of the DCX2496. Here's a measurement of Q=1 and Q=4 for a 1khz notch filter -6db. (The +6db measurements are exactly opposite of these as expected.)

The second measurement you asked for yields a perfectly flat response.......as expected.

Cheers,

Dave.
Dave - thanks for posting that. I've been able to check this against a new version of the Active Crossover Designer that I have been working up.

These EQ bands are "symmetrical", which is not the same as what you get when you design an analog EQ stage (one with negative gain at least).

The phase looks right to me (in response to O/P).

-Charlie

Last edited by CharlieLaub; 13th April 2013 at 09:22 PM.
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Old 13th April 2013, 10:27 PM   #9
wxn is offline wxn  Lithuania
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Not to be picky but what exactly did I do wrong this time? It's exactly the same, except for different scale...
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Old 13th April 2013, 10:33 PM   #10
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Quote:
Originally Posted by wxn View Post
Not to be picky but what exactly did I do wrong this time? It's exactly the same, except for different scale...
You didn't do anything wrong... Dave just posted a very clear plot without the funny skew. I needed to read off some values to compare with some of my own results and his plot worked well for me for that purpose.

Thanks for checking that the bands cancel. I needed to have that confirmed.

Cheers,

-Charlie
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