# DCX assistance - Q conversion from REW



## Warpdrv (Mar 3, 2007)

Is there a simple formula to convert the Q value that REW gives for the DCX. It seems that REW is really geared more toward the 1124 and the values are different. 

I'm going to be remeasuring with REW and setting up my 4 subs - 3 18" LMS's and 1 15" TC3K ti all sealed and scattered about the room. Delay and crossover are taken care of, but I need to go back to work out Phase and EQ - I got some cancellation that is killing my down low output which I figure is due to phasing but I'll have to start over and EQ the whole mess again... 

Any shortcut would be greatly helpful to lessen my time screwing around on this.


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## bjs (Jun 12, 2008)

Warpdrv said:


> Is there a simple formula to convert the Q value that REW gives for the DCX....
> ....Any shortcut would be greatly helpful to lessen my time screwing around on this.


Pick the device in REW (not the 1124p) that specifies Q the same way as in the DCX? 

It's been a while and I'm not near REW or my DCX, but I don't recall any issues with setting DCX Q values.


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## Wayne A. Pflughaupt (Apr 13, 2006)

REW doesn’t have a setting for the DCX. You probably don’t want to use the DSP1124 setting, its filters are a broad as the Mississippi (compared to other parametrics, including Behringer’s).

Your best bet is probably to EQ using the RTA feature. Then you can see in real time the effect the filters are having, as you adjust them.

Regards,
Wayne


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## Warpdrv (Mar 3, 2007)

Thanks Wayne....

Your saying use the RTA feature in REW... I'll have to figure out how to do that... any suggestions would be great.

Nuance has been helping me get my subs setup and he is more the REW pro but I hate to keep wasting his time, setting up 4 subs is no picknic...


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## bjs (Jun 12, 2008)

Since the OP doesn't have a 1124p this is going off-topic...but the 1124p minimum filter bandwidth is specified as 1/60 of an octave. This is pretty narrow and better than the DCX. A higher Q filter is very questionable in value anyway IMHO.

As for the DCX, most people just pick the right device in REW as I suggested when using the DCX. No fancy Q conversion required. So I don't see the problem. But apparently that suggestion is too simple... :scratch:


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## Wayne A. Pflughaupt (Apr 13, 2006)

> Your saying use the RTA feature in REW... I'll have to figure out how to do that... any suggestions would be great.


Look for the Spectrum tab in the REW window and select RTA 1/24-octave (or a lower-resolution if you want some averaging) . Use Periodic Pink Noise (Pink PN) from the Generator, with a Rectangular window, and a FFT of 65536, press the red button. Go to Settings / View Tab and turn off bars to get a smoother looking line. 

This thread shows other uses for the RTA feature.


Regards,
Wayne


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## Warpdrv (Mar 3, 2007)

bjs said:


> As for the DCX, most people just pick the right device in REW as I suggested when using the DCX. No fancy Q conversion required. So I don't see the problem. But apparently that suggestion is too simple... :scratch:


So if the DCX isn't listed then I choose what... :huh:

If I am to choose the "right" device and I am using the DCX what is the right device...? :scratch:


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## Wayne A. Pflughaupt (Apr 13, 2006)

bjs said:


> As for the DCX, most people just pick the right device in REW as I suggested when using the DCX. No fancy Q conversion required. So I don't see the problem. But apparently that suggestion is too simple... :scratch:


Unfortunately, it’s not quite that simple. The problem is that different equalizers “calculate” filters differently, which gives a wide variation in the amount of “real estate” a filter actually has an affect on.

For instance, here are graphs generated from REW’s EQ panel for three different parametrics, showing the path each cuts for a 1/3-octave filter.

*






Behringer DSP1124 
1/3-octave (20/60)








Behringer FBQ2496
1/3-octave (.333)








Velodyne SMS-1
1/3-octave (4.3 Q)​*

It should be plain to see, using a setting in the REW Equalizer Panel to predict filters, if your equalizer doesn’t correspond to those filters, the actual results you get will be off. 

Merely selecting an Equalizer setting that uses a similar method of identifying bandwidth is no guarantee. For instance, the “Generic” setting gives bandwidth as Q, as does the “SMS-1” setting. But the latter has much narrow filters for a given Q than the "Generic" setting.

This is the problem I found when I tried to use REW with my Yamaha digital parametric. I used the “Generic” setting (which gives bandwidth in Q, as does my equalizer) and manually dialed in a predicted response curve that I liked, but when I loaded the filters into the equalizer and took a sweep, actual response was not the same as what was predicted.

That said, if you have found an REW setting that seems to correspond favorably with the DCX, it might be helpful to others if you could let us know which one it was? People have been asking for an REW setting for both the DCX and the DEQ2496 for a long time.

Regards,
Wayne


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## Warpdrv (Mar 3, 2007)

Thanks Wayne... gunna look at some of this... I'll give it a try...


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## bjs (Jun 12, 2008)

The response from parametric equalizers is all basically the same except for the different ways of labelling the Q (for the same response). The generic device works for the DCX although with hardware limitations.


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## Andysu (May 8, 2008)

I thought I’d overlap the filters as you see they are still miles apart close but they don’t overlap with transparency.


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## bjs (Jun 12, 2008)

Andysu...I couldn't quite understand your sentence so probably am missing your point. I see that you combined Wayne's graph into one. Is there more?

I don't think there is any doubt that parameter equalizers are all over the place in terms of how they specify filter bandwidth (ie Q). Behringer themselves have at least 3 different ways they do it.

But they are all based on 2nd order biquadratic equations so in the end they all have basically the same response. So it's not really correct to say one's filters are wider than anothers. The 1124p response isn't any different than the DCX for example (they just specify things differently).

As an aside, if you want the same filter shape with different equalizers, I think REW will actually convert from one device to another for you automatically.


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## Wayne A. Pflughaupt (Apr 13, 2006)

What do you mean by "they all have the same response?"

Regards,
Wayne


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## JohnM (Apr 11, 2006)

Connect the DCX in a loopback to the soundcard, make measurements with a couple of filter settings and see which if the REW options comes closest to matching the measured shapes when using the values that were set up in the DCX.


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## JohnM (Apr 11, 2006)

Wayne A. Pflughaupt said:


> What do you mean by "they all have the same response?"


The filter shapes are all derived from a biquadratic transfer function (a ratio of two quadratic polynomials). Any particular shape results from a specific set of biquad coefficients. How those coefficients are derived varies from device to device - pretty much all of them handle gain and frequency the same way, but there are many different ways of specifying the response bandwidth - some formulations specify bandwidth in octaves, some in Hz, some use the filter sharpness or "Q", some measure the bandwidth or Q at the half gain points, some at the the -3dB point and so on. Many different bandwidth/Q figures can be used to express the same filter shape depending on the definition chosen, but they all end up with the same biquad coefficient set for a particular filter shape.


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## bjs (Jun 12, 2008)

What John said.


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## Warpdrv (Mar 3, 2007)

WmAx and another guy posted this link for a calculator that may or may not help...

http://www.sengpielaudio.com/calculator-bandwidth.htm


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## Wayne A. Pflughaupt (Apr 13, 2006)

JohnM said:


> The filter shapes are all derived from a biquadratic transfer function (a ratio of two quadratic polynomials). Any particular shape results from a specific set of biquad coefficients. How those coefficients are derived varies from device to device - pretty much all of them handle gain and frequency the same way, but there are many different ways of specifying the response bandwidth - some formulations specify bandwidth in octaves, some in Hz, some use the filter sharpness or "Q", some measure the bandwidth or Q at the half gain points, some at the the -3dB point and so on. Many different bandwidth/Q figures can be used to express the same filter shape depending on the definition chosen, but they all end up with the same biquad coefficient set for a particular filter shape.





bjs said:


> What John said.


So every parametric EQ that gives bandwidth as Q will have identical filter bandwidths for a given Q value? IOW, for 4.3 Q they would all “look like” what the graph above shows for the SMS-1?

Same with every parametric that gives bandwidth as octaves, etc.?

Regards,
Wayne


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## Andysu (May 8, 2008)

bjs said:


> Andysu...I couldn't quite understand your sentence so probably am missing your point. I see that you combined Wayne's graph into one. Is there more?
> 
> I don't think there is any doubt that parameter equalizers are all over the place in terms of how they specify filter bandwidth (ie Q). Behringer themselves have at least 3 different ways they do it.
> 
> ...


Not really I kinder looked at and saw the differences between each model and its Q width. I mealy overlaid them to show how different apart they look. Kinder surprised even me.


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## JohnM (Apr 11, 2006)

Wayne A. Pflughaupt said:


> So every parametric EQ that gives bandwidth as Q will have identical filter bandwidths for a given Q value? IOW, for 4.3 Q they would all “look like” what the graph above shows for the SMS-1?
> 
> Same with every parametric that gives bandwidth as octaves, etc.?


No, because they may also differ in the point in the response at which the bandwidth or Q is measured, some at half gain, some at -3dB etc.


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