# DCX2496 shelving filters...how?



## ssabripo (Nov 2, 2006)

replaced my broken DEQ with the DCX, and now I cannot find anywhere (manual, online, etc) on how to setup a simple stupid low pass shelving filter at 6db/oct

:hissyfit:

I know I'm being a m0ron and it's likely right in front of me...can someone be kind enough to show me how to do it from the front panel?


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## JohnM (Apr 11, 2006)

In the EQ filters the shelving filters are (confusingly) called low pass and high pass filters, the associated gain parameter sets the shelf 'height'. What one would more normally call low pass and high pass filters are in the crossover section. I don't have a unit, so can't talk you through the panel steps, but from the manual the EQ features look to be on the third 'page' of menus for each input and each output.


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## JonP (May 8, 2008)

OK, here you go... Hit the "Setup" button, then hit one of your channel buttons. Then (depends on where you were last in the menu) the 3d page will be EQ for that channel. (or group of)

In there, you will see the number of filter, far top right is the type, HP, BP or LP. HP and LP are the "shelving" filters. (BP is bandpass, for notches) Lower (mid) right side will have 6db or 12db as choices, that's the slope, and left to mid are the freq and gain. HP and LP of course are general direction, plus or minus gain will have it shelving down or up. Play around with these settings to get the shelf you need. Instead of a shelf with a set slope, at a corner freq, it's a filter at (sort of) a corner freq, with a positive or negative gain in it's passband area.

Now, what is a very good question that I would like to find out/figure out, is how much gain and what frequency setting create a "classic" 6db/oct (or 12db) for frequency X? There seems to be some, er, differences, than I'd expect... from my measuring with REW on my DCX2496 in loopback. 

John? Did you ever have to figure this out for making the filter control section for the DCX as a REW output device? I almost remember some discussion going on about this long ago, haven't been able to dig it up searching forums... I'm trying to use Charlie Laub's Active Crossover Designer spreadsheets, and there's a difference between the shelving he describes and the DCX's way of doing it.


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## JohnM (Apr 11, 2006)

I believe the DCX 6dB /octave and 12dB/octave shelf settings correspond to S settings of 0.5 and 1.0, where S (per Robert Bristow-Johnson's cookbook formulae) is:



> S, a "shelf slope" parameter (for shelving EQ only). When S = 1, the shelf slope is as steep as it can be and remain monotonically increasing or decreasing gain with frequency. The shelf slope, in dB/octave, remains proportional to S for all other values for a fixed f0/Fs and dB gain.


I don't have a DCX though, so haven't been able to verify the REW implementation against a unit.


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## JonP (May 8, 2008)

OK,sounds like what youl'd expect... though that might be a not entirely correct assumption.

I am not as up on this as I wish I was, will look over that paper. I guess my questions on how to achieve a particular shelf response are around how the DCX has just a single Fo and gain adjustment, rather than the ability to specify Fo/Fs. Or, to put it differently, what gain and Fo will equal a particular Fo/Fs? 

From what I've measured, It seems that the "Fo" in the DCX menu might be the center of the slope, instead of the rolloff knee. Or something else not quite expected.

I'm thinking there might be a conversion formula/method/table that someone (me?) will have to work out, I've been trying to see if someone else already has gone thru this. Will look at this again in the near future, maybe work out a table... 

Hope the original poster got his question asked... apologize for the threadjacking. Been meaning to start one on the topic, and it seemed like a reasonable place to bring it up..


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