How to extend the high pass filter below 20hz in DCX2496 - AVS Forum
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post #1 of 56 Old 03-03-2013, 11:54 PM - Thread Starter
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The DCX2496 only allows for corner frequencies down to 20hz, but many folks could use a high pass feature below that point. This is easily accomplished using a combination of a 20hz high pass filter and a negative gain high shelf filter.

Here is how it works illustrated in WinISD (divide the frequency scale by 10).

The green line is the target curve--a 2nd order high pass filter at 10hz.

Step 1: The red line is a 2nd order high pass filter at 20hz. Enter this in the DCX.

Step 2: The yellow line is a 12db/octave high shelf filter with negative gain of -12db also at 20hz. Enter this in the DCX.

Step 3: You're done. When these two filters are combined, they produce the target curve.

If you need a different high pass point, just adjust how many db of gain are in the shelf filter. For example, a setting of -6db should provide a high pass filter at around 14hz.



Step 1 detail.



Step 2 detail.





Update:

Many have asked how to set this up in the iNuke DSP. The filters are the same.

Here is an example of how to set the filters for a i would suggest a 2nd order 16hz high pass filter.

iNuke can't set filters below 20hz, but that is no problem. We just start with one at 20hz, then add a little shelf to push it down to 16hz.

Here is exactly what to put into the iNuke DSP.

Step 1: On the FILTER tab, enter 2nd order High-pass filter at 20hz (Butterworth, 12db / octave)



Step 2: On the PARAMETRIC EQ tab, enter Filter 1, Gain -4db, Frequency 20hz, type: HS12



Then use filters 2, 3, 4, etc. for any other EQ that you wish to add (e.g., to help correct for room modes).

To set their filter lower than 16hz, just add more "negative gain" in step 2. If adding a total of -12db negative gain, the 20hz high pass will be pushed all the way down to 10hz.
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post #2 of 56 Old 03-04-2013, 01:36 AM
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Thanks for illustrating, I read many times it could be done but was hard to visualise.

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post #3 of 56 Old 03-04-2013, 08:12 AM
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Quote:
Originally Posted by LTD02 View Post

The DCX2496 only allows for corner frequencies down to 20hz, but many folks could use a high pass feature below that point. This is easily accomplished using a combination of a 20hz high pass filter and a negative gain high shelf filter.

Here is how it works illustrated in WinISD (divide the frequency scale by 10).

The green line is the target curve--a 2nd order high pass filter at 10hz.

Step 1: The red line is a 2nd order high pass filter at 20hz. Enter this in the DCX.

Step 2: The yellow line is a 12db/octave high shelf filter with negative gain of -12db also at 20hz. Enter this in the DCX.

Step 3: You're done. When these two filters are combined, they produce the target curve.

If you need a different high pass point, just adjust how many db of gain are in the shelf filter. For example, a setting of -6db should provide a high pass filter at around 14hz.



Step 1 detail.



Step 2 detail.



Are you sure about the negative HSF part? the yellow line looks like like +6db second order low shelf filter to me. Would not a negative high shelf filter at 20Hz lower the output from 20Hz up?
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post #4 of 56 Old 03-04-2013, 08:46 AM
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Quote:
Originally Posted by zheka View Post

Are you sure about the negative HSF part? the yellow line looks like like +6db second order low shelf filter to me. Would not a negative high shelf filter at 20Hz lower the output from 20Hz up?
Yes, but to counter act the drop, add 12db of boost to the output or somewhere else in the pre/pro. It's not as perfect as setting a 10hz HP, but this is a viable work around.

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post #5 of 56 Old 03-04-2013, 09:05 AM
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Quote:
Originally Posted by Looneybomber View Post

Yes, but to counter act the drop, add 12db of boost to the output or somewhere else in the pre/pro. It's not as perfect as setting a 10hz HP, but this is a viable work around.

I think understand the idea. I just do not see how HSF, positive or negative, can be used to lower the HPF point since it would act only above the set frequency (20Hz in this example).
I suspect LTD02 meant to use positive Low Shelf Filter because that's what needed to boost output below 20Hz to counteract early roll-off from the 20Hz HPF, just like LT (yellow) curve in his simulation.
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post #6 of 56 Old 03-04-2013, 12:41 PM
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LTD, thanks, I was never entirely clear on how to do this either.
Quote:
Originally Posted by zheka View Post

I just do not see how HSF, positive or negative, can be used to lower the HPF point since it would act only above the set frequency (20Hz in this example).
I suspect LTD02 meant to use positive Low Shelf Filter because that's what needed to boost output below 20Hz to counteract early roll-off from the 20Hz HPF, just like LT (yellow) curve in his simulation.

Good point.

It might be just a function of the inherent LF rolloff of the electronics, but do we even know for sure that the boost persists below 20 Hz?

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post #7 of 56 Old 03-04-2013, 02:10 PM - Thread Starter
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"I suspect LTD02 meant to use positive Low Shelf Filter because that's what needed to boost output below 20Hz to counteract early roll-off from the 20Hz HPF, just like LT (yellow) curve in his simulation."

the *low* shelf filter is set by the low corner. that is the trick.

in this case, it would need to be 10hz, but corner settings below 20hz are not available.

the *high* shelf filter is set by the high corner.

in this case, it would need to be 20hz, and that setting is possible.

that is why a high shelf filter with negative gain is used instead of a low shelf filter with positive gain.

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post #8 of 56 Old 03-04-2013, 02:20 PM - Thread Starter
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"LTD, thanks, I was never entirely clear on how to do this either."

np. i have heard of folks using various methods to shape a signal below 20hz using the dcx, i just never saw the high pass application detailed explicity.

"It might be just a function of the inherent LF rolloff of the electronics, but do we even know for sure that the boost persists below 20 Hz?"

i asked about that previously and a few people confirmed that the filters do continue to work below 20hz.

there are people fudging parametric eq below 20hz with multiple filters above 20hz (i believe ricci has done this) and it would be odd if filters such as simple high pass filters stopped working below 20hz.

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post #9 of 56 Old 03-04-2013, 02:31 PM - Thread Starter
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here are the two filters:

(again, divide the frequency scale by 10)

1) low pass filter 12db/octave, +12db positive gain, corner set to 10hz (blue line).

2) high pass filter 12db/octave, -12db negative gain, corner set to 20hz. (yellow line).

in this case, 100db is the arbitrary "zero point". the low shelf with positive gain filter increases up to 112db while the high shelf with negative gain drops to 88db.

the revelation was that these two filters are exactly the same *shape* and shaping the signal is what we are trying to do. the level of the signal can easily be changed if necessary with gain.

since filter #2 only requires a 20hz corner setting, it is possible to use it. filter #1 requires a 10hz corner, so it is not possible. the difference is because the dcx uses the lower corner of a low pass filter to set the frequency/locate the filter, while the high pass filter uses the upper corner.


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post #10 of 56 Old 03-04-2013, 02:52 PM
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I am officially confused. it maybe that the setting and terminology in DCX and Inuke are different and that's why I am not getting it.

1. here is how negative (-6dB) high shelf filter @40Hz is represented in Inuke remote interface:


2. here is how positive (+6dB) low shelf filter at 4oHz is represented:


3. here is how a 40Hz second order high pass filter is represented :


I can see how a combination of 2 and 3 may result in a lower effective HPF. But I do not understand how combining 1 and 3 would.
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post #11 of 56 Old 03-04-2013, 03:32 PM
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Negative gain high shelfs don't work like that on the iTech DSP, Crown seems to have implemented a slope-inverter at the zero crossing point.

This is as close as I could get, using this method on that DSP. Close enough...


Looks like the iNuke is the same way as per zheka.
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post #12 of 56 Old 03-04-2013, 03:34 PM
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LTD, thanks, very cool!

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post #13 of 56 Old 03-04-2013, 03:50 PM
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This has got to be the first time I have ever seen Noah excited. biggrin.gif

Keep up the good work, John! smile.gif

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post #14 of 56 Old 03-04-2013, 04:35 PM - Thread Starter
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"I am officially confused."
"I do not understand how combining 1 and 3 would."

zheka, just for the sake of experimentation, change the gain to +/-12db so it will be easier to see the corners. think about what is going on in post #10, example 1. the signal is flat across all frequencies but reduced in magnitude from about 60hz up (in your example). below that point, the level begins to rise (just as it would with a low shelf with positive gain) up to whatever plateau you have set (6db, 12db or whatever). so you are *shaping* the signal in the same way.

if you use the low pass filter with positive gain and set it at 20hz, most all of the "gain" will have already occured by the time you get down to 20hz.

if you use the high pass filter with negative gain and set it at 20hz, most all of the "gain" gets stuffed under 20hz. that is the trick.

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post #15 of 56 Old 03-04-2013, 04:42 PM - Thread Starter
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"This is as close as I could get, using this method on that DSP. Close enough..."

bassthathz, it appears that itech dsp uses the front corner for the low pass shelf, so yours won't require using negative gain on the high shelf.

just for the sake of experimentation, try these changes.

1. change the high pass frequency from 275hz to 200hz.
2. on eq filter1 change the gain from 17.8 to 12.0.

the result should be a net 100hz high pass filter, 12db/oct slope, which has a -3db point at 100hz.

of course, for the 10hz filter, just divide all the frequencies by 10.

....

edit: after eyeballing it a little bit more, it appears the itech dsp uses the mid-point frequency to locate the shelf filter. let me see if i can cook up something that works with that.

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post #16 of 56 Old 03-04-2013, 05:09 PM - Thread Starter
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bassthathz,

if i am eyeballing it correctly, the itech dsp locates the shelf based on the midpoint of the slope. this is different from the dcx.

based on that, the best that you can do, in theory, is a 14hz high pass filter using the following configuration.

1. 12db/oct low pass shelf with +12db gain set at 20hz (the yellow line).
2. 12db/oct high pass filter set at 280hz (the red line).
3. the net is equivalent to a 12db/oct high pass at 14hz (the green line).



however, as you point out, it doesn't have to be perfect. so one method that gets you within 1.5db of a theoretically perfect 10hz high pass is:

1a. 12db/oct low pass shelf with +12db gain set at 20hz
1b. parametric eq, centered at 20hz, q=1.00, gain= -3.0db (the yellow line).
2. 12db/oct high pass filter set at 20hz (the red line).
3. the net is equivalent to a 12db/oct high pass at about 9hz with a smidge of boost before rolloff (the green line).

with model, we are using a parametric eq to beat down the front end of the shelf filter. it ends up being pretty close.


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post #17 of 56 Old 03-04-2013, 05:37 PM
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Quote:
Originally Posted by LTD02 View Post

"I am officially confused."
"I do not understand how combining 1 and 3 would."

zheka, just for the sake of experimentation, change the gain to +/-12db so it will be easier to see the corners. think about what is going on in post #10, example 1. the signal is flat across all frequencies but reduced in magnitude from about 60hz up (in your example). below that point, the level begins to rise (just as it would with a low shelf with positive gain) up to whatever plateau you have set (6db, 12db or whatever). so you are *shaping* the signal in the same way.

if you use the low pass filter with positive gain and set it at 20hz, most all of the "gain" will have already occured by the time you get down to 20hz.

if you use the high pass filter with negative gain and set it at 20hz, most all of the "gain" gets stuffed under 20hz. that is the trick.

I think I am beginning to understand. With regular LSF the resulting curve would have a hump before the HPF takes over and extension of the HPF will be minimal because, as you say, most of the "gain" is above the set point. The hump can be seen on the snapshot BTH posted only in case of Behringer products it will likely be more pronounced and the extension of the combined response - smaller, because the set frequency is not at the midpoint but at the upper corner.

The hardest part for me is to accept that from signal shaping point of view boosting signal from frequency X and below is the same as cutting the signal from frequency X up. But it's probably correct. It would require an input signal strong enough to offset the cut though. Are there any other downsides to this approach?
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post #18 of 56 Old 03-04-2013, 06:06 PM
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Quote:
Originally Posted by BassThatHz View Post

Negative gain high shelfs don't work like that on the iTech DSP, Crown seems to have implemented a slope-inverter at the zero crossing point.

This is as close as I could get, using this method on that DSP. Close enough...


Looks like the iNuke is the same way as per zheka.

I like how crossover and EQ filters are combined in this interface. I wish behringer did the same.
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post #19 of 56 Old 03-04-2013, 06:19 PM - Thread Starter
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"Are there any other downsides to this approach?"

the downside is that headroom/signal to noise ratio are lowered by the amount of gain used in the shelf filter. with a 10hz highpass setup, that is 12db. for the 14hz highpass setup, it is 6db. as looney mentioned, it is a "workaround" (i like that term), not an ideal solution. an ideal highpass 10hz would take up no headroom nor would it reduce the signal to noise ratio at all.

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post #20 of 56 Old 03-04-2013, 09:49 PM
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Quote:
Originally Posted by LTD02 
that is why a high shelf filter with negative gain is used instead of a low shelf filter with positive gain.
Somehow I missed the negative high shelf concept. -(-12db) = +12db. I must'a been tired.

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post #21 of 56 Old 03-05-2013, 10:20 PM
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Quote:
Originally Posted by zheka View Post

I like how crossover and EQ filters are combined in this interface. I wish behringer did the same.

+1

I just downloaded the inuke s/w and it apparently does not.

Is the itech dsp s/w downloadable w/o having an amp like the inukes? I wasn't able to find it.

Anyone know of something free or cheap that sums filters for messing around with this idea?

Noah
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post #22 of 56 Old 03-20-2013, 01:08 AM
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Quote:
Originally Posted by LTD02 View Post

"Are there any other downsides to this approach?"

the downside is that headroom/signal to noise ratio are lowered by the amount of gain used in the shelf filter. with a 10hz highpass setup, that is 12db. for the 14hz highpass setup, it is 6db. as looney mentioned, it is a "workaround" (i like that term), not an ideal solution. an ideal highpass 10hz would take up no headroom nor would it reduce the signal to noise ratio at all.

I am running up against this very issue, but I just have an epiphany... how about setting up a low shelf at 200Hz with a 12db gain? Would that not effectively offset the 12db negative gain of the HS while still leaving in place the 10Hz high pass?
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post #23 of 56 Old 03-20-2013, 07:55 AM
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Quote:
Originally Posted by DanLW View Post

I am running up against this very issue, but I just have an epiphany... how about setting up a low shelf at 200Hz with a 12db gain? Would that not effectively offset the 12db negative gain of the HS while still leaving in place the 10Hz high pass?

Interesting. I wish there was a way to model this in WinISD
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post #24 of 56 Old 03-20-2013, 08:28 AM - Thread Starter
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"Interesting. I wish there was a way to model this in WinISD"

of course there is or i wouldn't have been able to do it. :-)

the easiest way to get a feel for how to create a shelf filter in winisd is to:

first we need a flat line to model on.
1. plop some driver in a sealed enclosure, it doesn't matter which driver or which enclosure.
2. set the inductance to zero. this will flatten out the top end.
3. put on a linkwitz transform with the lower end set to 1hz.

now you should have a flat line to work with.

to model a shelf filter, add a second linkwitz transform, but this time set f0 to 200hz and fp to 100hz.
ensure that the q0 and qp are both about 0.707

bang, now you should have the shelf.

to create a high shelf, just swapt he 100 and 200hz frequency values.

other shelf filters can be created by changing the f0 and fp.

once you have the shelf filter modeled appropriately using the flat line model, you can just add that to whatever driver/enclosure model you are working with to see the net effect.

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post #25 of 56 Old 03-20-2013, 08:40 AM - Thread Starter
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also, i'm not sure if headroom is actually lost. since the combination of boosting and cutting will never exceed the level of the original signal, i'm thinking that no headroom will be lost.

not sure about that one though.

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post #26 of 56 Old 03-20-2013, 09:49 AM
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Can this negative gain high shelf filter trick be done with a Behringer DEQ2496 also or just the DCX?
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post #27 of 56 Old 03-20-2013, 02:44 PM - Thread Starter
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i don't know.

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post #28 of 56 Old 03-21-2013, 01:58 PM
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Quote:
Originally Posted by Linkwitz Riley View Post

Can this negative gain high shelf filter trick be done with a Behringer DEQ2496 also or just the DCX?

Well, I can confirm that it does indeed work on a Behringer iNuke DSP, so I don't see why not.
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post #29 of 56 Old 03-22-2013, 07:10 AM
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I though DEQ2496 is basically a PEQ unit with RTA. Does it actually have shelf and crossover filters?
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post #30 of 56 Old 03-22-2013, 07:30 AM
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Quote:
Originally Posted by LTD02 View Post

"Interesting. I wish there was a way to model this in WinISD"

of course there is or i wouldn't have been able to do it. :-)

the easiest way to get a feel for how to create a shelf filter in winisd is to:

first we need a flat line to model on.
1. plop some driver in a sealed enclosure, it doesn't matter which driver or which enclosure.
2. set the inductance to zero. this will flatten out the top end.
3. put on a linkwitz transform with the lower end set to 1hz.

now you should have a flat line to work with.

to model a shelf filter, add a second linkwitz transform, but this time set f0 to 200hz and fp to 100hz.
ensure that the q0 and qp are both about 0.707

bang, now you should have the shelf.

to create a high shelf, just swapt he 100 and 200hz frequency values.

other shelf filters can be created by changing the f0 and fp.

once you have the shelf filter modeled appropriately using the flat line model, you can just add that to whatever driver/enclosure model you are working with to see the net effect.

John,

The trick works well for +/-12dB second order shelves. But I could not get it to do shelves of different magnitude and shelves with 6dB/octave slopes. Can you do it? Thank you


this instructions tell how to create curves in WinISD that resemble shapes of the shelf filters in Behringer products. But I do not see how they can be used for actual modeling because they mirror the corresponding shelf filters. e.g. the curve that has a shape of the negative HSF in Behringer is actually a positive LSF. you cannot use to model excursion or SPL for example.
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