Cut or Boost? How Much - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 01-31-07, 01:16 PM
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Naut

Join Date: Apr 2006
Location: Richmond, VA
Posts: 606
Re: Cut or Boost? How Much

I should have added "at any frequency."

Keep an eye on the "filters" graph in REW and make sure that it doesn't go over the target level at any point.

You can use positive filters to help shape the EQ curve so that the wide cuts don't further attenuate your measured dips, but you can't use positive filters to boost the level at the dips.

For example here's one with two filters. Note that there is no net gain at 53.3 Hz despite the large boost, because the sum of the two filters is always less than (or equal to) zero. Thus, we do not further attenuate our original dip, but we don't boost out of it either.

41.5 Hz, -14 dB, 1/1 octave wide
53.3 Hz, +12 dB, 1/6 ocatave wide

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Old 01-31-07, 01:38 PM
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brucek

Join Date: Apr 2006
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Re: Cut or Boost? How Much

Quote:
I should have added "at any frequency."
Yeah, that's correct.

Quote:
perhaps you could share your thoughts on whether the following example (well below the x-over) would constitute a boost?
There's a couple points to make. Indeed, as ayreonaut implies, if I have a +10dB filter at 50Hz and a -10dB filter at 50Hz, the resultant effect on the output signal is 0dB. In that regard, although I gave an extreme example, I can use gain to shape a negative filter without providing any gain to the input signal and as such, the output signal will not clip (requiring a reduction in the input level to compensate).

My point in my upthread posts is that if you provide any resultant gain in the input signal, you will have to reduce your input level to the BFD (with your processor subwoofer output trim). This lowered input level to the BFD causes negative effects as I discussed upthread. I am simply trying to get the maximum signal to the A/D converter. You can't do that if you apply gain because the output will clip.

A +10dB filter at 50Hz and a -10dB filter at 50Hz doesn't constitute gain in the output. I might point out though, excessive filter gain or cut value are better served divided into smaller bites. There will be less internal DSP distortion to two -5dB cuts at 20Hz than a single -10dB cut at 20Hz. Divide your big filters in half.

brucek
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Old 01-31-07, 01:55 PM
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Bri

Join Date: Apr 2006
Location: San Jose
Posts: 131
Re: Cut or Boost? How Much

Good advice about splitting large cuts into several filters. I may try that with my -13 cut at 74Hz. I'll also look into adding a bit of boost to any areas around the dip/null around 85~95Hz that were lowered because of the cuts and see what happens.

With regards to input level into the BFD, I used ch5 of WotW DTS to set the input level by checking for the peak. I think I turned the sub off, but had the volume at the max level I would probably play that scene at (-12 to -10 or so, IIRC) and watched the input level meter on the BFD.

I ended up raising the speaker levels and lowering the sub levels in the receiver to get proper calibration with a low enough sub output into the BFD. Before the sub was 3~4 notches below the level of the speakers in the receiver, with the sub's gain at about 1/2 way between 0 and the first tick on the HSU gain knob. But since my HK puts out a very strong sub signal, I raised the speaker levels to about +2~+3 all around, and have the sub level at -9 for DD/DTS. This is re-calibrating the receiver so that -5 on the dial is reference level (-10 = 80dB with Avia, so 5 under) rather than 0 equalling RL 0.

This is all great advice. I'll re-evaluate my settings, although I think my small boosts are fine as the sub hasn't shown any stress on even The Haunting DTS or WotW DTS at high volume (max volume I would watch at).

My HT Gallery (Updated pics of my setup and room treatments)

My REW measurements with bass traps and BFD eq
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Old 01-31-07, 02:54 PM
dcanet
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Re: Cut or Boost? How Much

Yes! It seems we have a complete answer to the original question that started this thread.

Quote:
mpompey wrote: View Post
I've been looking at some of the graphs folks have been posting up here. How do you get your sub responses so flat by cutting freqs alone. Do you ever give any a boost? If so, by how much. Is there a dB limit for boosting a freq range? I'm not talking about a room null. I understand that you can't electronically even that out with an eq alone.

Quote:
Is there a dB limit for boosting a freq range?

Zero..........
and

Quote:
How do you get your sub responses so flat by cutting freqs alone.

Turn up the sub amp and cut....
There is really no upside to boosting in the BFD.... it decreases dynamic range, reduces headroom in the DSP and reduces signal to noise ratio. Any boost at any frequency before the crossover frequency requires reduction in the input level of the BFD to make room for the boost.
You can see how someone reading this thread from the top could get the wrong impression that any filter with “any amount” of gain is to be avoided…unless it were close to or above the x-over.

That’s why it seemed further clarification of what defines “a boost” was in need.
Many could assume that any filter with any amount of gain is a boost, when that is not necessarily the case.

Utilizing the filter graph in REW as a means to evaluate filters that employ a bit of gain (but no actual boost) to determine if you are within the safe zone is very useful technique in smoothing out your response after those larger cuts needed to tame peaks down to the target line have created some modest dips in the process.

Quote:
brucek wrote: View Post
A +10dB filter at 50Hz and a -10dB filter at 50Hz doesn't constitute gain in the output. I might point out though, excessive filter gain or cut value are better served divided into smaller bites. There will be less internal DSP distortion to two -5dB cuts at 20Hz than a single -10dB cut at 20Hz. Divide your big filters in half.

brucek
brucek, In dealing with that last question you brought up the technique of splitting larger cuts into two. Do you have any basic guidelines as to when a cut should be split?

I’ve found myself doing this of sorts by using two fairly close spaced cut filters when dealing with a large odd shaped peak. However, I never knew if that was good practice or not?
Old 01-31-07, 03:01 PM
Senior Shackster
lovingdvd

Join Date: Jan 2007
Posts: 246
Re: Cut or Boost? How Much

"A +10dB filter at 50Hz and a -10dB filter at 50Hz doesn't constitute gain in the output. I might point out though, excessive filter gain or cut value are better served divided into smaller bites. There will be less internal DSP distortion to two -5dB cuts at 20Hz than a single -10dB cut at 20Hz. Divide your big filters in half."

So in other words, if REW recommends say a -15dB cut at 25hz, use one filter for -7 and another for -8 at the same freq, correct?

Just how much difference does this make? I have lots of tweaks and as it is I'm always struggling to fit all the tweaks into the 12 filters. Speaking of which, is there a way to apply more than 12 filters to the same sub?
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Old 01-31-07, 03:02 PM
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Bri

Join Date: Apr 2006
Location: San Jose
Posts: 131
Re: Cut or Boost? How Much

For the second question, I believe it is possible to use both the left and right "engines" to have more filters.

My HT Gallery (Updated pics of my setup and room treatments)

My REW measurements with bass traps and BFD eq
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Old 01-31-07, 03:08 PM
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lovingdvd

Join Date: Jan 2007
Posts: 246
Re: Cut or Boost? How Much

Quote:
cyberbri wrote: View Post
For the second question, I believe it is possible to use both the left and right "engines" to have more filters.
I was under the impression that the left engine only applied to Output 1 and the right only applied to Output 2. No?
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Old 01-31-07, 03:34 PM
dcanet
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Re: Cut or Boost? How Much

Quote:
lovingdvd wrote: View Post
I was under the impression that the left engine only applied to Output 1 and the right only applied to Output 2. No?

That's why to use both you would simply connect the output of one to the input of the other with a short cable and run them in series. How much this would degrade the signal (and if it really matters for sub use) is something someone else might answer.
Old 01-31-07, 03:37 PM
Elite Shackster
brucek

Join Date: Apr 2006
Posts: 7,514
Re: Cut or Boost? How Much

Quote:
In dealing with that last question you brought up the technique of splitting larger cuts into two. Do you have any basic guidelines as to when a cut should be split?
Nope. But if I had filter slots available and I had a cut over 10dB, why not cut it in half.

Quote:
Speaking of which, is there a way to apply more than 12 filters to the same sub?
Yes, feed the output of one channel to the input of the other channel. Now you have 24 filters. You have also converted from analog to digital and back twice.... do you want to do that?

If you need more than 12 filters, you're doing something wrong........

brucek
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Old 01-31-07, 03:42 PM
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lovingdvd

Join Date: Jan 2007
Posts: 246
Re: Cut or Boost? How Much

Quote:
brucek wrote: View Post
If you need more than 12 filters, you're doing something wrong........
brucek
Part of what drives my filter needs up is that I lose the ability to Optimize my filters once they are in the BFD. This goes back to my whole measure vs. re-measure thread.

For instance I do a measure and then optimize it with filters according to REW. I copy all these filters into BFD. So far so good.

Now I remeasure and see I need a few tweaks. So then I make a few tweaks using new filters in the new Measurement window. REW doesn't know about the old filters so it can combine and optimize things based on the filters any more because the first set of filters is independent from the second set.

I guess the proper way to do this would be to remove all the filters in the BFD after each run, and go back to the original filters window and make the adjustments there, and then copy all x filters back into the BFD. A lot more labor intensive especially without the MIDI programming. But perhaps this would do the trick?
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