# BFD sub EQ with or without mains?



## Valorum (Apr 23, 2006)

First of all, Hi from a new member 

Second: REW is an awesome piece of software. It does something useful, and it does it well. Cudos to the programmer of this piece of excellent software :T 

I built my own sub, and i'm pretty impressed with the results. It's my first one though, so i don't have much to compare to  

I've done some measurements and quite a bit of tweaking all day yesterday using REW and my BFD to get my FR flat. I can get it flat pretty well. Seems i got fairly good at doing the filters manually to get a near-flat response.

Now that i'm in the tweaking stage, i ran into a couple things that confuse me though :scratch: 

Using REW, there's a difference between running PC analog to the BFD directly vs PC digital output to the AVR and AVR sub out to BFD.

- When i connected my PC using the sound card LINE OUT straight to the BFD, the stuff below 20Hz would be of a relatively lower frequency.

- When i connected my PC using SPDIF to the AVR and used its sub-out into the BFD, the stuff below 20HZ got an enormous boost. It's like the whole area below 20Hz is lifted by 10dB or so.

I checked to make sure that no filters or anything were turned on in the BFD, and i checked all the settings in the receiver to rule out any level settings or other EQ tweaks in the AVR were doing this. I neutralized everything. Still getting the boost below 20Hz though. Which is great i suppose, getting the loud low freq stuff for free.. Problem is that it's a pretty large boost and i can't touch it with the BFD.

Then there's the question of how to calibrate / tune / EQ properly: I did sweeps from 0Hz (well, 10Hz) to 100/150/200Hz. There are a few different ways i can do things, and i'm not sure which one is the correct one:

(I use my PC to do the REQ measurements. When i run my PCs output into the AVR, the AVR does the cross-over for me between the sub and the mains obviously. My mains are "active" speakers, so i run the line level output from my AVR to them. So theoretically i could hook up a BFD-like device between the AVR and the mains.)

1) I can set the cross-over to 200Hz, turn off the mains, and just measure the sub's output. This should show me how the sub handles output above the cross-over range. So it's like measuring the sub all by itself. I can then adjust the BFD filters to correct the sub output so that it's nice and flat for as long as possible.

2) I can do like #1, but set a target fall-off in REW that shows the target to be a line that slopes down to be -3dB at the cross-over it looks like. This should simulate the required dropp-off in output from the sub if it's to match the cross-over in the AVR. I don't think i should do this, since the AVR is responsible for managing this drop-off in the signal to the sub. The sub should be as flat as possible below, at, and above the cross-over point i think?

3) I can set the cross-over in the AVR to 80Hz (where i have it normally). leave the mains on, andmeasure the output "full range" so see how the output is in the cross-over region. This measures the output across the sub and mains. I can then adjust filters in the BFD to correct the sub output to get the best result in the overlap region so that the net result is flat. I think this is the way to dial in the smoothest transfer between the mains and the sub...?

The problem with #1 is that i get some summing with the mains resulting in a boost from 40Hz on up when doing a full sweep with the mains active. So that seems to suggest that #3 is a better way to do it.

BUT also... #3 is the way to go if you don't take the ".1" LFE channel into account i think...? #3 is important for a smooth transfer between sub and mains, which is only important for low freq sounds from the main channels being rerouted to the sub. In addition to that, the sub has a dedicated .1 channel in the sound mix, and that one doesn't have the summing problem because that sound never gets sent to the mains.

So, it seems that you can either

a) EQ the sub output by itself to be flat to ~120Hz ideally (max freq for LFE stuff as far as i know). In this case the ".1" output will be nice and flat. But the low freq stuff routed from the mains to the sub an cause summing, resulting in a not-so-smooth net FR. The only way to combat this would be with control of the mains by means of EQ? Use a BFD for each channel?? I would just need to buy one more BFD and i could do 4 channels (L, C, R, sub). Might not be a bad investment? Then again, BFD's sound quality is not -that- good according to some... it's ok for sub stuff, but audiophiles scuff at the idea of using it for the mains.

b) EQ the net output of sub+mains and EQ for a flat FR all the way. In this case the mains and the sub will blend in nicely, but if you have the summing effect going on, you'll end up "dow-EQing" the sub some towards the cross-over point, which means that pure LFE stuff in that same freq range will be of a lower output than the rest.

What am i missing here? What's the proper way to do this? :scratch:


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## Sonnie (Apr 11, 2006)

Welcome to the Shack, Valorum!

First read post #2 and #3 here. That will give you some guidance.

Measure your sub only and post your graph here. You can upload it as an attachment. That will get us started. We'll look at the sub + mains later. Right now, all you can fix with the BFD is the sub itself, so let's just look at the sub response initially. But make sure you read the posts in the above link so that you have your settings correct.


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## Valorum (Apr 23, 2006)

Read 'm, and found a few tips there that i hadn't used yet. I'll post graphs when i get a chance so you can provide me some feedback on what to try. Thanks!


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## JohnM (Apr 11, 2006)

Valorum said:


> Using REW, there's a difference between running PC analog to the BFD directly vs PC digital output to the AVR and AVR sub out to BFD.


I'm assuming you ran the soundcard calibration, which uses the analog connections. If you keep that cal file and use the SPDIF output you will be compensating for low freq roll-off in the analog output stage which is no longer there, hence the boosted displayed results there. If you have not run soundcard cal you will still see a boost (of half as much) at frequencies below the point the analog output's low feeq response starts to roll off.


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## JohnM (Apr 11, 2006)

Valorum said:


> What am i missing here? What's the proper way to do this? :scratch:


There is always a compromise between EQing for the combined sub+main response and EQ'ing the sub alone for best LFE response. Ideally all channels would have some low freq EQ.


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## Valorum (Apr 23, 2006)

JohnM said:


> I'm assuming you ran the soundcard calibration, which uses the analog connections. If you keep that cal file and use the SPDIF output you will be compensating for low freq roll-off in the analog output stage which is no longer there, hence the boosted displayed results there. If you have not run soundcard cal you will still see a boost (of half as much) at frequencies below the point the analog output's low feeq response starts to roll off.


Ah, very interesting, i hadn't thought of that. It makes perfect sense that that might have something to do with it. I do seem to remember that the roll-off of the soundcard was actually small, but still... 

Hmmm... could / should i try to do the soundcard calibration by running an output (such as "monitor") from the AVR into the soundcard's LINE IN? That would calibrate the whole "loop" ...?


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