Bass management & LFE interaction

[darrancummins]

Hi all,

using the (MTRX Studio) DADman bass management with a 7.1.4 system, I noticed that the LFE channel's Low Pass Filter seems to interact with the Bass Management LPF for the main channels. E.g., the LFE's 120Hz filter slope is overlapping the 80Hz LPF for the front L, C and R channels, and causing it to roll-off more than expected; (approx 6dB dip in the frequency response around 60 - 80Hz). If I increase the LFE cutoff up to 200Hz (max possible value), the bass management filtering then works as expected, and we get a smooth crossover response on the main channels.

(Same is happening on all surround / overhead channels also. Using the default BM preset, all 24dB/oct slopes, Linkwitz-Riley, 80Hz crossovers, 120Hz LFE).

It looks as though the bass managed channels are being passed through the LFE filter, rather than summed in after it? I.e. the "Send to Sub" level for each main channel is actually "Send to LFE"...?

Any ideas how to get around this, or am I completely mistaken? Should the crossover slopes be adjusted to allow for this?
Is there a signal flow diagram available for the bass management & LFE routing in DADman, so I can see exactly how all this is working?

[LDS]

I can't comment on DADman bass management, but I have been caught out by a doubling up of bass management when it was also activated inside the Dolby Atmos Renderer. Just a thought. If you suspect a bug in DADman, I would take it straight to an Avid support ticket.

[BScout]

An LFE channel is a discreet channel. The LPF on an LFE affects everything put out by that channel. As a discreet mix channel.

Bass management is splitting the screen/surr channels output and redircting. You are using an 80Hz as a HPF on the LCR (Not LPF as written?) and then sending the rest of the signal to an LFE channel? Well then that output it going to go through the eq on the LFE.
If you have a dual input sub, use the secondary input as your bass management input and set up a separate Sub out to go to that with no filter.

Yes, the most basic answer is yes; things are operating correctly. If you pass a signal through two filters, they will compound. Much the same as if you had a physical LPF on your sub (which is what all the eq on the outputs operate as.) Seems you are expecting the LPF on the LFE to be on the input fed but all the eq is on the output (as if embedded in the speaker.) So no, the bass management output from screen channels doesn't sum after the LFE input is filtered because the filter is at the output.

If you wanted to get really crazy and you don't have a secondary input on your sub, you could redirect the LFE input to loop through a fixed monitor with eq (LPF) applied and the output of that loop summed back (with now no filter on the LFE) to the main set of monitoring outputs. You would just have to set up a separate mon that is REF locked (and hopefully you only have a few 5.1+ inputs since all the LFE channels would have to be put through this for all sources and summed on that side)

[gives]

Originally Posted by BScout

An LFE channel is a discreet channel. The LPF on an LFE affects everything put out by that channel. As a discreet mix channel.

Bass management is splitting the screen/surr channels output and redircting. You are using an 80Hz as a HPF on the LCR (Not LPF as written?) and then sending the rest of the signal to an LFE channel? Well then that output it going to go through the eq on the LFE.
If you have a dual input sub, use the secondary input as your bass management input and set up a separate Sub out to go to that with no filter.

Yes, the most basic answer is yes; things are operating correctly. If you pass a signal through two filters, they will compound. Much the same as if you had a physical LPF on your sub (which is what all the eq on the outputs operate as.) Seems you are expecting the LPF on the LFE to be on the input fed but all the eq is on the output (as if embedded in the speaker.) So no, the bass management output from screen channels doesn't sum after the LFE input is filtered because the filter is at the output.

If you wanted to get really crazy and you don't have a secondary input on your sub, you could redirect the LFE input to loop through a fixed monitor with eq (LPF) applied and the output of that loop summed back (with now no filter on the LFE) to the main set of monitoring outputs. You would just have to set up a separate mon that is REF locked (and hopefully you only have a few 5.1+ inputs since all the LFE channels would have to be put through this for all sources and summed on that side)

My LFE is a Focal Sub 12 and it's handled in DADMAN with Smaart V8 by Rational Acoustics Room Tune. The Bass Management is handled for the Tops, (0verhead speakers) by my other Sub, which is a Genelec 7370A which is doing a great job here and Dolby used my room for a private event and they dug it!

G

[BScout]

Exactly. 2 subs or 2 inputs to one sub.
Either gives the flexibility.

[LDS]

Originally Posted by BScout

An LFE channel is a discreet channel. The LPF on an LFE affects everything put out by that channel. As a discreet mix channel.

Bass management is splitting the screen/surr channels output and redircting. You are using an 80Hz as a HPF on the LCR (Not LPF as written?) and then sending the rest of the signal to an LFE channel? Well then that output it going to go through the eq on the LFE.
If you have a dual input sub, use the secondary input as your bass management input and set up a separate Sub out to go to that with no filter.

Yes, the most basic answer is yes; things are operating correctly. If you pass a signal through two filters, they will compound. Much the same as if you had a physical LPF on your sub (which is what all the eq on the outputs operate as.) Seems you are expecting the LPF on the LFE to be on the input fed but all the eq is on the output (as if embedded in the speaker.) So no, the bass management output from screen channels doesn't sum after the LFE input is filtered because the filter is at the output.

If you wanted to get really crazy and you don't have a secondary input on your sub, you could redirect the LFE input to loop through a fixed monitor with eq (LPF) applied and the output of that loop summed back (with now no filter on the LFE) to the main set of monitoring outputs. You would just have to set up a separate mon that is REF locked (and hopefully you only have a few 5.1+ inputs since all the LFE channels would have to be put through this for all sources and summed on that side)

Almost 20 years ago the AES published a list of recommendations regarding bass management and LFE. This is what they recommended for accurate low frequency reproduction, as it results in only a potential 60 degrees of phase shift between bass managed and LFE signals. Summing the LFE and bass managed signals should happen at the output, post filtering:




Bass managed signals never become LFE. They might share a DADman output or use the same subs, but they are absolutely two separate things serving different purposes. Putting a 120Hz LFE LPF over the top of the bass management LPF will make it way more difficult to achieve a congruent, full range output cross your mains & subs.

A lot of people are forced to work that way as their subs have built in LPFs that can't be removed from the signal path. I don't see a single good reason why DADman should do it, and I would be very surprised if any of the competition did it. Trinnov definitely don't apply the LFE filtering to bass managed signals.

[BScout]

I can confirm -- just ran a test on a secondary output -- darrancummins situation. LP component of 5.0 channels run through the LPF on the LFE.
So LFE signal from input sums with 5.0 LP signal before hitting the output LFE's LPF filter.

From an EQ standpoint (in the EQ section) that makes sense as that's why it's shown and if you are correcting the sub's frequency response with eqs there (EQ section), it should apply to the output from the 5.0 LP signal too.

Now I've always been for DADman/SPQ adding some virtual busses (which would be necessary to do it the other way), so complain away.
(But this has been how it has always worked and not a "bug.")

[BScout]

Btw, found DAD's block signal diagram for the SPQ system



https://www.digitalaudio.dk/media/SP...-Diagram-2.jpg

Though slightly unclear, seems to be showing a 2 subwoofer/LFE setup and you can see two LPF being applied for both subs in series one pre and post summation.
(It is unclear, though. Not the way I'd draw it as there should be variable symbol and distinction on which LPF filters are which.)

[darrancummins]

Hi all,

thanks for the very detailed replies! I did some more testing, confirmed the two LPFs (BM and LFE) are indeed in series. I tried a couple of things - changing the LFE filter type to Butterworth gives it a steeper slope with harder corner / knee, so it has less effect down around the bass management crossover point. This, plus moving the BM crossover lower to about 70Hz seems to mitigate the overlap effects.

Originally Posted by LDS

Bass managed signals never become LFE. They might share a DADman output or use the same subs, but they are absolutely two separate things serving different purposes. Putting a 120Hz LFE LPF over the top of the bass management LPF will make it way more difficult to achieve a congruent, full range output cross your mains & subs.

... I don't see a single good reason why DADman should do it, and I would be very surprised if any of the competition did it. Trinnov definitely don't apply the LFE filtering to bass managed signals.

This is how I always understood it, was surprised to see it happening here.

Originally Posted by BScout

I can confirm -- just ran a test on a secondary output -- darrancummins situation. LP component of 5.0 channels run through the LPF on the LFE.
So LFE signal from input sums with 5.0 LP signal before hitting the output LFE's LPF filter.

From an EQ standpoint (in the EQ section) that makes sense as that's why it's shown and if you are correcting the sub's frequency response with eqs there (EQ section), it should apply to the output from the 5.0 LP signal too.

Now I've always been for DADman/SPQ adding some virtual busses (which would be necessary to do it the other way), so complain away.
(But this has been how it has always worked and not a "bug.")

EQing the sub to correct its overall response is fine, but that's a separate matter. The LFE LPF is just to remove any info above 120Hz from the LFE channel, (as per Dolby) - shouldn't impact the main channels at all. Looks to me like the LFE LPF should be pre-summing...?

[BScout]

Originally Posted by darrancummins

EQing the sub to correct its overall response is fine, but that's a separate matter. The LFE LPF is just to remove any info above 120Hz from the LFE channel, (as per Dolby) - shouldn't impact the main channels at all. Looks to me like the LFE LPF should be pre-summing...?

This is where my "kludge" method would work. For your main 5.1 make it a 5.2 so you can treat bass management as a different out than LFE out and send both of those .2 channels to an unused output. Then loop both of those back (physically since DADman doesn't have virtual busses.) It could be 1 AES cable (2 ch). Make a second mon entry that just sums the two loopbacked inputs and outputs to your actual sub output. On this one, apply your corrective eqs for the sub for the room. Ref lock it and then just leave it alone.

You'll still only need to manage the first 5.2 for the room for volume but the separate LPF will then be available without running in series for the bass management.