REW - Timing Reference for Sub-Woofer

[gibbo9000]

Have been using REW to attempt to get a time delay / phase correction for sub-woofer.  Both mains and sub-woofer are active with inbuilt DSPs (with their own delays) so pure physical alignment of speakers isn't sufficient.

The underlying challenge is that the limited frequency response of the sub-woofer stops it reproducing the timing reference signal for REW.  According to other posts I have searched it should still attempt to estimate the delay, but warn that it may not be accurate.  I can't get it to even provide an estimate.  So:

• Any thoughts on how to get it to estimate?
•  If that is not possible, is there an alternate approach to deriving it from the available data?

Failing that I guess I am back to SPL measurements as a surrogate. 

Any thoughts?

Gibbo

 

[AudioGeek]

What is your setup for REW? You need to send the sweep signal from REW and record on REW- so you dont need an accoustic timing reference.

Does your computer have a line out?

[gibbo9000]

46 minutes ago, AudioGeek said:

What is your setup for REW?

I am basically following this approach: https://www.minidsp.com/applications/auto-eq-with-rew/measuring-time-delay  which suggests using the timing reference in REW measurement.  Works fine for mains, but sub doesn't reproduce timing reference tone given its frequency response.

In my case REW on laptop  with out as USB into MiniDSP SHD Studio.  SHD provides two digital outputs - one to main speakers (active with digital in) and other via DAC to Sub.  Umik measurement mic connected back to laptop.

And I am only doing this in 2.1 config, not HT as per link.

Edited March 5, 2020 by gibbo9000

[Steffen]

I found that it helps to remove/bypass the low-pass filters in front of or inside the subwoofer, or adjust them to the highest frequency possible. Also, use a full range scan, not a limited (20-200Hz) one.

Edited March 5, 2020 by Steffen

[Snoopy8]

You must always use a normal speaker for the timing reference even if all you are doing is measuring the sub. Remember, the timing reference allows you to align measurements relative to one another.

[gibbo9000]

1 hour ago, Steffen said:

I found that it helps to remove/bypass the low-pass filters in front of or inside the subwoofer

I disabled all the filters into and on sub, and cranked the volume into/on sub as high as I could stand - and thought I heard something of the timing reference, but not enough for REW to recognise.

[gibbo9000]

3 hours ago, Snoopy8 said:

You must always use a normal speaker for the timing reference even if all you are doing is measuring the sub

There seem to be two concepts at play here from what I can tell, but using similar wording.

• First is, as you say, which speaker is the timing reference 'speaker'  from which relative delays are calculated.  For me that is main left.  That is set in REW and left the same for all 3 sweeps in a 2.1 system (L, R and Sub each done individually with only the relevant speaker active)
• Then there is the timing reference 'signal' that REW includes in each sweep (the chirp near the beginning).  I understand it uses that to calculate the 'system delay' for each speaker.  But with a sub, that 'chirp' isn't audible and REW complains and says it can't determine a 'system delay' for that speaker.

At least that is my understanding . . .

[Snoopy8]

28 minutes ago, gibbo9000 said:

There seem to be two concepts at play here from what I can tell, but using similar wording.

• First is, as you say, which speaker is the timing reference 'speaker'  from which relative delays are calculated.  For me that is main left.  That is set in REW and left the same for all 3 sweeps in a 2.1 system (L, R and Sub each done individually with only the relevant speaker active)
• Then there is the timing reference 'signal' that REW includes in each sweep (the chirp near the beginning).  I understand it uses that to calculate the 'system delay' for each speaker.  But with a sub, that 'chirp' isn't audible and REW complains and says it can't determine a 'system delay' for that speaker.

At least that is my understanding . . .

Not sure that REW can calculate and use the system delay for the timing reference?  The chirp comes from setting the timing reference, in your case the left.

 

Having said that, I am not an advanced user of REW, so perhaps someone else can chirp in. (Sorry, couldn't resist that  )

[gibbo9000]

Thanks for the input guys.  @snoopy8 was close.  As well as letting REW know the timing reference speaker, you need to leave that speaker turned on!!  REW will only send the timing reference 'chirp' to that speaker, not the full signal.  I was turning both mains off when measuring sub as sub takes signal from both mains.

So now I have a new problem - sub with its DSP is a good 200ms ahead of main speakers - more than MiniDSP of sub phase control can fix!

 

[Satanica]

3 hours ago, gibbo9000 said:

So now I have a new problem - sub with its DSP is a good 200ms ahead of main speakers - more than MiniDSP of sub phase control can fix!

That sounds very strange. I think some more investigation is necessary.

[gibbo9000]

11 minutes ago, Satanica said:

That sounds very strange. I think some more investigation is necessary.

Agreed.  But it appears real.  Main speakers are active with DSP built in - and in their most 'accurate' mode (they call 'Exact') they note a 90ms delay.  I would say there is more in addition to that.
I have tamed the timing reference/measurement issue and get consistent delays around 220ms.  MinDSP will take 30ms max out of that (by delaying signal to sub) and that reduces to 190ms as expected.  Sub delay/ phase can take a further 20ms out. 
So now looking at audio delay processors as an option into sub.

[Ninooe]

This guy has some good tutorials for home theaters. Ep 7 & 8 look into gain matching.

 

 

https://www.youtube.com/watch?v=_A6gPCczhuU&t

https://www.youtube.com/watch?v=tDaZT5G0Mwg

 

[andyr]

On 06/03/2020 at 8:23 PM, gibbo9000 said:

 

So now I have a new problem - sub with its DSP is a good 200ms ahead of main speakers - more than MiniDSP of sub phase control can fix!

 

 

Methinks you are using the wrong miniDSP unit?  The OpenDRC-DA8 has a full 3 seconds of delay! 

 

Andy

 

[gibbo9000]

58 minutes ago, andyr said:

The OpenDRC-DA8 has a full 3 seconds of delay! 

Great pick up - it seems to be capable of being much more 'intrusive' than the Studio SHD on many of the back-end functions - and in a similar sized package.  It seems to be a slightly different technology branch in that it is D/A with REW support whereas the SHD-Studio is purely digital domain with Dirac Support (and a Roon endpoint which is why I initially bought it). 

Ideally I need the 200ms delay in all digital domain but haven't found a device yet - will look further into miniDSP other products based on the OpenDRC.  For the time being I have had to put a DAC and analogue delay on the way to the DAC which is working fine. 

Gibbo

[gibbo9000]

On further looking the OpenDRC-D1 is the all digital version - but I was a bit hasty on what I need.  It is actually the DR8 that would apply the delay for Sub (in digital domain) and then do the D/A for output to sub . .

Gibbo