# UMIK-1 timing reference and Phase



## orangeart

John, I don't know if you'll see this but I wondered if in the future you'll have a way of dealing with this?

As REW adjusts t=0 to correspond with the impulse peak when using this mic, presumably that means that all of my phase measurements for different drivers is also off? As is it obviously not possible to have a loopback with a USB system users will need a way of being able to rely on phase measurements and viewing delay for different drivers in a speaker as this type of mic becomes more popular.

Maybe in the future it would be possible for the user to set t= ? If we know the delay for a signal to propagate through the mic and into the USB interface we could set it to that manually, or if miniDSP let you know what the delay is you could incorporate it into the auto detection routine setup for the UMIK-1?

Alternatively could we just assume that this propagation time is zero and set the t=0 at the first sample, This may not be entirely accurate but would at least be consistent across measurements and allow us to view the difference in delay between measurements.

Maybe I am missing something?

Many regards

Stefan


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## JohnM

A reference is needed to allow comparison between measurements because the operating system latencies for the audio replay and capture paths can be very different between measurements. If there are no (or only known delays) in the path after the audio output then the REW "Estimate IR Delay" feature can be used to remove the measurement delay, any known delays (e.g. time delays programmed into digital crossovers) could be added back manually using the t=0 offset adjustments in the Impulse graph controls. 

The long term solution for USB mics involves generating a reference signal ahead of the measurement sweep and using that for alignment. I have a scheme in mind for that but I'm not sure when I'll get to implementing it. A workaround is to place a tweeter very close to the mic and drive it with the same measurement sweep that goes to the unit under test, the impulse response will then have a peak from the tweeter and a later peak from the unit being measured (delayed by the additional time of flight). The tweeter peak in separate measurements can act as a reference to align them.


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## charlieblue

JohnM said:


> A workaround is to place a tweeter very close to the mic and drive it with the same measurement sweep that goes to the unit under test, the impulse response will then have a peak from the tweeter and a later peak from the unit being measured (delayed by the additional time of flight). The tweeter peak in separate measurements can act as a reference to align them.


Now I get it...

Thanks, John!

Charlie

PS. Eagerly waiting for your "long term" solution, though : )


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## orangeart

JohnM said:


> A reference is needed to allow comparison between measurements because the operating system latencies for the audio replay and capture paths can be very different between measurements. If there are no (or only known delays) in the path after the audio output then the REW "Estimate IR Delay" feature can be used to remove the measurement delay, any known delays (e.g. time delays programmed into digital crossovers) could be added back manually using the t=0 offset adjustments in the Impulse graph controls.
> 
> The long term solution for USB mics involves generating a reference signal ahead of the measurement sweep and using that for alignment. I have a scheme in mind for that but I'm not sure when I'll get to implementing it. A workaround is to place a tweeter very close to the mic and drive it with the same measurement sweep that goes to the unit under test, the impulse response will then have a peak from the tweeter and a later peak from the unit being measured (delayed by the additional time of flight). The tweeter peak in separate measurements can act as a reference to align them.


Hi John, I hadn't realised that there was so much discrepancy because of operating system latency, I can see how that would mess everything up!! I like the idea you posted in the other thread about 'keep audio path open' Hopefully you'll get to that quite soon, I'm sure the growing community of active speaker builders using your excellent software with a UMIK-1 would applaud that, there seems to be quite a lot asking very similar questions. I'd certainly be happy to make a donation to help as would others I'm sure. The tweeter thing isn't really doable for driver measurement as that tends to be done at least in part in near field and squeezing in another tweeter closer that the one I am measuring seems impossible!!

This part of your answer confused me a little, could you expand, is this something that could help me?



JohnM said:


> If there are no (or only known delays) in the path after the audio output then the REW "Estimate IR Delay" feature can be used to remove the measurement delay, any known delays (e.g. time delays programmed into digital crossovers) could be added back manually using the t=0 offset adjustments in the Impulse graph controls.



Many regards and I look forward to hearing back from you.

Stefan


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## JohnM

'Estimate IR Delay' attempts to remove pure time delays from measurements, so using it on a set of driver measurements should give results that can be cross-compared. If there were deliberate delays in the crossover (e.g. if the crossover included a time alignment delay) then they could be added back by applying an equivalent adjustment to the t=0 position for the corresponding driver's measurement.

Whether that would work in practise is hard to know for sure though.


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## orangeart

Hi John, I wondered if you'd had any mare thoughts on implementing the scheme you had in mind for timing references forbusb mics?

Stefan


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## JohnM

Sorry Stefan, haven't got any further with that yet. It is near the top of my todo list though.


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## orangeart

Xmas present maybe? Or further into the future than that?

I'm very pleased that it's in a list at all, that's more solid than just a thought. :smiley:

Stefan


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## orangeart

JohnM said:


> Sorry Stefan, haven't got any further with that yet. It is near the top of my todo list though.



Hi John,

Just in the throws of speaker building AGAIN!! Just wondered if you were any closer to having this functionality sorted out?

Many regards

Stefan


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## JohnM

Sorry, not yet.


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## orangeart

Ok, never mind, worth an ask. I might try learning holmimpulse just for that bit of the process.


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## natehansen66

I have issues with the time lock moving from measurement to measurement in Holm, but my pc based system is quite complex. The only solution for me is a regular mic and preamp in REW.


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## orangeart

That's interesting, how complex is complex?


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## natehansen66

Active xo/eq done in software on the same pc I do measurements. Depending on how I do it the audio is either grabbed from the Windows mixer or its sent through an ASIO driver.


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## AudiocRaver

I have done similar things, with the following issues:

it takes a more powerful machine
getting drivers to behave with each setup change can be a chore; finally resorted to using two machines for that reason (FYI)


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## orangeart

Hi John, I just wondered if you'd got to this yet? Or if it might be on your list of things to do soon?


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## JohnM

Not done yet, but yes, it is on my list to get to soon.


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## orangeart

That's great John. I know how long my 'soon' can often stretch to though. Are we talking this quarter or maybe he spring???


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## JohnM

Hoping to do it before year end.


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## orangeart

You star, I'll buy you a pint if you are ever near me. Doing a speaker measurement tutorial in March at an audio show and was hoping to be able to demonstrate that one. Brilliant.


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## IslandHydro

Can anyone point me to any kind of guide for doing time/phase alignment?


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## jtalden

For SW to Mains XO using a UMIK-1 mic?

There are several easier ways to find good alignment settings other than using phase. We can recommend one of those methods and help with any questions you have. The only practical disadvantage to those methods is that it is not possible to see the phase tracking chart and confirm that it is the optimized alignment. Functionally there is probably no sound quality disadvantage to any good alignment. That is, any alignment of the SWs to mains is good if the SPL support through the XO range is good (assuming the distances/delays are reasonably close to measured values). 

If you want to use the phase alignment method with that mic it may be best to wait for the locked/relative timing capability that JohnPM just indicated is expected soon. If you want to see what is involved when using this mic now *here* is a link to a thread to give some idea of that process. I can answer questions regarding that method if that is your interest. If you just want me to offer an optimized timing recommendation for phase tracking through the XO I can do that if given the needed measurements of your system.


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## IslandHydro

Both SW to Mains, and inter-driver mains (I'm fully active XO throughout), and yes a UMIK-1. At this point all I've done is run a sin wave at each XO frequency (driving just the pair being XO'd) and adjust timing to maximize gain. I know that is pretty simplistic, I guess I'll just hold out for John's hopefully upcoming mod.


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## jtalden

I would expect that sweeps or RTA measurements are more reliable although I have never tried to do it with a sine waves. That may work just as well. I am not sure. If the distances/delay settings are initially set to the measured distances then one easy and reasonably reliable method is to temporarily reverse the polarity of the one of the drivers and fine tuning the delay until there is a deep null at the XO frequency. This method is more reliable at the higher frequency XO where room modes are not a big issue and the mic can be placed relatively close to the main speaker.


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## PhilH

This is my first ever post on a forum! I'm not an expert but this is how I time aligned an active xo using rew , HDMI , minidsp and a umik1. I think it works. 

First I eq'd both drivers flat, an octave or so either side of the crossover. Then I ran three REW sweeps of that frequency range without moving the mic: one for each driver and one with the two drivers together (with no HP or LP filters on).
Next I set t=0 to the positive peak for each impulse. In Overlays in REW I then created an A+B of the two drivers.
This created an fr trace that looked nothing like the measured trace for the two drivers together. Why not? Because the A+B is time aligned but the real drivers weren't. 
So how to calculate the delay required to align them? In the IR screen I offset the t=0 for the woofer a little. Then I went back did the A+B thing again and compared to the measured fr. I repeated this process several times until, as if by magic, the A+B fr perfectly overlapped my original measurement.
From memory, this occurred with the woofer's positive IR peak set at around 0.2ms, which made sense. So now I knew that the tweeter was leading the woofer by 0.2ms without having to rely on any absolute timing locks. From there it was easy. Delay the tweeter in minidsp , apply the lp and hp filters and listen to some music.


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## orangeart

JohnM said:


> Hoping to do it before year end.



Hey John, just checking back in again. I guess you didn't get to this in the end, at least I can't see it in the change log.

Any further ETA's you can let us have?

Hope you have a great Christmas and Look forward to more great things from this great software next year!

Stefan


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## JohnM

Started on it once I got the 5.14 release out, first beta with acoustic timing ref will be sometime in Jan.


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## orangeart

WAHOOOO!

John, this piece of software has more than any other bit of kit or knowledge moved forward what has been possible in terms of home audio reproduction .

Well done.

Stefan


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## IslandHydro

orangeart said:


> WAHOOOO!
> 
> John, this piece of software has more than any other bit of kit or knowledge moved forward what has been possible in terms of home audio reproduction .
> 
> Well done.
> 
> Stefan


So agree with this, oh and Merry Christmas all!


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## orangeart

JohnM said:


> Started on it once I got the 5.14 release out, first beta with acoustic timing ref will be sometime in Jan.


Hey John, did the last beta have this, I don't generally load up the beta builds but will make an exception of it did 

Stefan


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## JohnM

Afraid not, work paused while I was away for xmas and new year and I haven't got back to it yet. Just back from some business travel but should be able to get going on it again this weekend.


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## orangeart

Great stuff. Running a speaker measurements forum at the Scalford Hall Audio show on the 20th March and this would really simplify the demo, so if you'd managed to do it by then I'd forever appreciate it :wink:

Stefan


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## charlieblue

My long wait is coming to an end… where is that heartbeat emoticon ?

There it is! :heartbeat:

Thank you John! :kiss:


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## orangeart

Hey John, any movement here?

Stefan


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## JohnM

Yes, going well. As of last weekend I have a build that measures with an acoustic timing reference and seems to give good results. I need to add a few convenience elements, like being able to set the timing ref level independently of the measurement sweep level and making it easier to switch the output being used for measurement, but it's working now.


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## lmagoo

Newb here...but just got a Dirac 88-A myself. Comes with the UMIK that seems to work pretty well....

Can't believe the difference Room Correction software makes!!! Well worth it!

Hope to learn more here..

Cheers!


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## JohnM

I have uploaded V5.15 beta 3 to the installers directory with initial support for an acoustic timing reference. The choice of timing reference is in the Analysis preferences. If an acoustic timing reference is selected the measurement dialog shows a control for the level of the timing reference signal and, when using Java drivers, which channel to use for the timing signal (with ASIO drivers REW uses the timing ref channel selected on the Soundcard preferences). A limitation of this initial implementation is that the reference channel is only used for the timing signal, it can't be measured while using it as the timing reference (e.g. if the left channel output is being used for the timing ref, the measurement sweep will only be on the right channel output). I'll look at addressing that in a subsequent build.

The timing reference signal is a linear sweep from 5 kHz to 20 kHz, so the speaker being used to provide the reference needs a tweeter (a sub cannot be used as the timing ref). Keeping it to high frequencies helps with accurate timing and means the signal will usually be reproduced by the tweeter alone, so the derived timing signal peak location will not be affected by timing differences that may be present between drivers in a multiple driver speaker. The sweep is quite short, about 0.7 seconds, so it shouldn't present any power handling issues. The actual peak levels detected for the timing reference and the measurement can be read from the "Source" section of the Info panel. The delay relative to the timing ref is shown in the "System Delay" section of the info panel, as a time and equivalent distances in metres and feet.

I have only had a chance to test this on a Windows machine. As ever, please let me know of any unexpected behaviour.

Here is the cumulative list of changes since V5.14:


Moved preview generator into a separate thread to avoid delaying display of file dialog
Added sample rate, start freq and end freq to default filenames for sweeps saved as WAV
Removed the confirmation dialog after signal WAV generated
When meas sweep is selected start and end frequency spinners reflect chosen sweep start/end rather than being half/double
Removed redundant check for 0..1 input volume range
Increased range of impedance Y axis to 10k
Changed signal generator linear and log sweep fade in and fade out to 10 ms (fade in was 8k samples at start freq, fade out was 8k samples)
Start delay is available for both SPL and impedance measurements, relocated the start delay controls below the Start Measuring button
Added an acoustic timing reference option

Bug fix: Drop Small filters used a threshold of 1 dB instead of half the flatness target in some circumstances
Bug fix: Apply Windows could fail when the overlays window was being used
Bug fix: Loading cal file could fail when using UMIK under Linux
Bug fix: Controls in graph axis limits dialog could disappear under some circumstances
Bug fix: Var, Psy and ERB smoothing did not work on RTA spectrum trace


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## orangeart

That is great John. Scalford hall Audio show is the weekend after next so this has come at a perfect time. Busy this week but will thoroughly test in next week thanks you so much.

Stefan


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## jtalden

Hi JohnM,
Thanks for another great update!


I see the measurement sweep signal is modified a little so I intend to update my external sweep file accordingly. I looked to see if the measurement sweep export feature now includes a new option to save a stereo version of the sweep with the new acoustic timing sweep on the reference channel. I don't see that option yet. I am thinking that it will make it possible to run external sweeps that provides the capability of the new acoustic timing option. 


I use the REW default of right channel for the measurement and left channel for the reference, but it would make no difference to me how the two channels is saved. They can even be separate files as I can use Audacity to combine two mono files to stereo, or to switch left and right if needed in a stereo file.


Thanks for considering this addition.


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## dziemian

Thank you very much. With this addition andyc56 (the creator of Multi-sub optimizer) can make his software work with USB mics which makes it much easier.


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## JohnM

jtalden said:


> I see the measurement sweep signal is modified a little so I intend to update my external sweep file accordingly.


The measurement signal hasn't actually changed, just the way that start and end are shown in the signal generator controls to make them consistent with the measurement window settings.



> I looked to see if the measurement sweep export feature now includes a new option to save a stereo version of the sweep with the new acoustic timing sweep on the reference channel. I don't see that option yet. I am thinking that it will make it possible to run external sweeps that provides the capability of the new acoustic timing option.


The meas sweep export needs an additional dialog to choose whether or not to include a timing signal and where, I haven't done that yet. It would also lend itself to a mode where REW can be set to start capturing and wait to see the timing signal before capturing the measurement, removing the complication of trying to time things by hand. On my todo list


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## jtalden

Thanks John!


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## AustinJerry

Very nice indeed, John! Now I need to go back and research all of the instances in which my measurement accuracy was called into question because of the lack of a timing reference. Serious homework to do!

And I feel an update to my beginners REW guide will be coming soon... :smile:


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## andyc56

dziemian said:


> Thank you very much. With this addition andyc56 (the creator of Multi-sub optimizer) can make his software work with USB mics which makes it much easier.


Hi all. I'm the guy who wrote the Multi-Sub Optimizer (MSO) software.

I mentioned this in another thread, but it's worth repeating here. I used to have big, bold warnings about using MSO with USB mics, which I've since updated to reflect the current REW status with regard to the new acoustic timing reference feature. I had those warnings there because if one attempted to use measurements taken with a USB mic without an acoustic timing reference, MSO would appear to work fine, but give wrong results. This is an insidious type of "silent failure" error that needs to be clearly warned against in the documentation, especially considering that USB mics now dominate the measurement scene. There are actually no modifications to MSO required for it to give correct results when used with measurements taken with a USB mic, as long as one uses the acoustic timing reference when performing the measurement.

Kudos to John, not only for the quality and breadth of features of REW, but also for his amazing commitment to it.


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## Mike2001

JohnM, many thanks for the acoustic timing reference addition for USB microphones. I had the chance to do some measurements today - in preparation for some MSO calculations (thanks andyc56) - and the new system is so much easier to use, previously I had to crawl behind my AV cabinet to disconnect speaker cables and connect up some pre-amp outputs to a Behringer ADC.
The quick high frequency sweep gave identical timing accuracy to my previous method - so I knew I was good to go!!!!

Now, how about a quick access panel for selecting the HDMI output channel, with inc/dec buttons???????????

Regards, Mike.


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## JohnM

A bit more progress on the acoustic reference support, V5.15 beta 4 (available from the installers directory) has added the ability to include the timing reference signal when saving a measurement sweep as WAV and there now is a "Wait for timing reference" check box on the measurement dialog. When that box is checked REW will delay capturing the main measurement sweep until it has seen the timing reference signal, which allows coping with unlimited amounts of latency and makes it easy to measure by replaying the measurement sweep WAV if the REW output cannot be used directly.

I've also made some small changes to the waterfall and spectrogram plots.

The changes compared to beta 3 are:

Added option to add timing reference signal when saving measurement sweep as WAV
Made the signal generator export filenames more descriptive
Show a warning if using a timing reference with ASIO but haven't selected a timing ref output
Removed the fade to graph colour at the bottom of the spectrogram to better define the lower parts of the plot
Restored Violet as the bottom colour for the Rainbow spectrogram colour scheme
Added a cursor frequency line over the surface of the waterfall plot
Added a measurement option to wait for the timing reference signal to be detected when using an acoustic reference
Bug fix: Spectrogram contours were not redrawn when plot range was changed
Bug fix: Frequency axis grid lines were not drawn on floor of waterfall when using linear frequency axis


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## jtalden

JohnM,
Thanks!
I spent the day figuring out how to get this setup correctly. I am using the 'acoustic timing' feature and the 'hold for trigger' function when using an external signal from my DLNA file server. After some confusion and dumb mistakes I finally got it all sorted out. I now have accurate IR timing with foobar fed REW signal files and can thus confirm that close phase tracking is properly retained when the ReRhase FIR phase correction filter activated. Very nice!


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## orangeart

Had some success and some failure with this so far. One one of my setups it worked flawlessly once I figured it out. On a second setup I get the test signal also coming from the channel I have setup as the timing reference. Obviously a full range signal from the tweeter  Might be I've messed something up in the setup but pretty sure I mirrored the working setup on the other PC. I used the same gear.


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## AustinJerry

John, just to be clear, when selecting the channel to use for timing reference, there is no requirement that the speaker on that channel be closer to the MLP than the speakers being measured, is there? For example, when measuring my left, right, and center channels, I am using a side surround as the timing reference. That speaker is ~.5 ft further away than the mains from the MLP.


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## JohnM

It doesn't matter whether the timing reference is nearer or further away.


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## 3ll3d00d

JohnM said:


> A limitation of this initial implementation is that the reference channel is only used for the timing signal, it can't be measured while using it as the timing reference (e.g. if the left channel output is being used for the timing ref, the measurement sweep will only be on the right channel output).


Would you expect a meaningful increase in precision over just measuring that channel without a reference? Put another way, what are the variable latency sources that this feature would eliminate (in the case of measuring the reference channel)?


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## JohnM

Using the timing reference doesn't make any difference to the accuracy of the measurement, the method copes with static latencies. In any case, I have removed the restriction on measuring the ref channel in beta 6, which is now in the usual place. 

Changes in beta 6:

Added a left/right output channel selector in the Soundcard preferences when using Java drivers, meas sweep only appears on selected channel unless using loopback, when it appears on both
When using acoustic timing reference can now also measure the timing ref channel
Added an equaliser setting for the Anti-Mode 2.0 Dual Core


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## JohnM

A few more changes for beta 7:


Rearranged the input and output selectors on the Soundcard preferences panel
Added a "Both" option to the output channel selector when using Java drivers
Added output selectors on the measurement dialog
Added display of timing reference headroom below the timing ref level control on the measurement dialog
Added the timing reference mode and timing reference output name to the measurement Info panel
Added a display of the CEA-2010 peak level on the RTA graph


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## Mike2001

Although I have not had a chance to try out Beta7 (with output selection on the Measurement dialog) I felt that a 'like' simply did not go far enough.
Many, many thanks John for all the steady improvements to REW - absolutely unbeatable.

Best regards, Mike.


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## Mike2001

I will generously assume that other people have already realised an issue with using the new acoustic timing reference - but just in case you have not, I'll try to explain what I have realised.

A proper electrical timing reference will generally allow you to measure the flight time from speaker to microphone. Moving the mic relative to the speaker will show a change in timing due to the distance change - all good, and expected. Moving the mic along an arc, while keeping the speaker distance constant - will show no change in timing, again all good, as expected. Now try the same thing with an acoustic timing reference (using a surround speaker as your reference for example) - if the mic remains stationary the relative timing of various speakers will be accurate - but if you move the mic relative to the reference speaker, all the timings will change - even if you maintained a constant distance to the measurement speakers.

Therefore, if taking single point measurements - acoustic timing works just as well as electrical. But if you plan on taking multiple position samples (that require accurate timing) - don't use acoustic, stay with electrical. It is possible to correct for the reference movement - but it's a bit of a pain. I will grudgingly return to electrical loopback for my next session.

I'm not being critical of John's work - it works as intended, I've used it - just trying to let others know of an inherent issue. That's if others can understand my ramblings :smile:

Regards, Mike.


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## andyc56

Mike2001 said:


> Therefore, if taking single point measurements - acoustic timing works just as well as electrical. But if you plan on taking multiple position samples (that require accurate timing) - don't use acoustic, stay with electrical. It is possible to correct for the reference movement - but it's a bit of a pain. I will grudgingly return to electrical loopback for my next session.


Hi Mike,

Are you AV_mike over at AVS? I was asking because you mentioned MSO in an earlier post. Anyway, I thought about this very thing a few weeks ago. At first, I thought it might invalidate MSO results. But then I realized that the acoustic timing reference allows the summation of complex acoustic pressure amplitudes of multiple subs and main speakers taken individually _at a given listening position_ to match what the measurement would be if it were performed with simultaneous excitation. Since each summation of an MSO measurement group normally corresponds to only a single measurement position, the integrity of each summation (the MSO measurement group graph trace) is preserved. If the measurements in a given MSO measurement group were taken at different listening positions, there will be a problem though, and the summation won't be valid. MSO is flexible and allows measurement groups to be defined in this way, but such a summation has no useful physical interpretation that I am aware of.

So it's all good as far as normal usage of MSO is concerned. Normal usage would be the typical scenario for which all the measurements in each MSO measurement group were all taken at the same listening position. Taking phase differences _between_ measurement group summations won't be valid, but I can't see any reason to do that. Also, MSO uses only the magnitude, and not the phase, of the complex summation of the components of a measurement group in its calculations.


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## Mike2001

Hi Andyc56,

This is my alter-ego...............................

I must admit, I thought MSO needed the phase to perform the complex summation - as phase and delay share similarities, and would affect how two or more sources interact within the room?

I have been using MSO to combine dissimilar subs - never even got close trying to do it manually - many, many thanks Andy.
I have also been using MSO to get the best sub-sat splice in two listening positions - this is where microphone movement is required. I noticed that my center channel impulse had shifted by over four feet - and yet the center speaker is equi-distant from each listening position. The change was because the mic moved relative to the surround speaker (timing ref source). My initial MSO analysis gave some pretty poor results - until I manually corrected for the impulse timing movement.

While you're here Andy, any changes to MSO on the horizon? I have tried the Target Curve option - but it does not operate as I expected (I'll try to make some notes). A permanently visible RMS error display, and iteration counter would be useful.

Regards Mike (Mike2001, AV_mike)


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## andyc56

Mike2001 said:


> I must admit, I thought MSO needed the phase to perform the complex summation - as phase and delay share similarities, and would affect how two or more sources interact within the room?


Yes, MSO does need the phase to perform the complex summation. However, it does not require that the _absolute_ delay of each measurement be correct. It only requires, for a given complex summation, that the measurements preserve the delays _relative to one another_ of each speaker or sub measured.

This is not unlike how an AVR works internally. An AVR uses zero delay for the most distant speaker to the MLP, then delays closer speakers based on their relative distance to the MLP from the most distant one. So the AVR only uses the relative delays for this calculation. It's a similar idea with MSO.



Mike2001 said:


> I have also been using MSO to get the best sub-sat splice in two listening positions - this is where microphone movement is required. I noticed that my center channel impulse had shifted by over four feet - and yet the center speaker is equi-distant from each listening position. The change was because the mic moved relative to the surround speaker (timing ref source).


If all else is done correctly, this should be okay, as the sub distances should all shift by four feet also, thus preserving the delays relative to one another of each speaker and sub. The latest REW beta allows using a channel under test as the acoustic timing reference, so if this shift in center channel distance bothers you, you can use the center as the timing reference.



Mike2001 said:


> My initial MSO analysis gave some pretty poor results - until I manually corrected for the impulse timing movement.


I would absolutely recommend _against_ any second-guessing of REW data by manual time shifting in this way. As long as the measurements were performed correctly, and there's no bug in this new REW feature, REW should take care of all of this internally. In this case, "taking care of it" should manifest itself in a four-foot shift in the impulse responses of the subs at the new position, preserving the relative delays of the subs to one another and to the center channel.



Mike2001 said:


> While you're here Andy, any changes to MSO on the horizon? I have tried the Target Curve option - but it does not operate as I expected (I'll try to make some notes). A permanently visible RMS error display, and iteration counter would be useful.


For the time being, I'm not working on it, as I'm dealing with home repairs due to water damage from an unfortunate plumbing incident. I'll add those requests to the list, and I have some other planned improvements too. But they may be a while. I can help you with the target curve in the main MSO thread.


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## JohnM

Not strictly related to acoustic timing reference, but I have extended the output selection to the signal generator and made a few other changes in beta 8, available in the usual place.


Added output selection to the signal generator
Added bit length selection to signal generator WAV export (16, 24 or 32-bit WAV files can be generated)
Rearranged the signal generator controls
Added next and previous 1/3rd octave frequency buttons to the CEA burst generator
Added the output name to the measurement Info panel


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## JohnM

I have uploaded beta 9 to the installers directory. This version is earlier than intendede due to an error in the way beta 7 and beta 8 were built. Files made in beta 7 will not open in beta 8 or later and files made in beta 8 will not open in beta 9 or later. The workaround is to open files made in beta 7 or beta 8 in beta 6 or earlier, then save them again. The saved files will open in any later version. There are a couple of small changes in beta 9, long file names should not cause wrapping in the measurement panel and the CEA peak value turns red if any harmonic limits are exceeded. I have removed beta 7 and beta 8.


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## brumledb

Would someone please outline how to use the Acoustic Timing Reference? I understand how to set everything up properly (I think). My main issue is how to set the reference channel? Currently, it seems like the reference channel changes with each measurement since the timing reference sweep gets measured each time.


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## AustinJerry

brumledb said:


> Would someone please outline how to use the Acoustic Timing Reference? I understand how to set everything up properly (I think). My main issue is how to set the reference channel? Currently, it seems like the reference channel changes with each measurement since the timing reference sweep gets measured each time.


First, go to the REW Preferences screen and click on the Analysis tab. Under Impulse Response Calculation, make sure "Use acoustic timing reference" is selected.

On the Preferences screen click on the Soundcard tab, select a speaker to use for the timing reference in the "Timing Output Reference" box, and then select the speaker you want to measure in the "Output" box. The same two selection boxes now appear for convenience on the Measurements screen (in the latest REW beta versions).

For example, if you select HDMI2 as the timing reference, then when you click the Measure button, you will hear a brief burst of sound from the right speaker before the measurement sweep is conducted for the speaker you are measuring. As you select additional speakers to measure during a measurement session, it is important not to change the timing output reference speaker so that the timing reference is consistent from one measurement to the next. If you are using the right channel as the timing reference, the right channel can be measured as well by simply selecting HDMI2 as both the timing reference and the output.


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## brumledb

AustinJerry said:


> First, go to the REW Preferences screen and click on the Analysis tab. Under Impulse Response Calculation, make sure "Use acoustic timing reference" is selected.
> 
> On the Preferences screen click on the Soundcard tab, select a speaker to use for the timing reference in the "Timing Output Reference" box, and then select the speaker you want to measure in the "Output" box. The same two selection boxes now appear for convenience on the Measurements screen (in the latest REW beta versions).
> 
> For example, if you select HDMI2 as the timing reference, then when you click the Measure button, you will hear a brief burst of sound from the right speaker before the measurement sweep is conducted for the speaker you are measuring. As you select additional speakers to measure during a measurement session, it is important not to change the timing output reference speaker so that the timing reference is consistent from one measurement to the next. If you are using the right channel as the timing reference, the right channel can be measured as well by simply selecting HDMI2 as both the timing reference and the output.


I just want to make sure I have this straight so I don't waste more time trying to do this properly. First of all, I have v15.5 Beta 9. I do not see the option to set the "Timing Output Reference" on the Soundcard tab. Ah, I see now. You have to be using ASIO soundcards to get the option on the Soundcard tab to set the "Timing Output Reference".

1. When I select "Measure", I get the option to set the "Timing Output Reference" as either "Left" or "Right". I leave it as default "Left". I take measurement.
2. I select "Measure" again. Now I can select either Right, Left, or Both for the "Output" channel and select either Right or Left for the "Timing Output Reference". 
3. I would want to leave the "Timing Output Reference" set to "Left" and select the "Right" for the other output channel I will be measuring? 

Each time I take a new measurement, will the "Acoustical Timing Burst" play before the regular sweep? And that is how each measured channel is compared to the Reference channel?


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## AustinJerry

brumledb said:


> I just want to make sure I have this straight so I don't waste more time trying to do this properly. First of all, I have v15.5 Beta 9. I do not see the option to set the "Timing Output Reference" on the Soundcard tab. Ah, I see now. You have to be using ASIO soundcards to get the option on the Soundcard tab to set the "Timing Output Reference".
> 
> 1. When I select "Measure", I get the option to set the "Timing Output Reference" as either "Left" or "Right". I leave it as default "Left". I take measurement.
> 2. I select "Measure" again. Now I can select either Right, Left, or Both for the "Output" channel and select either Right or Left for the "Timing Output Reference".
> 3. I would want to leave the "Timing Output Reference" set to "Left" and select the "Right" for the other output channel I will be measuring?
> 
> Each time I take a new measurement, will the "Acoustical Timing Burst" play before the regular sweep? And that is how each measured channel is compared to the Reference channel?


You have it correct.


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## mvaud

Sorry for borrowing this thread. I have been trying to time align my tweeters and mids for a while. I'am using REW V5.15 Beta 12 and UMIK-1 usb-mic. I have measured all drivers separately and using tweeters as a timing reference.

Only problem is that I dont know what to look for. Trying to read delays from impulse response graph but I just dont get it. 

Could someone please help me to read tweeters delays so I can set up my distances right? I am using Audison AP8.9 dsp (yes, this is car audio system... sorry). 

Measured distances from LP to tweeters are:
LP - left tweeter 85cm
LP - right tweeter 126cm

Thanks!


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## jtalden

Basics for measurements needed to align phase of Mid to TW: [one good option]

> Mic about 1m from the baffle and located on the listening axis.
> Use a different speaker channel for the acoustic timing reference signal. So if the Mid to TW timing is to be determined using the left main speaker then use the right main (or any other main speaker) for the acoustic reference channel.
> Measure the Mid full range with all XOs active.
> Measure the TW full range with all XOs active.
> Measure the Mid+TW full range with all XOs active.

Using that file the delays can be determined. 
If you post that file and if needed, I can help provide the steps required to do that.


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## mvaud

jtalden said:


> Basics for measurements needed to align phase of Mid to TW: [one good option]
> 
> > Mic about 1m from the baffle and located on the listening axis.
> > Use a different speaker channel for the acoustic timing reference signal. So if the Mid to TW timing is to be determined using the left main speaker then use the right main (or any other main speaker) for the acoustic reference channel.
> > Measure the Mid full range with all XOs active.
> > Measure the TW full range with all XOs active.
> > Measure the Mid+TW full range with all XOs active.
> 
> Using that file the delays can be determined.
> If you post that file and if needed, I can help provide the steps required to do that.


Thank you for your reply!

So if I make measurements from LP the data cant be used to determine delays and distances? If so then many hours has been wasted  But learning is fun. 

I will make those measurements later. I try to solve this on my own but some tips are more than welcome. 

Thanks!


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## jtalden

mvaud said:


> So if I make measurements from LP the data cant be used to determine delays and distances? If so then many hours has been wasted  But learning is fun.



If you have the appropriate measurements at the LP those can be used. LP measurements are necessary for SW to Mains timing. For drivers mounted in the same box it is helpful to move the mic closer to the speaker. This reduces the effects of the room reflections. The results should be the same, but the SPL and phase traces will be much cleaner and easier to read with lower room impact. Depending on the room this may be a big help particularly for the beginner. It's your choice.


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## mvaud

jtalden said:


> For drivers mounted in the same box it is helpful to move the mic closer to the speaker. This reduces the effects of the room reflections. The results should be the same, but the SPL and phase traces will be much cleaner and easier to read with lower room impact. Depending on the room this may be a big help particularly for the beginner. It's your choice.


Okay... thats why my measurements are so messy. But is there any other reasonable place for mic than LP because mid and tweeter points different directions? Is this a problem? Allthought there is a same thing when measuring from lp. But as you said closer location should produce cleaner trace. 

And yes, I am a beginner


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## jtalden

Place mic at the LP if there is a question.

Drivers pointed in different directions! It depends... What is the situation?

If the Mid is pointed up like the Linkwitz Pluto/LX types, the XO is low, and there is some space to the nearest walls, then that may not be an issue. If it is much different than I have seen or studied then I cannot predict the results. If the TW is pointed away from the LP then I expect that looking to achieve a phase tracking alignment will be pointless. 

In most any case I expect it is possible to pick a 'better' alignment than other possibilities just by maximizing the SPL reinforcement in the XO range. That is always an option.


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## mvaud

jtalden said:


> Place mic at the LP if there is a question.
> 
> Drivers pointed in different directions! It depends... What is the situation?
> 
> If the Mid is pointed up like the Linkwitz Pluto/LX types, the XO is low, and there is some space to the nearest walls, then that may not be an issue. If it is much different than I have seen or studied then I cannot predict the results. If the TW is pointed away from the LP then I expect that looking to achieve a phase tracking alignment will be pointless.
> 
> In most any case I expect it is possible to pick a 'better' alignment than other possibilities just by maximizing the SPL reinforcement in the XO range. That is always an option.


Drivers have different alignment because this is car audio system. Mid is about 90deg and tweeter is pointing to lp. Thats one of the reason why measurements have so much reflections. 

This time, aligning mids is impossible thing to do. Sorry for asking car audio things but I just cant find suitable information anywhere else.


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## jtalden

I see. Others with car experience will need to help out. My attempts to help with car applications have not worked out very well.


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## mvaud

jtalden said:


> I see. Others with car experience will need to help out. My attempts to help with car applications have not worked out very well.


I try to make measurements you advised. Maybe I get time later this week. If you could take a look and give some advises. I would really appreciate your help. 

When I get required measurements what is the first thing to look for?


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## jtalden

Sure I'll take a look at the measurements. but expect much help from me. As I have no personal experience with car applications and was not able to help much on the 2 or 3 car applications that I saw posted here.


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## mvaud

Okay, here it is. I have been spending many hours watching these graphs and everytime I think that I just figured out how this works I got stuck. 

I think that LeftWO and RightWO time difference is about 1,2ms or 0,42m, 14ft. That makes sense because measured distances are LeftWO 1,07m and RightWO 1,42m (from LP to driver). 

So when watching impulse graph should I be able to read time difference between left wo and tw and right wo and tw? I dont have a glue that to do next. Any tips?

I have TWs hp-filter set to 3kHz and WOs bp-filter to 20-3kHz.


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## jtalden

I reviewed your data.

> There is a lot of SPL overlap of the drivers in XO range.
> There is very little improvement in SPL support when the delays are shifted from the current settings due to the strong SPL fill of late arriving reflections.
> When a FDW of only 5 cycles is used then there is improved SPL support and phase tracking in the Left Channel when the tw is delayed 0.45ms.
> The right channel does not show good phase tracking or Improved SPL support with any delay setting that I could find. There are just too many strong reflections present even with the FDW set to 3 cycles.

I thus have no expectation that any particular change to the delays will result in an improved sound quality.

Possibly others with car audio experience will have more helpful thoughts.


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## mvaud

jtalden said:


> I reviewed your data.
> 
> > There is a lot of SPL overlap of the drivers in XO range.
> > There is very little improvement in SPL support when the delays are shifted from the current settings due to the strong SPL fill of late arriving reflections.
> > When a FDW of only 5 cycles is used then there is improved SPL support and phase tracking in the Left Channel when the tw is delayed 0.45ms.
> > The right channel does not show good phase tracking or Improved SPL support with any delay setting that I could find. There are just too many strong reflections present even with the FDW set to 3 cycles.
> 
> I thus have no expectation that any particular change to the delays will result in an improved sound quality.
> 
> Possibly others with car audio experience will have more helpful thoughts.


Sad text to read 

My current XOs are Linkwitz slope 12. Maybe slope 24 would reduce overlapping. 

Dont know what to do with strong reflections... The right channels has always been the biggest problem.

Thank you for your help! I'll try to solve these reflection problems and then lets try again.


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## FargateOne

Maybe you already know this forum specialized in car audio. If not, I found there a lot of help with REW even if I do not have car audio experience:

http://www.diymobileaudio.com/forum/


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## mvaud

FargateOne said:


> Maybe you already know this forum specialized in car audio. If not, I found there a lot of help with REW even if I do not have car audio experience:
> 
> http://www.diymobileaudio.com/forum/


I'll check that forum. Thanks for information!


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## AndrewUK1990

First of all, thanks John for your amazing efforts with REW, it is an excellent bit of software. 

I have a question regarding the new acoustic time reference feature if anybody may be able to help.

I am in the final stages of producing a 3-way pair of stereo loudspeakers which have a flat baffle. I wanted to try and use the new acoustic reference feature in REW to allow me to accurately measure the impulse response and phase of each driver at the microphone reference point (1m from baffle and in line with tweeter) with my UMIK-1 USB mic.

I have 2 main aims here:

1. To determine exact time offset of each driver on baffle when measured at the mic ref point. In doing so I will be able to time align each driver by applying the relevant time delay for each individual driver in my Mini-DSP DRC-DA8.

2. Once the Tweeter, mid and woofer pair are perfectly time aligned, I want to accurately measure the phase of each driver at the mic ref point. The goal here is to input the accurate measured phase of each driver into the RePhase software so that I can use an FIR filter on each driver to linearise the phase in order to allow each driver to be in phase with one another and the loudspeaker as a whole to be of linear phase from 30hz to 20khz.

Many thanks,

Andy.


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## jtalden

Welcome Andy,
That is the process I use frequently so I have a lot of experience with it. The other more common method is detailed at the rePhase and MiniDSP forums. I think that either method can provide excellent and similar results. If you move forward with this method I can assist. I suggest that you start a new thread for that purpose as it is a bit out of scope in this one.


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## AndrewUK1990

jtalden said:


> Welcome Andy,
> That is the process I use frequently so I have a lot of experience with it. The other more common method is detailed at the rePhase and MiniDSP forums. I think that either method can provide excellent and similar results. If you move forward with this method I can assist. I suggest that you start a new thread for that purpose as it is a bit out of scope in this one.


Thanks jtalden,

I dont want to take this thread off topic so I have started a new one here with my previous question in a bit more detail:

http://www.hometheatershack.com/for...ment-fir-filter-linear-phase.html#post1449377

Thanks again


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