REW Soundcard Calibration & Loopback Questions While Using Active Monitors w/AES Input

[justbob]

Hello,

So I'm trying to wrap my head around how to best configure REW while using a pair of active Genelec monitors in combination with a Lynx Hilo as the audio interface feeding the Genelec's. The Lynx Hilo I am using contains a USB Card and uses the Lynx ASIO driver for Windows (Windows 11 Pro x64)

I am using the AES Output on the Lynx Hilo >>>>> TO >>>>> The AES Inputs on the Genelec's. Since there is only one physical AES Output Interface on the Lynx (carrying AES channel 1/2 data) that means there is a single AES cable leaving the Lynx Hilo to ONE of the Genelec monitors (lets call it monitor A).

From Monitor A, there is a AES Out/Thru interface which then connects to monitor B to feed the second channel of the AES signal to monitor B.

The channel designations are saved to the Genelec monitors using GLM so each monitor knows whether its job is to be AES Channel 1 or 2.

Lastly, as part of the connection chain, I am using an external Mic/Pre box (Earthworks 1021 ZDT & Earthworks M30) which outputs an XLR Analog signal into the Left XLR Analog Line Input on the Lynx Hilo for A/D conversion


So with that connectivity explanation out of the way I'm struggling to understand how to perform the "Soundcard Calibration" within REW. When I click that button within REW it pops up a message talking about using a Lookback connection. As I read it, my confusion sets in from the highlighted text shown below.

Taking my use case into consideration with an AES Output to a pair of active monitors with built-in DA conversation......where/how on earth does one setup this Lookback connection?

I think I have many questions about this Loopback option, but I will stop here for now to keep each question/issue better separated in my head.

Thanks for any thoughts on how to do this.

 

[John Mulcahy]

You'll be fine without a soundcard cal, the monitor roll-offs will be much narrower than any contribution from the mic pre and the Lynx input.

 

[John Mulcahy]

Do make sure the input from the mic isn't mixed back to the output though.

 

[justbob]

Do make sure the input from the mic isn't mixed back to the output though.

Hello Sir, thanks for the response. I will skip the sound card calibration process then. Good to hear.

Can you clarify the quoted reply above though. Are you referring to the Loopback configuration section below? If so, should I just leave these two options not configured? So no Loopback and no timing reference output here and also use no timing reference during the sweeps?

Thank you

 

[John Mulcahy]

I'm referring to the Lynx interface mixer settings.

 

[justbob]

I'm referring to the Lynx interface mixer settings.

OK I will be sure to keep this in mind when setting up the Lynx.

I should have wrote these two questions in my last reply a bit differently in hopes they could be addressed.

What would be the best way to handle the Loopback/Timing reference settings with a setup like mine? Am I correct that there wouldn't be a Lookback in use and as a result there is no need to configure the Loopback Input dropdown menu?

Similar question for the Timing Reference since it seems these two things (Loopback & Timing Reference) are like peas and carrots...they seem to go together.


Thank you again

 

[John Mulcahy]

If you need a loopback as a timing reference you could make one locally on the Lynx. The loopback settings don't matter unless you select loopback as a timing reference. You could alternatively use an acoustic timing reference. See the help for more on that.

 

[Keith_W]

The loopback timing reference measures the discrepancy in time between two circuits:

1. The loopback (reference) circuit. Your Lynx might have a software loopback. If it doesn't, you will need a physical loopback (i.e. a cable connecting an output to an input).
2. The measurement (test) circuit). This is the cumulative delay of ALL the delays of everything downstream from your interface, which may include: DSP processing latency, amplifiers, the group delay of speakers, the time of flight delay between speaker and microphone, and any ADC latency.

As an example, here is a measurement I made using a loopback timing reference. I was testing the frequency response of my speaker at different distances on-axis to the speaker, at 50cm intervals and up to a 3m distance. I can't use a tweeter chirp timing reference since the timing reference (tweeter chirp) will have the exact same delay as the output from the speaker. Note the 2.7 second latency. This is because I had to pipe the output from REW into a convolver to apply DSP so my measurement circuit was quite complex. In the end, you can see the impulses separated quite nicely by about 50cm.

Anyway, there are reasons why you would choose one timing reference over another. For any situation where you might want to move the speaker or the microphone, AND want to combine or sum the graphs, you need a loopback reference. Example: measuring the output of a port and summing it with the woofer output. If your mic and speaker are static, either reference is fine.

 

[justbob]

If you need a loopback as a timing reference you could make one locally on the Lynx. The loopback settings don't matter unless you select loopback as a timing reference. You could alternatively use an acoustic timing reference. See the help for more on that.

OK, so let me ask this.

Based on your response about setting up a Local Loopback within the Lynx (I assume you mean using the routing matrix only to create a virtual Loopback?)

Create a route within the internal matrix using USB Playback Channel 5 >>>> TO >>> USB Record Channel 5 and then choose those two paths for these two drop down options within REW? Just to note, these two Channels (Ch5) would not be used in my setup for any future use so they are just considered unused Channels in my configuration. Would this be considered a valid Loopback/Timing Ref configuration?

Thank you

 

[justbob]

The loopback timing reference measures the discrepancy in time between two circuits:

1. The loopback (reference) circuit. Your Lynx might have a software loopback. If it doesn't, you will need a physical loopback (i.e. a cable connecting an output to an input).
2. The measurement (test) circuit). This is the cumulative delay of ALL the delays of everything downstream from your interface, which may include: DSP processing latency, amplifiers, the group delay of speakers, the time of flight delay between speaker and microphone, and any ADC latency.

View attachment 96076

As an example, here is a measurement I made using a loopback timing reference. I was testing the frequency response of my speaker at different distances on-axis to the speaker, at 50cm intervals and up to a 3m distance. I can't use a tweeter chirp timing reference since the timing reference (tweeter chirp) will have the exact same delay as the output from the speaker. Note the 2.7 second latency. This is because I had to pipe the output from REW into a convolver to apply DSP so my measurement circuit was quite complex. In the end, you can see the impulses separated quite nicely by about 50cm.

Anyway, there are reasons why you would choose one timing reference over another. For any situation where you might want to move the speaker or the microphone, AND want to combine or sum the graphs, you need a loopback reference. Example: measuring the output of a port and summing it with the woofer output. If your mic and speaker are static, either reference is fine.

Thanks for the additional explanation and details

 

[justbob]

Hello, back again.

I was playing around with creating a Loopback within the Hilo last night and it appeared to be working as expected based on the measurement results once the sound card calibration completed. The results were ruler flat across the board which, according to the Help file, was a valid measurement.

If I save the results as a CAL file within REW, is there a need to keep the Loopback path/route within the Hilo in place/present at all times or is that setup a one time thing that will live within the CAL file and after that point any sweeps/measurements wont need to send a signal thru it going forward?

Also, one question on REW Sweep Level control of volume output sent to the speakers during measurements. I am using the Hilo ASIO driver provided by Lynx on my Win 11 computer. There is mention within the Help file that volume isn't controlled within REW when using ASIO. I found this odd because in the past I recall it did work and based on some initial testing last night (without the speakers connected as a safeguard), when I changed the Sweep Level buttons UP/DOWN while watching the level meters within the Hilo Remote App, I could see that the levels did go UP/DOWN in unison with any level adjustments made within REW.

It appeared as if REW was in fact controlling the Output levels just based on watching the Level meters. Has anyone else seen the same while using a Hilo with REW? Is it possible that Lynx's USB ASIO driver is providing the necessary into to REW to allow volume control to occur despite the Help file saying that it wont work?

Thanks

 

[John Mulcahy]

I was playing around with creating a Loopback within the Hilo last night and it appeared to be working as expected based on the measurement results once the sound card calibration completed. The results were ruler flat across the board which, according to the Help file, was a valid measurement.

If that's an internal loopback it is just sending back the same data it receives, so it should be completely flat. No need for a cal file for that and that's not your measurement path, so again no need for a cal file.

If I save the results as a CAL file within REW, is there a need to keep the Loopback path/route within the Hilo in place/present at all times or is that setup a one time thing that will live within the CAL file and after that point any sweeps/measurements wont need to send a signal thru it going forward?

For cal it would be a one-time thing, but it doesn't reflect your measurement path so it should not be used for a cal file. The loopback would only be needed if you want a timing reference.

There is mention within the Help file that volume isn't controlled within REW when using ASIO.

That means REW doesn't try to access the Windows volume controls since they aren't applied to ASIO paths. The sweep data is generated at whatever level you select.

 

[justbob]

If that's an internal loopback it is just sending back the same data it receives, so it should be completely flat. No need for a cal file for that and that's not your measurement path, so again no need for a cal file.


For cal it would be a one-time thing, but it doesn't reflect your measurement path so it should not be used for a cal file. The loopback would only be needed if you want a timing reference.


That means REW doesn't try to access the Windows volume controls since they aren't applied to ASIO paths. The sweep data is generated at whatever level you select.

Hello John,

I've been under the impression that someone should strive to use a timing reference (be it an acoustic one or lookback) whenever possible. For my purposes, I am intending to measure the Room and use the results to make decisions on tweaking correction filters and where to place speakers/subs within the room being measured. Am I mistaken in thinking that a timing reference is needed based in these use cases?

Does there need to be an actual audible acoustic output in room during the Soundcard Cal process (ie..do the speakers need to be hooked up)?

Lastly, you had mentioned that I could use a local Loopback on the Hilo but unfortunately I couldn't determine if your use of the word "Local" meant within the Hilo Routing Matrix only or if you meant by connecting a physical cable from Line-In 1 to Line-Out 1? If you meant the later, then how would that work when my actual signal Output under normal conditions would be via AES Out? I imagine that it would be unwise to connect AES Out to Line-In to use the physical cable Loopback. Would using Line-In >> TO >> Line-Out via physical cable still be good enough to determine the latency of the Hilo for REW measurement purposes even if I would be using AES Out? If so, I will just do that then.

Thanks

 

[John Mulcahy]

Does there need to be an actual audible acoustic output in room during the Soundcard Cal process

No, but you do not need a soundcard calibration.

The internal routing loopback should work as the timing reference. It is something that gets used every time you make a measurement, it is not a one-time process.