# Time alignment & multichannel query with usb devices



## 3ll3d00d (Jun 6, 2006)

From what I have read accurate time alignment is not possible with a USB mic. Am i right in thinking that a USB mic amp is fine though? eg if the path is emm-6 mic -> behringer q502usb (or similar) with loopback on spare channel -> USB to computer

In this setup, what connects to the processor? The q502? In that case is the only way to send a signal to 1 channel at a time to manually move the connection from one analogue input to the next?

Is there any device [that would replace the mic amp in this chain] that provides multichannel connection in one wire?

If one has a built-in soundcard with optical or HDMI then is it still a valid setup to loopback the analogue line in/out but send a signal using the optical/HDMI output?

Thanks
Matt


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## EarlK (Jan 1, 2010)

(A) You're assuming that the *Behringer q502usb* can be successfully used with REW ( all the while avoiding circuit feedback ) .

Since no one has currently posted ( here at HTS ) any reports about their success ( using the mixer with REW ) , I'd advise that you proceed with caution .

At the very least , only buy from a store with a liberal return policy ( who won't blink if you need to return the mixer ) .

(B) The necessary loopback circuit ( to create a timing channel for REW to use ) must be made between an Analog out & an Analog in ( both, BELONGING TO the SAME SOUNDCARD that the MIC is PLUGGED INTO/THRU ) .

- It must free of extraneous signals / such as the live mic pickup .

:sn:


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## 3ll3d00d (Jun 6, 2006)

I just used q502 in my post as it was the model number I could remember. I realise they are unproven hence the "or similar" 

Re the loopback for timing, basically I want to confirm whether it is independent of the connectivity to the computer and the processor.

Cheers
Matt


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## Scubasteve2365 (Mar 15, 2013)

3ll3d00d said:


> I just used q502 in my post as it was the model number I could remember. I realise they are unproven hence the "or similar"
> 
> Re the loopback for timing, basically I want to confirm whether it is independent of the connectivity to the computer and the processor.
> 
> ...


I can't answer your questions specifically with regard to time alignment. I don't know why a USB mic wouldn't work. As I think any USB bus lag could be compensated for. Anywho, unless you want to buy a Q502USB for other purposes I don't think it's going to work out for the REW measurements. Reason being is that the mic input signal cannot be separated from the main line out of the Q502 itself meaning that whatever the mic is reading is going to get pumped right into the lineout which is going to go into your AVR and that loop repeats.

Now, I'm not exactly sure why you couldn't just use the line/out headphone out of your laptop and bypass the Q502USB altogether.

I researched this pretty well because I really wanted to use a Q502USB as I have other purposes for it. I settled on buying a UMM-6 USB Mic from Cross Spectrum because at the end of the day, the cost isn't really any different. I can still buy a Q502 (or Q302 more likely in my case).

In fact, it wasn't until reading your post that I was aware of any possible issues with time based measurements and USB. Do you have any links to posts detailing this?


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## 3ll3d00d (Jun 6, 2006)

Scubasteve2365 said:


> In fact, it wasn't until reading your post that I was aware of any possible issues with time based measurements and USB. Do you have any links to posts detailing this?


There is this post from JohnM.



Scubasteve2365 said:


> Now, I'm not exactly sure why you couldn't just use the line/out headphone out of your laptop and bypass the Q502USB altogether.


The only reason I was looking at the q502 is because it seems to be quite cheap in the UK, less than 1/2 the cost of the other commonly mentioned devices (e.g. ART USB Dual Pre, tascam 144 mk2). Thanks for the info that it is useless for this use. I am looking for an external solution for flexibility really (and none of my laptops have line out).

Cheers
Matt


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## EarlK (Jan 1, 2010)

Hi Matt,

Back to your first post ;



3ll3d00d said:


> From what I have read accurate time alignment is not possible with a USB mic. Am i right in thinking that a USB mic amp is fine though? eg if the path is emm-6 mic -> behringer q502usb (or similar) with loopback on spare channel -> USB to computer


I think you are best buying a USB based mic like the *miniDSP UMIK-1* or the *UMM-6 ( as calibrated by Cross Spectrum )* .

One won't be able to do timebased alignments with these / but then, 99% of the people posting here don't use the timing chnl feature for EQing their systems or subs .



> In this setup, what connects to the processor? The q502? In that case is the only way to send a signal to 1 channel at a time to manually move the connection from one analogue input to the next?
> 
> Is there any device [that would replace the mic amp in this chain] that provides multichannel connection in one wire?


If your current computer doesn't have an analog output then you need to get a small USB based ( 2-chnl ) soundcard to acquire output capability .
The Behinger UCA202 is popular around here . With it, you would need to move the output ( RCA ) cable from input channel to input chnl of your AVR .  *UCA202*



> If one has a built-in soundcard with optical or HDMI then is it still a valid setup to loopback the analogue line in/out but send a signal using the optical/HDMI output?


The loopback ( for the timing chnl ) in this case would need to include ( one of ) the AVR's analog outs for the timing chnl to be of any use . Just remember, if you buy a USB based microphone, this is all a moot point . 

FYI ( some perspective ), REW only added ( relatively recently, by including the "new" ASIO setting ) the ability to address chnls, higher the standard (1 of ) 2 choices ( that JAVA is limited to, when selecting input or output chnl-routing ) . As such, sending REW to an HDMI enabled sound-output is still a relatively new topic. . ( You should search this forum for clues on this question .)  *HDMI Search* 

If any of your laptops have an HDMI output on them, then you should experiment with REW ( & it's tone generator ) to see if you can access these outputs from within REW's preferences window ( & actually get output from REW , to your AVR ) .

You'll need ( assuming you're Windows based ) to download ( & install ) from *ASIO4ALL*  their universal ASIO driver ( this is assuming your current soundcard within your laptop doesn't have an  *ASIO driver* ) .

Here's the direct download of the ASIO driver ;  *ASIO4ALL*


:sn:


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## 3ll3d00d (Jun 6, 2006)

EarlK said:


> The loopback ( for the timing chnl ) in this case would need to include ( one of ) the AVR's analog outs for the timing chnl to be of any use . Just remember, if you buy a USB based microphone, this is all a moot point .


I decided on the EMM-6 as I will need a fairly long run to the mic and didn't want to risk any issues with using a usb repeater. I don't think I understand why the loopback needs to be on the processor though if the onboard soundcard is doing digital out. It's probably a moot point though as I'll probably stick to analogue out & was thinking of picking up the tascam 122 mkii to drive it (the mic).

Thanks for the links about ASIO & HDMI, I will install & have a play. My laptop does not have hdmi out but my server does. I last tried HDMI with REW a year or so ago and it was not entirely happy, at least I couldn't get it work consistently despite usual windows (or linux, it's dual boot) audio working ok through it. I will give ASIO a try though to see if that works more reliably. 

If I wanted to use HDMI, and given the use of non USB mic, then I think that means using the tascam purely as a mic amp and connecting it to the PC via a line input. However the benefit of HDMI is the ability to drive individual channels via software alone (if it works) but the main reason I want to drive individual channels atm is to investigate timing issues and that requires a loopback over an analogue interface to get the right data. Is that correct or am I missing something there?

Cheers
Matt


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## EarlK (Jan 1, 2010)

Matt said:


> If I wanted to use HDMI, and given the use of non USB mic, then I think that means using the tascam purely as a mic amp and connecting it to the PC via a line input. However the benefit of HDMI is the ability to drive individual channels via software alone (if it works) but the main reason I want to drive individual channels atm is to investigate timing issues and that requires a loopback over an analogue interface to get the right data. Is that correct or am I missing something there?


If you buy the Tascam ( & find a way to use the HDMI output from your server ), the ASIO4ALL driver allows one to cobble together a composite driver, using different audio devices ( ie ; Tascam 122 for input , on-board graphics-audio card for output ) . This isn't ideal ( due to clock-drift from the 2 devices ) but it works well enough for REW ( due to the short durations of the test signal ) .


For a timing chnl to be accurate ( made via a loopback cable ), it must take into account the circuit ( A-D / D-A ) delays caused by the conversion circuits that are in *actual use* ( REW subtracts one channels delay from the other, to arrive at the delay of speaker to mic value ) . Therefore, the cable ( ins & outs ) must mimic your choice of where you are obtaining your analog, "ins & outs" .

:sn:

PS: A headphone output is the same as a line out .


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## 3ll3d00d (Jun 6, 2006)

EarlK said:


> If you buy the Tascam ( & find a way to use the HDMI output from your server ), the ASIO4ALL driver allows one to cobble together a composite driver, using different audio devices ( ie ; Tascam 122 for input , on-board graphics-audio card for output ) . This isn't ideal ( due to clock-drift from the 2 devices ) but it works well enough for REW ( due to the short durations of the test signal ) .


Is there a reference available for what the options are with the ASIO driver? e.g. what is the meaning of the "timing reference channel" & what should it be used for? is there a way to correlate the named channels in the dropdown to the same named things in some soundcard config? I get 8 channels per hdmi device though, atm, I only get a signal channels 1 & 2 which equate to L and R. This might be processor config though it's not obvious what it could be (will need to experiment further).



EarlK said:


> IFor a timing chnl to be accurate ( made via a loopback cable ), it must take into account the circuit ( A-D / D-A ) delays caused by the conversion circuits that are in *actual use* ( REW subtracts one channels delay from the other, to arrive at the delay of speaker to mic value ) . Therefore, the cable ( ins & outs ) must mimic your choice of where you are obtaining your analog, "ins & outs" .


OK that makes sense. Why doesn't that mean one usually loops back across the soundcard & processor analogue interface? Is it just because the additional delay of an analogue connection is trivial? Just to be clear this would mean I need to connect from an processor analogue output (any unused one will do?) to the soundcard line input?

Thanks
Matt


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## 3ll3d00d (Jun 6, 2006)

3ll3d00d said:


> I get 8 channels per hdmi device though, atm, I only get a signal channels 1 & 2 which equate to L and R. This might be processor config though it's not obvious what it could be (will need to experiment further).


quoting myself to pull out this part... I was able to successfully drive individual channels using the asio driver after updating the nvidia driver to latest (hdaudio 1.3.21.1, video 314.22, card is a gt240), forcing the processor to pass hdmi only & switching off the other devices (there is onboard realtek hd audio hdmi out) in the asio control panel. There was some interference (intermittent clicks) until I bumped the "kernel buffers" option up in the asio control panel (no prior knowledge, just systematic twiddling of the obvious options). This looks v promising as an all in one solution if I can get a workable loopback going.


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## EarlK (Jan 1, 2010)

3ll3d00d said:


> quoting myself to pull out this part... I was able to successfully drive individual channels using the asio driver after updating the nvidia driver to latest (hdaudio 1.3.21.1, video 314.22, card is a gt240), forcing the processor to pass hdmi only & switching off the other devices (there is onboard realtek hd audio hdmi out) in the asio control panel. There was some interference (intermittent clicks) until I bumped the "kernel buffers" option up in the asio control panel (no prior knowledge, just systematic twiddling of the obvious options). This looks v promising as an all in one solution if I can get a workable loopback going.



I'm glad you got ASIO4ALL working . Can I assume that you saw the full slate of available output chnls ( 8 ? ) from within REW's preferences window ?


:sn:


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## EarlK (Jan 1, 2010)

3ll3d00d said:


> Is there a reference available for what the options are with the ASIO driver? e.g. what is the meaning of the "timing reference channel" & what should it be used for? is there a way to correlate the named channels in the dropdown to the same named things in some soundcard config? I get 8 channels per hdmi device though, atm, I only get a signal channels 1 & 2 which equate to L and R. This might be processor config though it's not obvious what it could be (will need to experiment further).
> 
> 
> OK that makes sense. Why doesn't that mean one usually loops back across the soundcard & processor analogue interface? Is it just because the additional delay of an analogue connection is trivial? Just to be clear this would mean I need to connect from an processor analogue output (any unused one will do?) to the soundcard line input?
> ...


Yes your instincts are right , the electronic delay from the AVR ( if it is an all analog circuit path ) is quite trivial ( being in micro seconds rather than milli seconds ) .

Any analog out will do ,( as long as you can route signal to it from within REW ) .

As you found out, the ASIO4ALL driver's buffering can be tweaked by it's controls . The only real reference for the driver is the info available on the site .

FWIW, when first opened , the driver seems to evaluate the situation & "guesses" at what settings to use ( more times than not, the guessed parameters need to be changed by increasing the buffering controls ) . 

You'll need to solve the channel routing ( through HDMI ) & the naming conventions through trial & error .

:sn:


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## 3ll3d00d (Jun 6, 2006)

EarlK said:


> Can I assume that you saw the full slate of available output chnls ( 8 ? ) from within REW's preferences window ?


yes, there are 8 channels.



EarlK said:


> You'll need to solve the channel routing ( through HDMI ) & the naming conventions through trial & error


in my case it goes 1=L, 2=R, 3=C, 4=SW, 5=SL, 6=SR. I have 5.1 connected so can't confirm 7 & 8 but would expect them to 7=xL & 8=xR (height?) based on the pattern above. 



EarlK said:


> Any analog out will do ,( as long as you can route signal to it from within REW )


I found this post from JohnM in which he describes the "reference timing" channels as follows



> They are used for a loopback connection if the Analysis preference setting "Use loopback as timing reference" is selected. This is only required if absolute timing of the IR peak is needed (for example when aligning multiple drivers).


am I correct in thinking this means, given my 5.1 setup, I can select channel 7 as the timing source and loopback from the corresponding pre-out on the processor to the line in on the soundcard?

btw, you've been v helpful. Many thanks!

Cheers
Matt


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## EarlK (Jan 1, 2010)

Matt said:


> am I correct in thinking this means, given my 5.1 setup, I can select channel 7 as the timing source and loopback from the corresponding pre-out on the processor to the line in on the soundcard?


- Yes ( assuming the AVR hasn't "muted" outputting signal on #7 for some reason / such as turning off all channels that it "senses" aren't being used ) .

:sn:


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## Scubasteve2365 (Mar 15, 2013)

For those of us new to taking room measurements, what exactly does the loopback timing channel do for you? If one were to have a USB measurement mic, that can't be utilized, could they use any generic microphone for timing related functions (i.e., is frequency response critical for these measurements)?


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