new here - time alignment with REW and UMIK

st0nehead_ss

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hey everyone
ive sent a few days reading most everything on here with regard to time alignment using the above.
I have a few questions, maybe im getting confused.

I am trying to test a 4 way reggae sound system (4 scoop subs, 4 es18bph kicks, 2x vertically stacked 2x12_hf paraflex line array style cabinets) they are set up in a mono stack.

I understand that the loopback isnt possible when using a USB mic, as the sources are different. its a shame because loopback is making more sense to me in determining IR delay than does the acoustic timing reference (mainly as i dont know how to use that in my case, except for maybe placing a monitor speaker off of the mixer's "booth" channel and placing it very close to the mic...)
where i am getting confused is that the "chirp" that would play through the Midtops for example, or the full range monitor speaker if i were to use it, also would be playing the full range sweep to be measured..thus interfering with the graph, as i wouldnt be able to see what is the timing speaker and what is the (for example) sub?
or is it likely to have a discernable IR gap between the two on the time domain/IR window when i look at it?

we spent a day fidding and didnt come up with much but i assume this is because of the setup errors...

below is a pic of the stack in case it helps...it is powered and controlled DSP wise via powersoft x8 with armonia+

54884
 

skid00

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I'm not sure how much help I can be... Can you send the sweep thru just the subs, just the kicks, just the HF? You would need to add a full-range (placed equi-distant with the subs or kicks (what freq range are those?)) for the chirp to be useful. Are you trying to time-align everything?

But you might be able to use pink noise, if the stack doesn't have much in the way of reflective surfaces (for EQ).
 

st0nehead_ss

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I'm not sure how much help I can be... Can you send the sweep thru just the subs, just the kicks, just the HF? You would need to add a full-range (placed equi-distant with the subs or kicks (what freq range are those?)) for the chirp to be useful. Are you trying to time-align everything?

But you might be able to use pink noise, if the stack doesn't have much in the way of reflective surfaces (for EQ).
Hey, thanks for your reply.

The data we obtained I am fairly certain did not work out as expected. We erroneously measured with crossovers in place. The speaker for the acoustic time signal was set beside the mic to try and make it a 0 time red for example. We had found a fellow via speaker plans who made a document on this.
As the rig is fairly large and heavy I can't drag it out and take them again without help from 1-2 friends.
Pink noise did make sense to me if we were to leave crossovers on, in order to get a time domain IR alignment match but we didn't get to that...
 

John Mulcahy

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The timing reference is played before the measurement sweep, it doesn't produce any change in the measurement itself. REW uses it to synchronise capture, it doesn't appear at all in the result. If you use a separate speaker for the timing reference that is placed much closer you will need to use a timing offset to remove the difference in distance between the speaker reproducing the timing signal and the speakers being measured. REW can estimate that for you using the "Estimate IR delay" feature on a measurement of the system to be measured, that timing offset should then remain the same for all the subsequent measurements.
 

st0nehead_ss

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The timing reference is played before the measurement sweep, it doesn't produce any change in the measurement itself. REW uses it to synchronise capture, it doesn't appear at all in the result. If you use a separate speaker for the timing reference that is placed much closer you will need to use a timing offset to remove the difference in distance between the speaker reproducing the timing signal and the speakers being measured. REW can estimate that for you using the "Estimate IR delay" feature on a measurement of the system to be measured, that timing offset should then remain the same for all the subsequent measurements.
Thankyou very much for your response, John.
So the chirp plays, but is not recorded back via the mic?
It is sent to, for example, the full range speaker being used for reference, as well as the sub...but mainly probably only audible on full range reference and mid range/tweeters being tested?

Essentially I know I am going to have to delay kicks and midtops to the sub delay to time align, I was just getting lost.
I understand now this needs to be done without any crossover filters in place.
Once they are aligned with ms delay the filter choices can be determined around their effects in phase trace?


Sidenote, as you can see each "way" is playing a different frequency if the music. Whilst the stack is aligned in space, each cabinet has a different horn path and of course the way sound travels the lower freqs will be lagging so I need to align to the subs which play 30-80hz..
 
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John Mulcahy

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The timing reference signal is captured and processed along with the subsequent measurement sweep, but only the measurement part of the signal is used to generate the response. The timing signal and any timing offset specified on the Measure dialog are used to work out where t=0 should be placed. If you use a separate speaker for the timing reference (probably easiest for your setup) it would typically be connected to a different channel than the system being measured, so the timing reference would only ever go to the reference speaker. The "Ref output" line on the Measure dialog is used to select where the timing reference gets sent. You can then measure the individual parts of the system, which is best done with the crossovers in place unless you plan to use the measurements to decide on the crossovers. Once you have the measurements you can also use the Alignment tool in the All SPL graph controls to help work out the delays, for the sub that's usually the easiest method.
 

st0nehead_ss

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Thankyou, that's helpful.
So I could send left output to the timing ref speaker (active), right out to the amp and main speaker stack (using it's DSP to mute channels accordingly for testing) and use the umik as the only input.
In this case the left output takes the place of what would otherwise be the loopback? But would the active full way speaker on the left channel still receive the entirety of the sweep?
Or is this what the selection you mentioned does, selects it only for the "chirp"?

Is this correct?
 

st0nehead_ss

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And somewhere inside the software I can determine that yeah?
 

sm52

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st0nehead_ss
to do it right, you need two output channels of the sound card. One to which the reference signal will be applied, whistling (ref output), and the second, to which the sweep tone will be applied to capture the response.
 
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