REW + USB interface loopback question

Hi All,

I am trying to setup my USB interface to allow loopback as time reference with REW.

I have Audient iD4 mk1 and I wonder if it is possible to set it up with analogue cable connecting some inputs and outputs to achieve analogue loopback to be used as time reference for REW and measuring mic?

If so could someone advise which cable I need and which inputs outputs should I populate with it?

I have tried to find this info online but so far was unlucky.

Time stamped video showing what I am talking about on example of a different interface.

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Thanks

You should be able to link one of the speaker outputs to the instrument input and use that pairing as the loopback reference.

John Mulcahy said:

You should be able to link one of the speaker outputs to the instrument input and use that pairing as the loopback reference.

Click to expand...

Thank you John for coming back to me.

If I would get Id4 MK2, which allows to setup digital loopback via PC software - would that work as well with REW?

It should, but what you have should be fine.

John Mulcahy said:

You should be able to link one of the speaker outputs to the instrument input and use that pairing as the loopback reference.

Click to expand...

When I looked up what type of connections are these I learnt that Speaker Outs are balanced TRS and DI Instrument input is 1/4 mono jack.

So I check for cables and this is what I found - which is TRS to 1/4 Instrument Cable. But it has 2 1/4 Jacks. I would obviously plug only one as I have only one DI instrument input. Would it work in mono to stereo for the purpose of loopback and REW (would it be ok to tape to prevent electrical interference the other loose 1/4 instrument cable jack)?

I have just spoken with Audient about Id4 MK1 and they said that despite analogue loopback would work using DI port they don't feel it is a good option to use due to frequency shift on DI port and levels differences between speaker out and DI input sensitivity meant for guitar etc.

In relation to frequency shift I deducted that it should probably not matter as it is only used as timing reference so as long as shift is constant the signal should be suitable to establish timing reference?

But they also said that relative high output of speaker out and high gain of DI would lead to huge distortion of the signal.

They felt that ID14 is the right interface for the job (out of ID lineup) as it has 2 mic inputs so problem is resolved as speaker output can be looped back to spare mix input to mitigate potential problems with DI input.

But they said that it is best to check with REW team, so I wonder what is your view @John Mulcahy on it?

Another question that I asked and they were not sure about is if I would be using ID14 MK2, it has option to digital loopback via software. Would that work for REW timing reference? They were not sure as they digital loopback will be almost instant with close to 0 latency while analogue one will have something in range of 1-4ms and they were not sure if REW is using and needing this latency for the timing reference (especially that digital loopback signal never leaves the interface while analogue is looped through PC which causes the latency).

Finally is there a benefit other than convenience of having loopback timing reference vs audio reference via one of the speakers? I guess one is that we are eliminating time drift from USB Umik-1 (by not using UMIK-1) caused by it not having it's own clock and relaying on a PC clock? Is that even the case with external audio interface and XLR mic connected to it?

Or is CSL UMIK-1 perfectly fine to pre-shape and align multiple DIY subs with use of something like MiniDSP and time drift is not an issue here or not much of an issue?

I hope you can clarify some or all of it for me

Thanks

sebna said:

When I looked up what type of connections are these I learnt that Speaker Outs are balanced TRS and DI Instrument input is 1/4 mono jack.

So I check for cables and this is what I found - which is TRS to 1/4 Instrument Cable. But it has 2 1/4 Jacks. I would obviously plug only one as I have only one DI instrument input. Would it work in mono to stereo for the purpose of loopback and REW (would it be ok to tape to prevent electrical interference the other loose 1/4 instrument cable jack)?

Click to expand...

That cable won’t work. It’s either an insert cable (for a professional mixing console) or a headphone splitter.

The TRS output is: Signal (+) = tip; Signal (-) = ring; cable shield = sleeve.
A headphone output is: Signal (R) = tip, Signal (L) = ring; Signal (-) = sleeve.

See the problem? The TRS is balanced; trying to use that connector will get you on the other end: Signal (+) = tip; cable shield = sleeve. (Or the reverse - signal (-) and shield.) IOW, you'll get nothing because the input will not have both signal (+) and (-) present.

This is a typical point of confusion from an audio industry that for decades recycled the same connectors for multiple applications.

Actually, you can use a standard TS (aka “mono”) cable for the loopback. TRS jacks work fine with them. It will result in an unbalanced signal, but that is of no consequence for this application.

sebna said:

Finally is there a benefit other than convenience of having loopback timing reference vs audio reference via one of the speakers?

Click to expand...

If you intend to do acoustics measurements (such as ETC), you need the timing reference. If you only intend to take frequency response measurements, you don’t need the timing reference.


Regards,
Wayne

Thank you @Wayne A. Pflughaupt for clarifying about the cable. So for ID4 if I want to use DI interface for audio loopback I would just buy 1/4 Mono jack to 1/4 Mono jack and that would work as intended for Speaker Out to DI port?

And would you say that gain mismatch and possible distortion between the two ports should be ok for time reference signal with REW? Or could I just adjust gain down for DI port to mitigate?

I am really sorry for total novice questions. I have little experience with interfaces etc.

Wayne A. Pflughaupt said:

If you intend to do acoustics measurements (such as ETC), you need the timing reference. If you only intend to take frequency response measurements, you don’t need the timing reference.

Click to expand...

I plan to use this setup to pre-shape my 4x DIY subs with MiniDSP and to time align and further EQ correct them with same MiniDSP. So I believe I will need time reference for this.

For the above scenario is there a benefit to go with audio interface and XLR mic vs something like USB UMIK-1 (which I read time drifts due to not having it's own clock and using PC as a master clock)?

Other than possibly eliminating time drift of UMIK-1, the other benefit would be not needing a speaker to be used as acoustic time reference.

But purely on performance level, would there be a benefit of eliminating time drift of UMIK-1? And if so can it be quantified / estimated as 5%, 10% more accurate time alignment in the end?

Thanks

UMIK-1 doesn't have "time drift". It relies on the USB clock, which is not as stable as a dedicated clock though the variations would have no impact at all on subwoofer measurements. If you intend speaker build projects in future a setup allowing a loopback connection would be easier to use.

John Mulcahy said:

You should be able to link one of the speaker outputs to the instrument input and use that pairing as the loopback reference.

Click to expand...

Bit confused here. I just picked up an Audient ID4 MkII to do some REW speaker measurments and in process of setting up and wanting to do the loopback test.

Would not a TS cable from one of the speaker outs into the mic/line in work for the loopback? My confusion is why the FET instrument input on the front and not the mic preamp input?

Or perhaps you generically used the term instrument input and I'm being too literal?


The instrument in is a FET input which colors the sound and seems to be at odds with calibrating the sound card, so the reason for my question. Sorry if I'm being pedantic, just want to be off to a good start and order the right cables.

Picture of the front and rear panel of the ID4mkii in question. Front panel has the instrument input.







ps... tried the digital loopback feature, but that's not working for me as the gain on the mic input unable to match the output for some reason in that mode. (on a mac os, so set by configuring the midi speaker outputs). Could be a config issue, so all ears, if that's the case. Set loopback input channel as 1 in REW, which is the mic input and gain knob set to max only brings the level up to about -60 db. Midi speaker config had loopback going to input 1/mic instead of speaker out. Hopefully not a problem with my mic preamp, but seems to be working fine with my dbx RTA xlr mic with decent levels, so have ruled that out.

The post you quoted was in response to the OP's question about how to set up a loopback to use as a timing reference, not for measurement.

Soundcard cal is measurement output to measurement input. Loopback timing ref is a different output to a different input, or the digital loopback.

John Mulcahy said:

The post you quoted was in response to the OP's question about how to set up a loopback to use as a timing reference, not for measurement.

Soundcard cal is measurement output to measurement input. Loopback timing ref is a different output to a different input, or the digital loopback.

Click to expand...

Gotcha and many thanks for the fast response and clarification! I overlooked that bit being a bit myopic. Will use the mic input/ts male/ts male.

Hmm, now wondering if this interface wasn't the best choice if I wanted to to speaker impedance measurements in the future due to the differing inputs.

No, not suitable for impedance measurement.

Two matching inputs is ideal, for impedance or measurement with electronic loopback. For Audient products, look at ID14 or ID24 instead. They may bit a bit overkill though, Motu M2 is a solid high price to performance option.

Thanks dcibel.

Worst case, I can probably get by with a multimeter and the stepped sine function and endure the tedium of keying in data points, if I do end up doing an impedance 'scan'. I end up focusing on the speaker measurements at the end anyway, since I'm not doing ground up designs.

Thanks on the tip for the Motu M2, I did cross shop those and the SSL mk ii units, but got this one because it was local, new in box and $120, so jumped on it before doing the research.

For most of my equipment, adjustments are one channel at a time anyway, so I suspect I should be ok.

Hopefully this helps others who may be considering this unit, based on what they want to do.