# Using M-Audio Fasttrack Pro loopback with S/PDIF



## kromkamp (Jan 12, 2012)

Hi, I am using the M-Audio Fast Track Pro combined with a Behringer ECM8000 mic plugged right into the front of it. I am using S/PDIF digital out from the M-Audio to my receiver. Its not clear to me how to set up for loopback mode in this case?

If anyone could walk me through this it would be greatly appreciated.

Thanks!


----------



## SAC (Dec 3, 2009)

Not having used the M-Audio front end (and having a well-earned aversion to anything AVID in general), you _may_ be able to use the S/PDIF output. 

But I might suggest instead simply following the standard wiring guide and configuring the unit using the channel 1 & 2 (right and left) outputs for the feed to the receiver and for the loopback as is documented and supported in the configuration tutorial.

In these configuration guides, the 'left out' is looped back to the 'left in', and you can ignore the "optional Y cable" input to the receiver as you will only want to drive one speaker at a time for frequency response or ETC measurements in order to avoid establishing a comb filtering lab.


----------



## kromkamp (Jan 12, 2012)

The biggest confusion for me is that this unit does not offer RCA analog inputs, only a pair of balanced inputs that may or may not support line level. Also, it has two pairs of RCA analog outputs and I don't know whats what. I simply don't understand what the specific combination of I/O is for this device. There are a lot of switches and gain controls on the device - again in analog I don't really know how to set them properly.

Currently I am using it in a non-loopback mode with S/PDIF and its working well. The benefits of loopback may not be worth it to me (I can figure out reflection boundary locations pretty easily without having an absolute number) but if someone were able to hold my hand a little bit I would certainly give it a shot.

Are there any advantages to loopback mode other than direct time measurement for the ETC graph?

Cheers


----------



## Wayne A. Pflughaupt (Apr 13, 2006)

Hey krom,

The loopback (aka soundcard calibration) “tells” REW what the frequency response of the sound card is, so it can add corrections to assure flat response. This probably isn’t important for ETC measurements, but it is for frequency response and perhaps other measurements.

M-Audio’s website says the Fast Track Pro has balanced/unbalanced ins and outs. The front connector is a “combi-jack” that accepts XLR or 1/4” plugs. For loopback calibration, just use a 1/4” (mono) to RCA cable and you’ll be good to go.

As far as not knowing “what’s what” for the RCA outputs – don’t you have an owner’s manual? :scratch:

Regards,
Wayne


----------



## EarlK (Jan 1, 2010)

> The biggest confusion for me is that this unit does not offer RCA analog inputs, only a pair of balanced inputs that may or may not support line level. Also, it has two pairs of RCA analog outputs and I don't know whats what. I simply don't understand what the specific combination of I/O is for this device. There are a lot of switches and gain controls on the device - again in analog I don't really know how to set them properly.


> Do yourself a favour, calibrate the soundcard by including the mic pre-amp within the ( temporary ) loopback . 

> Here's the reason ( this is from my Fast Track Ultra ) ;









> What you'll need ( to execute the SoundCard calibration ) is a TRS to Male XLR adapter cable like this ( this is from Parts Express ) ;



> The reason there are 4 RCA outputs ( apart from the 2 SPDIF outputs ) is that the card has 4 chnls of analog output .

> Other queries about your card can be answered simply by reading ;  *The Fast Track Pro Operators Manual*  

:sn:


----------



## kromkamp (Jan 12, 2012)

I can see that in theory - however any deviations from flat FR from the sound card (particularly using digital I/O) should be so trivial that they would be dwarfed by the room, no?


----------



## SAC (Dec 3, 2009)

kromkamp said:


> I can see that in theory - however any deviations from flat FR from the sound card (particularly using digital I/O) should be so trivial that they would be dwarfed by the room, no?


So you are simply going to utilize the ETC response to address specular reflections that occur above the modal region most directly affected by the soundcard calibration? In that case you will need the hardware loopback left in place to compensate for the hardware propagation latency as well.

Otherwise, if you are going to attempt to address modal behavior and or employ any degree of EQ to augment the below 80 Hz region, you might note the effected region that the calibration of the soundcard most typically addresses. (And ironically, here we simply ignore issues of phase response in the preamp and of the noise floor... which the 'pro' platforms do not...).

Seeing as how the region that most here address is simply in the baby steps afforded by EQ of the bass, and how the effected region affected by soundcard calibration directly affects this region, I would suggest that calibration is a necessary pre-requisite - even if you are not going to explore the region dominated by specular reflections above ~250 Hz or have reason to employ a professional grade pre-amp and mic which exhibit even higher levels of performance.


----------



## EarlK (Jan 1, 2010)

> I can see that in theory - however any deviations from flat FR from the sound card (particularly using digital I/O) should be so trivial that they would be dwarfed by the room, no?


> "In Practice", the circuit needing the most correction ( that's found in the typical soundcard ), is the mic pre-amp .

> As long as you are using an analog mic pre-amp within your test circuit / I would recommend including a correction file for that pre-amp .

> You're right, Digital I/O doesn't ( usually ) require correction .

> The problem with the "Digital I/O" argument is that *users don't have a digital front-end * due to the mic not being "digital" & the mic pre-amp not being "digital" .

> What *one typically has is an analog front-end* that'll need some correction .


:sn:


----------



## kromkamp (Jan 12, 2012)

Sorry - I posted without having read Earl's message. If that is the roll off of the M-Audio its a little surprising to me, and I do see some value in the calibration. 

Well, I could still come up with reasons why its not necessary but that would make me sound like I'm beating a dead horse  I'll grab a 1/4" to RCA cable and give it a go tonight.

Cheers


----------



## kromkamp (Jan 12, 2012)

Ah, you know what - I'm going to beat the dead horse  I don't have a calibration file for my mic, wouldn't the FR of that still be the weak point?


----------



## SAC (Dec 3, 2009)

kromkamp said:


> Ah, you know what - I'm going to beat the dead horse  I don't have a calibration file for my mic, wouldn't the FR of that still be the weak point?


You need both.
Is there a point to seeing how many ways one can compromise the measurement?


----------



## kromkamp (Jan 12, 2012)

Well, I don't have calibration data for my microphone so that's that I suppose.

Lets flip this discussion around. If you have a look at my uncalibrated, non-loopback graphs as an example:


















Would you expect a substantially different result with a calibrated system? For example, could it be possible I dont have a room peak at 55Hz? Or that there's one that I'm not seeing? Seems very unlikely to me. Its possible the rising HF response at the end is due to some non-flat response of the sound card or mic though.


----------



## kromkamp (Jan 12, 2012)

Okay, so I calibrated the soundcard last night (mic is uncalibrated still) and did some more measurements.

The response curve of the soundcard is +/- 3dB, with a roll off below 27Hz and roll up at the top end (ie. higher then relevant for room testing purposes). 

The calibrated impulse response starts at 40ms, which is clearly erroneous. The first reflection spike is still around 1.5ms after. This impulse response was a little bit cleaner though - showing me some spikes around 15dB down at 6ms after and 14ms after. Mostly the same though.

I'll try to post some new pictures tonight.


----------

