Understanding Output and Ref settings

[neodog]

I've observed a few idiosyncrasies in REW Output and ref signal configurations, I just want to bring them up to clarify or change my understanding.

In preferences, it is possible to configure an output to use L+R, and a loopback ref to use either L or R. In a stereo system, isn't L+R fundamentally incompatible with a loopback at all? REW pops up an alert if you try to measure with Output and Ref set to the same channel. It does not complain if output is L+R and Ref is also L or R. Shouldn't it present an alert for that as well? I'm guessing that the design intent is that if you use L+R, then you do not use a loopback timing reference at all?

When doing measurements, it sounds on my headphones like there is no operational difference between the outputs generated with Output=L and Ref=R, compared to Output=R and Ref=L; the signals are the same either way. Perhaps the only difference is which letter is prefixed on the measurement title? Would it be possible to add an "opposite of output" choice so that they don't have to be manually kept in sync? (and of course it would have to grey out or something if L+R was chosen)

 

[neodog]

Related question: The Preferences window requires both Output/Input and Ref IO channels to be specified. Suppose you have a soundcard with just a mono input (as with a laptop's headset jack), but which presents in Windows as a stereo input L and R, being copies of the same input. For measurement time, it seems like this can be accommodated by using alternative timing approaches. However, when calibrating this soundcard with the calibrate button, the same signal appears to be sent to both channels, and there doesn't seem to be a way to turn off the loopback for timing. Is the resulting calibration going to be valid? Is it possible to calibrate this sound path properly, or is timing not really important to calibration?

 

[John Mulcahy]

Soundcard calibration is measurement output to measurement input, timing references are not used.

isn't L+R fundamentally incompatible with a loopback at all?

No, an output can go to more than one place.

 

[neodog]

Soundcard calibration is measurement output to measurement input, timing references are not used.

Perfect.

No, an output can go to more than one place.

I see... this actually tells me everything I needed to know. It means you can use an output as a ref input, which confirms that we can safely assume that the Ref and Output signals are always 100% identical. I had thought the ref signal might be somehow unique, as with the accoustic reference, e.g. to include sentinel signals not related to the measurement. But there must be only one signal, and so there would be no drawbacks of a collision between output and ref output settings. It seems the Ref Output setting isn't technically necessary at all, except as a convenience so that you don't have to split an output. In that light, it would be nice if also had an "L+R" option, so that you could use the Output setting just to declare intent of the measurement.