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Audio Interface (USB Pre2) "flatness"

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Erik Jerde:
I've never bothered to measure the flatness of my USB Pre2 till today when I didn't have a cable tester on hand and I figured that if I compared the suspect cable to a known good one it would become obvious if the suspect one was bad.  It was immediately obvious. :)

I was somewhat surprised to see what I've understood to be a very good measurement interface wasn't phase flat.  I'm thinking that since real systems aren't going to be phase flat and since this really isn't a huge deviance that it isn't an issue but I'd love the thoughts of more experienced folks.

Frank Koenig:
That's a surprisingly large phase difference between the reference and measurement channels. Something is wrong. Or is it possible that you're using an "internal" reference through the software that doesn't use a hardware reference input to cancel the interface's frequency  response?

--Frank

Erik Jerde:

--- Quote from: Frank Koenig on March 04, 2019, 06:54:48 PM ---That's a surprisingly large phase difference between the reference and measurement channels. Something is wrong. Or is it possible that you're using an "internal" reference through the software that doesn't use a hardware reference input to cancel the interface's frequency  response?

--Frank

--- End quote ---

Smaart v7.5.3.2 mic input 1 to generator.

Frank Koenig:

--- Quote from: Erik Jerde on March 04, 2019, 10:43:45 PM ---Smaart v7.5.3.2 mic input 1 to generator.

--- End quote ---

Erik, set up Smaart to use one input of the interface for the reference and another for the measurement. Connect both inputs to the generator output with a Y-cord. You should see at most a few degrees difference at 20 kHz. --Frank

Russell Ault:

--- Quote from: Erik Jerde on March 04, 2019, 10:43:45 PM ---Smaart v7.5.3.2 mic input 1 to generator.

--- End quote ---

From the measurement you've done it's impossible to say whether the phase deviation you're seeing caused by the interface's input, output, or a combination of the two. One of the reasons to use a Y-cable for a transfer function instead of comparing to the generator internally is that it controls for these things: it doesn't matter what distortion the outputs have because transfer functions are source-independent, and it doesn't matter what distortion the inputs have (as long as it's the same distortion) since the TF will negate them out.

-Russ

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