S/PDIF and USB Pre-Emphasis

B

bend3333

New Member
Just running some tests with an Axe FX II for a report for my degree.

I'm noticing that running white noise from my DAW through the Axe FX via USB and S/PDIF (using my Focusrite Liquid Saffire 56) and returning the signal to my DAW in the same way gives different frequency responses. I assumed that, since both are in the digital domain, these would be identical.

Just wondering if there was some reason for the difference (I'm assuming some sort of pre-emphasis) and if one is superior to the other and if so why?

In case any of you are interested, with the input routed straight to the output (no blocks in the chain, just a straight line from input to output), the S/PDIF signal comes back in pretty much flat and the USB comes back in with what looks like a 6dB/8ve rolloff from 10kHz up.

Cheers!
Ben
 
The USB I/O is the same exact data as the SPDIF I/O. Any differences are due to your DAW. If everything is set correctly you should see a completely flat response from 0 to 20 kHz. If you are seeing rolloff then your DAW is probably resampling the data with a poor SRC algorithm or you've got your sample rate set wrong somewhere.
 
Certainly sounds interesting. I'm using exactly the same setup for testing both ways. Not sure why my DAW would be resampling one strangely and not the other.

EDIT: Ran test again and both returns are now identical. I think there was an input level difference with the two initially. All better now!
 
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I have seen this phenomen in logic as well. Some of the meters shows flat response and some show a pink noise response even If they are connected to a white noise generator.
It must be a bug in the daw meters of some kind.

bend3333 said:
Just running some tests with an Axe FX II for a report for my degree.

I'm noticing that running white noise from my DAW through the Axe FX via USB and S/PDIF (using my Focusrite Liquid Saffire 56) and returning the signal to my DAW in the same way gives different frequency responses. I assumed that, since both are in the digital domain, these would be identical.

Just wondering if there was some reason for the difference (I'm assuming some sort of pre-emphasis) and if one is superior to the other and if so why?

In case any of you are interested, with the input routed straight to the output (no blocks in the chain, just a straight line from input to output), the S/PDIF signal comes back in pretty much flat and the USB comes back in with what looks like a 6dB/8ve rolloff from 10kHz up.

Cheers!
Ben
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