Audio Card Calibration

[Sliderule]

I have been having some high frequency measurement issues (excessive rolloff). I have been using a Motu 2 for audio interface. I have attached a couple off screenshots. Screenshot 71 is my old mobile Pre audio card. The phase is linear to nearly 22KHz. Screenshot 69 is my Motu 2. The phase goes craxy after 10KHz. Is this normal? The frequency response is fairly flat +0.2dB, -1.6dB. What would be expected values for phase and frequency response?

Tia sliderule

 

[Sonnie Parker]

I don't think it would be an issue, as it's not uncommon to see phase shifts with different filter designs in that last octave from 10kHz to 20kHz. It could be a more aggressive anti-aliasing filter in the Motu 2. Again, with the response looking good, I probably wouldn't be too concerned about it.

Perhaps someone else may chime in who is more technically minded about it than I am.

You could also measure a speaker with each, and see what the phase looks like wth each card and the same speaker. I'd be highly surprised if you heard any differences.

 

[Sliderule]

Thanx for your response. I have spent the last 8 months trying to find out why my very expensive speakers had a huge roll-off above 10KHz. I have tested everything with multiples - computers, audio cards, cables, you name it. The frequency response dies after 10KHz. No matter what test equipement is used. I would love an explanation why the motu 2 phase response is so bad compaired to my old Mobile Pre.

 

[Sonnie Parker]

What microphone are you using, and do you have a calibration file for the mic?

 

[Sliderule]

Yes, I am using the Behringer ECM800 which calibrated by Cross-Spectrum and loaded into REW. I have been using REW for over 12 years and made many hundreds of measurement and never had an issue until I got the Motu M2.

 

[Sonnie Parker]

That is rather odd since the loopback response is smooth out to 22kHz.

Are there any filter adjustments available on the Motu M2?

If not, I'd probably try a different audio card.

 

[Sliderule]

I agree about the loopback response being flat. Motu specs it as +0dB, -.01dB 20Hz to 20KHz. The phase response is what is causing the high frequency rolloff in my speaker measurements. FWIW I tested everything including multiple computers, Motu's, cables, ASIO vrs Java, - you name it and I tested it. Right now my objective is to get an audio interface card I can rely on for accurate measurements. i then plan to ask Motu what is the reason for the terrible phase response. I can onlu guess it is to minimize recording noise.