Newbie > first measurement > Presonus Audiobox

[andretoscano]

Hi everyone!
Nice forum, lots of nice info, I can see it will be a pleasure to be a part of this community.

This is not actually a measurement per se, it's just the first frequency response graph I managed to create using REW.
It's from a Presonus Audiobox USB. It kind of caught my eye that this test was made using a 48000 Hz sampling rate, but the frequency response cuts down abruptly before the 20 kHz mark. Even considering the Nyquist rules, shouldn't this happen a bit further up? Am I doing something wrong here? I tried with a different Audiobox (I have a lot in stock, since I'm a Presonus distributor) and mostly they all behave the same way. So it's definitely not a anecdotic issue.



(for some reason, image previews but doesn't load in the post thread. But you can find it at this URL: http://www.audiolog.pt/rew_tests/audiobox.jpg )

You can also spot some weirdness going on (pardon my language, but I'm not that much of a technical person...) in the lower frequencies up until the 60 Hz mark. Don't know if this kind of low end rambling is normal or not.

Also, I can see on the lower left footer of the REW main page the indication "48000 Hz, 16 bit". This soundcard is specified as a 24 bit/ 44.1/48 kHz product. Is there some place where we can change this in REW? Or are all measurements made with a 16 bit by default?

I'm having some difficulties configuring my trustworthy RME Fireface400 to work with REW, though. I will leave that to my next thread.

Cheers!

André Toscano

[brucek]

for some reason, image previews but doesn't load in the post thread

Five posts are required (spammer rule). Use the post padding thread to add dummy threads to get over five..

Even considering the Nyquist rules, shouldn't this happen a bit further up? Am I doing something wrong here?

The cutoff would be a result of the line-in and line-out amplifiers response characteristics.

It's not that bad, being down about 6dB at 20Hz.

Don't know if this kind of low end rambling is normal or not.

Yeah, it happens sometimes. Usually, to correct this in the final soundcard cal file, you need to extend the Impulse Response window so that more of the response is included. Extend it until it's a bit smoother and then save the file.

To do this, run the soundcard cal routine and get to the spot where you are about to click Finish. Don't click Finish, instead click the IR Windows ICON in the upper right corner of the REW screen. A popup will arise and it will show the pre-window as 125msecs and the post window as 500msec. Extend the post window longer, and as you do, you should see the Frequency Resolution result lowering. Change the window to something like 1000msec and click Apply Windows while watching your squiggles. See how it's smoother now? Hit Finish on the soundcard cal routine and save the file. Your measurements will be more accurate down lower now.....

Also, I can see on the lower left footer of the REW main page the indication "48000 Hz, 16 bit". This soundcard is specified as a 24 bit/ 44.1/48 kHz product. Is there some place where we can change this in REW? Or are all measurements made with a 16 bit by default?

There are only provisions to set the sample rate in REW.

brucek