Oscillations on loopback impulse measurement

[tof_sten]

Hello,

My system is MacBook Pro M1 + Audient ID44 interface running REW 5.20.4 under macOS Big Sur 11.6.

I did an impulse response measurement while using REW in loopback (ID44 output 2 directly into ID44 input 4).
What I observe is large oscillations. It looks like I have an issue, but I don't know what to do:

I hope someone can help me!

 

[tof_sten]

The oscillation period is close to 50us which means 20KHz.

 

[John Mulcahy]

That is a normal impulse response for a system with a minimum-phase style reconstruction filter. It rings at the Nyquist frequency, all band limited systems have an oscillatory impulse response. Only a system with infinite bandwidth has an 'ideal' impulse response.

 

[tof_sten]

Does it mean, I can perform my measurement at higher sample rate (96KHz) and then filter above 20K to see the real impulse response?
Is there a way to do that?

 

[John Mulcahy]

That is the real impulse response for the sample rate you used. It is completely normal.

 

[tof_sten]

Ok thank you John. Do you have technical read to share that explain this behaviour? I'm a software and electronic engineer, but it is a shame I don't really get what is this phenomenon

 

[John Mulcahy]

It may help if you use the option in the graph controls to show the sample points. A linear phase brickwall reconstruction filter would have an impulse response that looks like a sinc function, theoretically the correct form to use. Nowadays other filters are in fashion, minimum phase reconstruction filters have all their oscillatory features after the initial peak. For the general topic see https://en.m.wikipedia.org/wiki/Nyquist–Shannon_sampling_theorem

 

[tof_sten]

I know pretty well shanon nyquist theorem, it’s more the reconstruction filter oscillation I would like to understand.
I will try to find something to read!
Thank you very much John!

 

[DanDan]

Hi Tof, do you need Loopback?

 

[John Mulcahy]

The sinc oscillation is illustrated in the article I linked, worth a scan.