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Problem with THD and THD+N measurement

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Hi,

 

I'm trying to a SNR measurement from an audio signal in LabVIEW. However, I cannot measure SNR correctly. After investigating I found that sometimes I can have a higher THD than THD+N. As I believe that this is not possible there should be a mistake somewhere in my VI.

 

Here the analysis part of my VI :

 

And sometimes, here is the result when distortion is high :

 

Any idea for resolving this ?

 

Thanks

 

Message Edité par Alexandre.DaCosta le 01-16-2009 09:28 AM
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Nothing about this ?
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Bon soir Alexandre,

 

are you using simulated input signals and could post your signals? How many harmonics are you calculating?

 

Best regards

 

MarianMO

Marian Vorderer
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Hello Marian,

 

No, I'm not using simulated input signals but audio input signal from a usb sound card.

 

The Distortion measurement VI is set to calculate 20 harmonics.

 

Thanks for you help.

 

Have a nice day.

 

Alexandre

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Solution
Accepted by adcpc

It seems to be resolved now. The problem could appear when audio acquisition was too short (under 50 ms).

 

If audio acquisition is long enough then THD+N is always higher than THD.

 

Thanks,

 

Alexandre

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