• Registered: 2012-05-07
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Topic: Fireface 800 analog inputs have unexpectedly high harmonic distortion


I am using Fireface 800 for an audio measurement setup. I tried characterizing the THD of Analog input channel 1,2.
I was surprised to measure a THD of -40dB.

Description of my measurement:
Reference tone 1kHz, 1Vrms -> Fireface analog channel 1 -> MacOSX RME driver -> Audacity.

I do frequency analysis on the wav recorded by audacity. The analysis shows a 2kHz harmonic component 40dB down from the 1kHz component. 3rnd and 4rth harmonic are prominent. 5th onwards are not.

I have the limiter, instrument mode and speaker emulation turned OFF.

Any suggestions as to what I might be missing?

Thanks.

2 Reply by MC

  • MC
  • MC
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Re: Fireface 800 analog inputs have unexpectedly high harmonic distortion

You tried a different channel?

Regards
Matthias Carstens
RME

MC's Website

  • Registered: 2012-05-07
  • Posts: 4

Re: Fireface 800 analog inputs have unexpectedly high harmonic distortion

Yes, I tried analog channels: 1,2, 3, 5, 7. All show same problem.
I also tried sampling rates: 32K, 44.1K, 48K and 96K. Problem exists at all sampling rates.

4 Reply by MC

  • MC
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Re: Fireface 800 analog inputs have unexpectedly high harmonic distortion

So how you know your source is not the problem?

Regards
Matthias Carstens
RME

MC's Website

  • Registered: 2012-05-07
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Re: Fireface 800 analog inputs have unexpectedly high harmonic distortion

MC,
I don't have the right equipment to verify if my source is good.

So, I did a separate experiment. I looped back  Fireface analog out ch1 to analog in ch1.
I am measuring following (combined) THD values:

Vpeak (V)    THD (dB)
0.000484    -22.2
0.00242    -37.3
0.00484    -42
0.00968    -50.2
0.0242    -58.3
0.0484    -64
0.0968    -68.8
0.242      -77.5
0.484      -83.9
3.42188      -98
4.84      -98

Do these make sense?

Is it ok to conclude that THD spec is only met at close to full scale? Lower signal levels will experience much higher THD?

Thanks.

6 Reply by MC

  • MC
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Re: Fireface 800 analog inputs have unexpectedly high harmonic distortion

They don't have higher THD - the distortion is masked by the noise floor, so you can not measure them at lower levels anymore.

Why not use something simple to check, like http://audio.rightmark.org/index_new.shtml or http://www.hpw-works.com/ or one of the other numerous tools available...Audacity is NOT a measurement instrument, it's an audio editor.

Regards
Matthias Carstens
RME

MC's Website

  • Registered: 2012-05-07
  • Posts: 4

Re: Fireface 800 analog inputs have unexpectedly high harmonic distortion

Thanks MC.
yes, Will try to re-measure with known-good sources and measurement instruments..