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Old 2nd July 2012, 12:21 AM   #111
Pano is offline Pano  United States
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Friday I was playing around with V.A. and an HP 8903B distortion analyzer.
You might have noticed that there is a THD tab in the analyzer section. Using the 8903B as an external source, I got the number to agree between the HP and V.A., sort of. Both said about 0.002% THD if VA was switched to "None" for window smoothing. Anything else and VA reported much lower THD than the HP did. Of course I was using a soundcard for input to VA and the 8903B's own input.

What was also interesting is that the HP 8903B was measuring THD+n when reporting 0.002% and VA had to be in THD (only) mode to get the same number. I know there are some questions and concerns about how software measures THD, so I thought the comparison was interesting to see the two approaches.

FWIW, here is an ARTA spectrum of the HP distortion as seen by my M-Audio card. Orange is the HP signal, green is the M-Audio DAC itself.
Click the image to open in full size.
I used ARTA here because it runs faster than VA for FFT plots.
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Old 2nd July 2012, 05:24 AM   #112
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Does VA agree with ARTA?

And is anyone able to use the 192kHz capability of a soundcard, as opposed to the earlier mentioned apparent software imposed limit of 96kHz??

Pano - are you using the M-Audio Audiophile 192 or some other??

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Old 2nd July 2012, 08:22 PM   #113
Pano is offline Pano  United States
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M-Audio-Fast Track Pro. Though I do have a couple of other M-Audio USB gadgets. Can maybe borrow a PreSonus usb card to try. These USB top out at 96 Khz or 48 Khz full duplex.

A couple of people I know here have the internal Audiophile 196 KHz card.
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Old 2nd July 2012, 11:59 PM   #114
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Pano, it would be interesting to also compare the two instruments measuring higher levels of distortion. If you put one or a pair of diodes (in parallel but one reversed) across the source output, you can tweak the level for something like a few percent distortion. Probably good practice to put in some R to avoid embarrassing the source.

I find 5% a good figure - easy to hear and see, and well above the residuals in both systems. If you set the frequency low, you'll get a lot of harmonics in below the sound card upper frequency limit.

I guess the differences between the instruments might be due to such things as:
- the hardware device might have a wider bandwidth and higher self noise?
- the hardware device probably operates by nulling out the fundamental and measuring what's left? (Could you be seeing some residual fundamental?)
- VA operates by measuring the harmonics individually and summing them?
- VA will presumably ignore card artefacts that are not numerically related to the fundamental in the THD measurement, but will include them in the noise section?

I tried it comparing VA with the N&D set I built back in the 70's. 5% on the N&D set read as 4.86% on VA. That's probably within the accuracy of the N&D set's analogue meter!

It would be fun to hear how you got on.

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Old 3rd July 2012, 12:07 AM   #115
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Good ideas. The HP rather conveniently provides a filtered output that contains (mostly) everything but the fundamental. I played around with that a bit.

I'll try the 2 diode test later this week and see what I get. Thanks for the suggestion.
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Old 3rd July 2012, 01:17 AM   #116
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Hmmm, although REW computes it at about 6.2%

What other THD software is out there?

Terry
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Old 3rd July 2012, 02:08 AM   #117
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RMAA? Does it allow an external signal?
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Old 3rd July 2012, 04:40 AM   #118
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I don't think so. It sends its own packet of signals at various levels, frequencies, etc.

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Old 3rd July 2012, 06:36 AM   #119
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Quote:
Originally Posted by Pano View Post
RMAA? Does it allow an external signal?
Yes, easy.
There is a button on the bottom of the window where you can save the .wav, and burn it on CD or so.
All you have to do is to start replay at the correct moment, but RMAA is not picky, it somehow recognises when the test signals are starting.
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Old 3rd July 2012, 06:39 AM   #120
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by Pano View Post
M-Audio-Fast Track Pro. [...]
And this can record at 96kHz? Did VA record at 96kHz?

I have a ESI Juli@ which is able to record at 192kHz (RMAA has no problems with it) bit VA stubbornly says that 96kHz is maximum.
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