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Old 23rd February 2003, 09:16 PM   #1
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Default Software Analysis Testing

I am trying to use the spectra plus analysis software to test the distortion of my soundcard(soundblaster live). I know it is not a good card but the tests done athttp://www.pcavtech.com/soundcards/ct4620/index.htm show that .01% THD+N using the loop back test can be achieved. However, when I send a 1kHz sine tone(-0.6dB) I get about .09% THD+N and when I test my signal generator(1kHz) I get over 2% THD+N. These seem too high, could these errors be caused by other components in the computer? I have been working at it for over a day now and can not seem to get the correct results. Any help would be greatly appreciated!
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Old 23rd February 2003, 10:49 PM   #2
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If your signal generator is a "function generator" set on sine, rather than a true sine-wave generator, 2% distortion is not out of the ballpark. Also, though you've no doubt done this, check for ground loops. A few mV of 60 Hz can really screw up a distortion measurement.
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Old 23rd February 2003, 11:19 PM   #3
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I just down loaded the spectra plus demo program.

What kind of signal generator are you using? Ive seen some w/ horrible specs. Depending on what it was made for distortion might not have been a huge issue with its design
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Old 24th February 2003, 12:04 AM   #4
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My function generator is very very old Mark I. I didn't realize that 2% THD was not out of the question for old equipment. On that note, can you use your sound card as a signal generator sending it through the unit under test back into the SC's input to analyze it. If so, I wouldn't need a new signal generator right? I hope....
I may be recieving a Lynx One Sound Card next week so that will hopefully double as my signal generator and analyzer.
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Old 24th February 2003, 12:48 AM   #5
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I don't have any experience using sound cards as a signal source, but it is routinely done. Actually, a test CD does wonderful sine waves, up to the Nyquist frequency anyway. That might be a cheap'n'easy workaround.
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Old 24th February 2003, 01:16 AM   #6
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A long time ago I had a Tek SG501 on my workbench. It was the companion to the 501 distortion analiser. Wish I still had that sertup, it was simple to use and very precise. I recall that the oscillatorís sine wave output was good .001% thd.

I also had this HP Function generator that did a lot of nice sweeps and odd waveforms. Besides being 3ru high, the other trade off for all that neat functionality was that the sine wave was not all that clean.

But jeessh 2% is ruff. Unless the unit is failing I would suspect that there may be a gain stage issue in your test setup. Other than that, all I can think is that it may be an adapter or cable issue... Sounds like youíve been driving at that issue for some time now.

Having messed with the software for all of 10 or 15 minutes now, I would say that your suggestion of using the sound card output to feed signal to the device under test, out of the dive under test, and back into the soundcard for analysis, is perfectly sound (sorry for the pun). The only caveat I can think of is that the PC is fast enough to perform the function generator and analysis functions in unison. I donít think that that should require too much horsepower.

I like the software, couldnt find the price. Can I ask what it costs?

Performing the loop test provides you with the base line test system performance. Breaking the loop and inserting the device under test will show the value of the test apparatus plus the device under test. In theory, you then subtract the loop test value to get the value for the device under test.

A few years ago I had a nice talk with Sam Berkow, who writes the SMART software distributed by JBL. Sam is a very sharp acoustician. Heís also not at all to remember talking to me, Iím not. The bulk of our conversation was about using PC sound cards test purposes. No reason not too. If there is something ugly in the cards design, itíll show up in the loop test.

I tried looking up the Mark I. I'm afraid that I'm not familiar with it, and couldnít find out much from googling. Was that HP, Tek...?

I'm also not sure that Iíve read your posts closely enough. Hope I'm not telling you and bunch of stuff you already know.

-Dave
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Old 24th February 2003, 01:35 AM   #7
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I just downloaded the program for a trial. I've e-mailed the company for the price so I should have it soon.
The Mark I was made by Energy Concepts Inc., I can find any info on it either.
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Old 24th February 2003, 03:53 AM   #8
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If it's like most function generators, it's actually a triangle wave generator with a differentiator to make the triangles into squares and a sine-wave converter which more-or-less crudely curves the side of the triangle to simulate a sine wave. And often that converter has a trimpot which you can use to minimize the distortion, though it will never be very low.
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Old 24th February 2003, 09:36 AM   #9
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Default Integrators and differentiators

Absolutely. Function generators usually produce about 1% THD and a "Norman arch" can generally be seen at the peaks of the sine wave. However, they have their uses. The response flatness when sweeping the frequency of a function generator from end to end of its range is much better than a typical Wien bridge oscillator (which struggles to better 0.1dB). You almost always need a frequency counter with a function generator because their frequency calibration is poor, whereas the frequency calibration of valve Wien bridge oscillators is generally very good.
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Old 24th February 2003, 10:37 AM   #10
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Default Re: Software Analysis Testing

[QUOTE]Originally posted by RobPhill33
[B]I am trying to use the spectra plus analysis software to test the distortion of my soundcard(soundblaster live). I know it is not a good card but the tests done at[
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Go to http://audio.rightmark.org and download the analyser suite.

You are right about the Sound Blaster. Performance is dismal and well below 16 bit. Overloads beyond 60% volume badly, high intermodulation, harmonic artifacts all over the place at around -70dB. Throw it away!
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