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Old 21st August 2012, 09:42 PM   #21
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Quote:
Originally Posted by ra7 View Post
I was having a strange problem using the XMOS USB2.0 Eval board with ARTA. In Spectrum Analyzer mode, it measures superbly, less than 0.0045% THD. But in impulse mode, I can't get it to do a full 20 Hz - 20 kHz sine sweep. The sweep doesn't produce any sound until after 1 kHz. Now, if I set the averaging to 2 or more, then the second sweep begins at 20 Hz, but the first one still comes out as no sound until above 1 kHz. And funnily enough, the pink noise seems to work fine.
Sounds like there is some delay between the sound device driver being opened, and the sound card starting to work ?

Are you using WDM or ASIO ?
Quote:
Also, ARTA starts doing all sorts of funky things, like showing two impulses, one after the other, when really there should be one averaged impulse from 2 sweeps.
Are you sure that's not analogue passthrough enabled from line in to output causing an echo ? Harder to spot in a loopback test but with a speaker/mic there will be obvious discrete echos.
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I've put it down to the async clock on the XMOS that ARTA cannot get a good handle on. I read in the documentation that is designed to work with Synchronous cards only.

Any thoughts?

Oh btw, same problem with Holm. So, it's definitely the soundcard.
Doing single channel measurements on an asynchronous card is going to be inherently problematic, is there some reason why its not synchronous ? A hardware limitation, or just a driver limitation ?

You might have better luck with the dual channel mode in ARTA, although that has limitations of its own.
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Old 21st August 2012, 09:51 PM   #22
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Originally Posted by TheGimp View Post
At some point the input will exceed the acceptable input for all of the audio interfaces, and they will begin clipping. When they do your distortion will go up, and no longer represent the input signal.
That's true, but many sound cards can't reach 0dbFS without a small amount of clipping - for example the harmonic distortion whilst still low will jump up by maybe 10-15dB at FS compared to what it is at -2dBFS.

Also I've sometimes found that the full scale output and input don't match - for example on the Audigy 2 ZS the line output with output volumes set to maximum actually exceeds the clipping point for the line input also to 0dB. So a simple analogue loopback test will clip on default mixer settings.

In the case of this sound card the line input level slider controls an analogue attenuator before the ADC so by turning that down a couple of dB digital clipping is avoided in this situation. (It can also be set for gain, increasing the sensitivity beyond the normal 2.8v)

The analogue inputs have a clipping threshold that is (from memory) about 6dB higher than the ADC, so you can apply up to 6dB of attenuation in the input slider before the analogue stages start clipping before the ADC...

On some sound cards the input level slider is simply a digital attenuator, so in these cases it should be left on maximum.

Theres a lot of gotchas and tricks with different sound cards, it takes a little while to learn the strengths and weaknesses of a given card, so its very important to do plenty of loopback testing at different sample rates, with the mixer app set to different settings and so on...
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Old 21st August 2012, 09:57 PM   #23
RNMarsh is offline RNMarsh  United States
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Default need best --

[QUOTE=5th element;3125417
But if you want it to be able to assure you that your 0.0003% DAC/headphone amp, or ultra low distortion blameless power-amp and a matching pre-amp are working to spec, then you'll need to get something better.[/QUOTE]

I need something 'better' -- .0003 or less. no internal sound card - must be portable/external. What do you suggest as options with and without ARTA - other?

Thx, Richard Marsh
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Old 21st August 2012, 09:58 PM   #24
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Originally Posted by dennismiller55 View Post
If reduced the Master output and record levels I can reduce the harmonics. But I thought the goal was to increase the levels to peak the 1khz tone somewhere close to -3 dBFS. I thought I read that somewhere in the ARTA manual.
You want to maximise SNR by having the input signal be near maximum when possible, but not at the expense of introducing distortion. Only a few types of acoustic measurement require large dynamic range mind you.
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It would seem to me that one would have to establish a consistent reference level from one measurement to the next.
Yes. Part of the calibration process is to find what mixer settings work best with your sound card, then taking note of the exact mixer settings, and either leaving them alone, or verifying that they're still correct before a measurement session.

If you share the PC with other uses (playing music, gaming etc) then you may need to do an analogue loopback test before starting a measurement session to check your calibration is consistent, as some other app may have messed with your mixer settings. (Skype, games, etc)
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Old 21st August 2012, 10:21 PM   #25
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Originally Posted by RNMarsh View Post
I need something 'better' -- .0003 or less. no internal sound card - must be portable/external. What do you suggest as options with and without ARTA - other?

Thx, Richard Marsh
Actually I have no idea when it comes to external devices, sorry.
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Old 21st August 2012, 11:15 PM   #26
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Originally Posted by dennismiller55 View Post
Thanks everyone for your comments and help.

On many analog spectrum analyzers you can place markers at key spots on the trace. The markers display precise measurements that can compare with other markers.

My next question is, does ARTA (or any of the other popular software programs) provide this capability?

I know you can move the cursor to a point of interest, annotate the reading and then move to another point of interest.

Anyone?

I am just exploring what ARTA can do (or can not do).
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Old 22nd August 2012, 12:27 AM   #27
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I'm not sure Dennis, it doesn't seem immediately obvious and I don't think I've encountered it before, but then I've not been looking either. I know of what you speak though because my electronics spice simulator will allow you to set markers and then it will tell you the difference between them etc. Maybe someone else can chime in if they know of a similar feature in ARTA. I mean you can set overlays but the cursor wont compare one from the other.
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Old 22nd August 2012, 01:57 AM   #28
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Thanks DBMandrake and 5th Element. I am slowly getting the fell for this ARTA Software. I think it will be a great tool on my bench.
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Old 22nd August 2012, 07:05 AM   #29
RNMarsh is offline RNMarsh  United States
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Originally Posted by 5th element View Post
Actually I have no idea when it comes to external devices, sorry.
Rats! OK I'll buy a cpu in a box -- now what would do the job?
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Old 22nd August 2012, 09:01 AM   #30
ODougbo is offline ODougbo  United States
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dido here 5th, thanks!

Dennis, could you provide link to sound card plz?


Quote:
Originally Posted by dennismiller55 View Post
I agree, this is just the help I was looking for. Thanks for taking the time to explain it 5th.

I do have a new usb m-audio sound card coming. It should show much better results.
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