Soundcard performace .......FFT pics and problems. - diyAudio
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Old 26th June 2004, 02:54 PM   #1
A 8 is offline A 8  Sweden
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Default Soundcard performace .......FFT pics and problems.

I've been tweaking my USB 2.0 creativeLab soundcard in order to lower the noisefloor, focusing on powersupply and shielding in the AD converter and digital processing paths.

This morning I decided to take it to the next level and try to push the noisefloor down even further, or get rid of the crap @ 5kHz and up.

This is where I started today, at this point the laptop and soundcard are on batteries with a bit of powersupply (oscons & coils where they made sense) tweaking since before.
Basically up until this morning getting the cards AD path down from around -100-110dB SNR to -130dB SNR.

Here is a FFT screendump of this morning. The conditions are ; analog in shorted, soundcard and spectraLab set for 24/48 displaying -120 to -150dB.
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File Type: jpg a_shorted_rev1.jpg (34.7 KB, 455 views)
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Old 26th June 2004, 05:34 PM   #2
A 8 is offline A 8  Sweden
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Default -131dB SNR is pretty good...

But working on it for an hour or so more, killing the internal switchmode supply and shielding the dac/ad chip gives this...-135.3dB.
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File Type: jpg a_shorted_rev2.jpg (34.8 KB, 374 views)
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Old 27th June 2004, 09:33 AM   #3
A 8 is offline A 8  Sweden
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Default -136.2dB @ 8Hz

This dump shows the soundcard after adding shielding to all digital processing chips and specific shielding around the spdif transformer which was very close to deoupling and filter caps for the AD converter......gives a reading of -136.27 now @ 8.06Hz.

This brings me to my problem, I can't seem to get rid of the spikes at 5.4, 15kHz and around, they are slightly damped compared to the start but I don't seem to have found the cause for them.
If I add a oscons or mkt caps at the wrong places the spikes could increase with up to 10dB (which so far, for me indicated the powersupply)
They are only visible when I monitor analog in, using spdif and running zero's does not reveal the noise.

Any advice/experiance would be welcome!

/Michael
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Old 27th June 2004, 11:55 AM   #4
Bricolo is offline Bricolo  France
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I think that many ICs are powered from the same regulator, is that it?

If yes, you could try to implement Guido Tent's power supply decoupling


The 5.4 and 15kHz spikes can be due to an IC working (or asking for current) at this frequency, that causes some noise on the supply, and degrades the DAC's (or the ADC's) performance
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Old 27th June 2004, 12:19 PM   #5
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Default What is the ADC

Which is the ADC chip?? And what is there before it??
Thanks
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Old 27th June 2004, 02:30 PM   #6
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Default Re: -136.2dB @ 8Hz

Quote:
Originally posted by A 8
This brings me to my problem, I can't seem to get rid of the spikes at 5.4, 15kHz and around, they are slightly damped compared to the start but I don't seem to have found the cause for them.

What about noise from the computer itself, or the USB connection?
Do you get a difference if you place the soundcard at another position relative to the computer?
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Old 28th June 2004, 10:08 AM   #7
hjelm is offline hjelm  Sweden
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Have you checked that it isnt a frequency that is folded around fs/2. At these levels it could be that the filters do not cope with filtering the noise above fs/2.

The frequencies to look for would be then fs/2+n(fs/2-freq).

Where n is most likely to be 1 but any integer is possible.
Are you sampling at 48k or is it 96k?
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Old 28th June 2004, 02:26 PM   #8
A 8 is offline A 8  Sweden
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Quote:
I think that many ICs are powered from the same regulator, is that it?
Yes the chips analog supply share the same 5v supply as a few logic chips and the toslink units however there are ferritecoils in between. I also tried disconnected the spdif and toslink units but it did not make a difference. As I don't have the schematics there could ofcourse be more.

Quote:
Which is the ADC chip?? And what is there before it??
Its a sigmatel URL=http://www.sigmatel.com/documents/stac9460-ds.pdf]STAC 9460[/URL] that is not supposed to come close to where its at right now and the signal goes directly into the chip, the only komponents between input and the chip are two caps for dc filtering.

Quote:
What about noise from the computer itself, or the USB connection?
The only issue that changes depending on where I put it is that it sometimes picks up 50Hz if I am not careful.

Quote:
Have you checked that it isnt a frequency that is folded around fs/2. At these levels it could be that the filters do not cope with filtering the noise above fs/2. The frequencies to look for would be then fs/2+n(fs/2-freq). Where n is most likely to be 1 but any integer is possible. Are you sampling at 48k or is it 96k?
The pics up until now was sampled at 48kHz and I thought about it before posting. It could explain the peak at 5350Hz (ie 48kHz/9 equals 5333Hz) but not the one at 14362Hz as far as I understand.

I've attached the same dump(closeup) but showing both 44 and 48kHz, yellow is 48kHz and purple is 44kHz.

I guess I am trying to ***** if this is as good as this chip goes or if it is likely that I can get rid of the crap over 1kHz.

/Michael
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Old 28th June 2004, 03:20 PM   #9
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Default M-Audio Sonica Theater

Hi,
I've just started looking at doing something similar with my USB M-Audio Sonica Theater, as with the amp turned up I can hear all sorts of crap when the computer is busy. RMAA also shows very bad S/N ratio etc.

What I did was to buy a powered USB 2.0 hub and hang the sound card off of it, so that as well as cleaning the power, hopefully the data sinals will be buffered by going through the hub electronics.

This had a huge effect, esp. in the noise floor (dropping from around -70dB to -96dB in the bass region). I will post graphs when I get a chance soon.

The enxt step is to see if the hub needs any more filtering, then to open the card and see what can be done with power inside it.

Arnie
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Old 28th June 2004, 04:15 PM   #10
ojg is offline ojg
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Quote:
Originally posted by A 8

Its a sigmatel STAC 9460 that is not supposed to come close to where its at right now and the signal goes directly into the chip, the only komponents between input and the chip are two caps for dc filtering.
Hi Michael,

You have made some very good progress in minimizing the noise-level of this ADC! However in my experience, the tricky part is minimizing THD. I think you will see that when you start measuring sinewaves with amplitudes close to full scale that the linearity of the ADC becomes the limiting factor.

This thread have got me interested in buying an external soundcard. Does anyone have an overview of what ADC/DAC chips the different USB/Firewire soundcards use?

Let's ask Google first:
Creative Labs Audigy2 NX: Sigmatel STAC9460
M-Audio Sonica Theater: AKM4529
M-Audio Firewire 410: AKM AK4381, AKM AK5380, AKM AK4355
M-Audio Firewire Audiophile: AKM AK4628
Terratec Aureon 7.1 Firewire: WM8770

Any more?
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