I have worked with audio before, and can confirm internal audio codecs
are very good for... trash them.
If quality is of any concern for you, just try another adapter, i.e. an
inexpensive Behringer UCA 202 USB audio interface.
I've tried it with great results on OpenBSD, and you can buy it in
Europe for as low as 27eur in Thomann.
Regards, and good luck.
I've a PMR/O
El 16/06/2010 15:05, Paul M escribis:
On 16/06/2010, at 6:45 PM, Alexandre Ratchov wrote:
On Wed, Jun 16, 2010 at 12:06:40AM +1200, Paul M wrote:
On 15/06/2010, at 11:18 PM, Paul M wrote:
On 15/06/2010, at 8:25 PM, Alexandre Ratchov wrote:
On Mon, Jun 14, 2010 at 08:20:57AM -0600, Ted Roby wrote:
Sound cards just get too much noise off the motherboard.
well, it depends on the sound card; properly engineered
cards don't get noise, including pci ones.
It seems the best I can get with the built-in sound on this computer
is about 36dB S/N. If the various input gain stages are not set quite
Sorry, that's a typo - should be -46 dB
that's not very good; 8-bit samples correspond to 48dB; I
mean with 48dB S/N, only 8 higher bits are significant.
Such a low S/N ratio makes me wonder if your cables, power
supply or whatever are ok.
BTW, how did you measure the S/N ratio?
No, it's not.
I measured by writing samples to a file then examining the file.
For a source, I used a 440 Hz sine wave.
With all the input gains set to minimum, the noise was negligible
(< -60 db, which is the smallest I could measure). This is pretty much
just the noise of the chip itself.
I then tweaked the various input levels (preamp, input stage, adc) till
the output of the preamp was just below clipping of the first input
stage, and the samples in the file were also just below clipping.
This gave me my signal level.
Then I disconnected the input and repeated, this gave me my noise -
shorting the input may give better results.
I repeated this over and over to find the best balance between input
stage gain and adc gain. I found the best result was to set the input
stage as low as possible, and adjust the adc gain to give full output.
(dont forget the input signal is as high as it can be without clipping)
Update: Today I reduced the input gain stage to 0 (I didnt try this
yesterday) and got about another 6-8 dB, so now I'm < 50 dB.
Jacob mentioned in and earlier mail that '0' for the input stages on
this codec is 0dB gain. FWIW the adc gain is set to 96 (+12dB).
Since my moise is measured with no input connected, this rules out
any external problems such as cables.
The power supply in the computer may be moisy, but I would guess
that crappy circuit layout is more likely. (though I will try another
power supply some time and see if it helps)
paulm
