The soundcard input is a high pass at a few tens of
Hz. It doesn't work at DC and badly at low frequencies
(<~50Hz).
It's maybe not as bad as you think... I tried an experiment a while
ago. Audition lets you zoom in on the amplitude scale and take very
accurate measurements of the signal level. I set a signal generator so
Audition read -6dB at 1kHz and swept the frequency while monitoring
the sig gen voltage with a 'scope to make sure it didn't change.
The table below is a listing of amplitude error (in dB) vs. frequency.
The two left hand columns are for a 'Soundblaster Live Platinum 5.1'
and the two right hand columns are for a Soundblaster USB widget I got
for about 30ukp.
Freq Right Left Right Left
10 -0.13 -0.13 -1.88 -1.88
20 -0.09 -0.09 -0.57 -0.58
40 -0.08 -0.08 -0.18 -0.18
70 -0.04 -0.04 -0.05 -0.06
100 +0.00 -0.01 +0.0 +0.0
200 +0.00 +0.01 +0.04 +0.04
400 +0.00 -0.01 +0.02 +0.01
700 +0.00 +0.00 +0.01 +0.01
1000 +0.00 +0.00 +0.0 +0.0
2000 -0.03 -0.02 -0.04 -0.04
3000 -0.09 -0.09 -0.06 -0.06
4000 -0.08 -0.08 -0.01 +0.0
5000 +0.02 +0.02 +0.02 +0.03
6000 +0.07 +0.07 -0.03 -0.01
7000 +0.08 +0.08 -0.08 -0.06
8000 -0.04 -0.04 -0.08 -0.05
9000 -0.10 -0.10 -0.02 +0.01
10000 -0.13 -0.13 -0.01 +0.03
12000 -0.01 -0.02 -0.12 -0.07
14000 +0.02 +0.01 -0.07 -0.02
16000 -0.11 -0.11 -0.20 -0.15
18000 -0.04 -0.05 -0.27 -0.24
20000 -0.54 -0.54 -0.92 -0.92
21000 -1.44 -1.59 -2.20 -2.20
22000 -3.8 -4.0 -8.60 -5.40
23000 -13.0 -9.0 -16.0 -12.5
24000 -23.0 -22.0 -24.0 -24.0
That looks pretty good to me! Notice the VERY sharp antialias filter
that starts at 20kHz.
It's not as high resolution as you may think. 16 bit
is not really achievable inside a PC and most soundcards
are not even close, anything below the first 12 bits or
so is random numbers.
This is true, I can see around 3 bits worth of noise - around 120uV. I
wonder about the 24bit Audigy devices, the are external so it would be
possible to get the noise down very low. I would hope to see >16bits
of resolution from one of these. They also sample at 192kHz but I
don't know if you still get the full 24bits of resolution at that
rate.
Dave.