11 Jan
2014
11 Jan
'14
12:20 a.m.
On Fri, Jan 10, 2014 at 10:48:15PM +0000, James Stone wrote:
It means something... but what it means is another question.
Assume
* the noise is more or less white (if not: real trouble)
* the spectrum analyser uses an 1024 point FFT (typical),
* and a raised cosine window (also typical),
Then the full bandwidth (1/2 Fs) is covered by 512 filters,
and each of these has a bandwidth that is 1.5 times that of
a rectangular filter.
So each filter sees 1.5 / 512 of the total power.
10 * log10 (1.5 / 512) is -25.3 dB
So the noise level would be around -93 + 25 = -68 dB.
You could check this with jnoisemeter.
Do you mean
the spectrum analyser showed a trace at -90 dB ?
Yep - the spectrum analyser showed a trace at around -93dB across all
bands. Is this meaningful? With further testing, I think I can detect a similar
change in gain
effect from what Robin reported - I seem to be getting between -13.0
and -13.6 db at the lowest end of the spectrum for the snippet that is
being played back. Seems a bit random where it comes though.
That could as well be due to the random alignment of the signal and
the periods of the analyser.
I think
they are analogue pots, so maybe this might account for this
variance?? - anyway, I think it is too subtle to notice in normal use.
Unless you pay at least 10 times more they will analog pots, and they
all show this effect to some degree.
Ciao,
--
FA
A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)