On Wed, 18 Oct 2006 20:34:24 +0100, Rob
wrote:
Don Pearce wrote:
Apropos of some stuff we were doing a while ago, here is the pink
noise track from th HFN-RR test disk played on my system, which is:
Systemdek IV
SME 3009ii arm
AT-OC9 microline cartridge
V.nice indeed.
http://81.174.169.10/odds/dspkr/atoc9.gif
It shows an essentially flat response from about 12Hz, with a slight
lift at 10kHz, then returning to the proper level above that. The
recording is made at 96kHz sampling, and it is clear that the test
record cuts off sharply at 20kHz. The cartridge claims to go to 60kHz
(ahem!)
I've offset the levels of the left and right tracks slightly so they
are more visible.
d
I'm quite used to being alone on this, but could you explain a couple of
things please:
How is time represented on the graph?
Time isn't represented on the graph. The whole thing is about 20
seconds of pink noise, converted by FFT to a dB vs frequency graph.
I look at the graph and I can't see the things you describe:
I'd have expected the db (loudness?!) to remain constant for a flat
response (that is, a straight horizontal line)?
Pink noise isn't flat for an FFT. It drops as you go up by 3dB per
octave, or 10dB per decade. If you put a ruler along the average of
the graph, you can see the deviations above and below the straight
line.
If you have another look, I've drawn in a line that represents what a
flat response should be.
I wouldn't have commented on the 10k blip, but consider the 10-20k range
to roll off?
The line rises above the mean diagonal before it drops again - in fact
it drops back to just below the wanted line at 20kHz.
I've often seen these things in magazines and frankly they've always
been a mystery!
And, clearly not ashamed of public disclosure here, I tried to 'tune' my
system adopting (I think) the same twisted mentality associated with my
interpretation of your graph, and it sounded totally bizarre - bass just
swamped the entire thing.
Thanks, Rob
Much easier when you know what is happening. To be technical, with
pink noise you have equal power per relative bandwidth - so as you go
up in frequency the power gets more spread out, and there is less of
it in each finite frequency bin that the FFT produces.
d
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Pearce Consulting
http://www.pearce.uk.com
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