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Scott Dorsey Scott Dorsey is offline
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Default Why don't these signals null?

James Price wrote:

The recordings weren't played by a human twice. A 10 sec. *recording* of a
guitar DI was played into an amp/cab twice, consecutively, one after the
other, the output of which was recorded and cut up into separate tracks.
Both tracks were time-aligned and the phase inverted on one.


Ahh, I was thinking you were comparing the original DI'ed signal with the
reamped signal.

If you're comparing two reamped signals, the difference is likely the
difference between the two reampings, and the differences are likely
more than 30 dB down.

What is different between the two reampings? Well, the noise is different,
because noise always is (that's the definition of noise... it's not correlated
with signal). Likely the microphone position is a little different (maybe
less than a millimeter), the air currents in the room are a little different
(which is a big deal with a source that has an uneven radiation pattern like
a guitar amp), and if that's a tube amp it's likely warmed up a little more
or a little less and so slightly inconsistent.

You'll know for sure when you listen to the difference signal. Is it all
120 Hz hum? Is it all hiss? Is it correlated enough that you can hear a
guitar note somewhere in it? The best thing about the null test is that you
can use your ears (and an FFT) to figure out what the difference is, as well
as just measuring the absolute difference as a scalar number.

Remember that if you're looking at a difference that is 30 dB down from the
signal, you're seeing a difference of 1/1000th of the signal. It does not
take much.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."