Hi, Maxime. I'm not sure exactly what you are looking for,
and I don't
have any references to provide. But if you are looking for
a perceptual
description, here's what I know:
"Birdies" are little
whistling sounds that are related to the
program material, but are not
harmonics of it. They used to
be a serious problem in sigma-delta
converters, which compare the
input signal to a reconstruction of the output
signal, and generate
a "higher than" or "lower than" response on each
sample. That
1-bit stream is then used to create the reconstruction for
the
comparison (and the eventual output). Nowadays, this is all
done
at very high sample rates and then ultimately converted
down to a nominal
rate, and the reconstruction processing is
very sophisticated. But
simple early schemes had interactions
between the input signal and the sample
frequency that caused
"birdies" at sum and/or difference frequencies.
The birdies might
be only 40 dB down, but even if they were much softer than
that
they were clearly audible, especially on sparse program material
like
simple sine waves, flutes, etc, since they appeared in
non-harmonic locations
and were not masked by the program
itself. They also often had the
annoying habit of sweeping in the
opposite direction to a sweep in the signal
frequency, which made
them really obvious.
Hope that
helps!
Best regards,
Bob
Masta