"But simple early schemes had
interactions
between the input signal and the sample frequency that
caused
"birdies" at sum and/or difference frequencies."
I realise now "birdies" might not be
exactly what i meant.
I will rephrase then, if you ever looked
closely at the spectrogram of a sample of music encoded at low
bit-rate (20-64Kbps)by either mp3 or AAC codecs, you might have
noticed dark spots in some places where it is obvious the energy of the
signal is not suppose to be so small. I suppose that artifact is due to
the richness of the signal at this precise moment (in comparison with the
bit-rate) and therefore bit allocation can not cope with the demand. Then the
coder being unable to encode leaves a hole in the spectrogram.
If i'm correct with the above assumption,
what i'd like to know is if there is any documentation or perceptual
intepretation of this problem of coding.
Regards,
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