Thank you Bob for you comments,
"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,
Maxime
________________________________
De: AUDITORY Research in Auditory Perception de la part de Bob Masta
Date: mer. 15/03/2006 14:30 À: AUDITORY@xxxxxxxxxxxxxxx Objet : Re: -
"Birdies"
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