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Re: RE : - "Birdies"

the description below was my understanding of birdies. I think there
must be documentatino on perceptual effects in AES papers

Dr. Peter Lennox
Signal Processing Applications Research Group
University of Derby
Int. tel: 1775

>>> Maxime Leroy <m.leroy@xxxxxxxxxx> 15/03/2006 19:34 >>>
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.


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
have any references to provide.  But if you are looking for a
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

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