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"Spectral holes" not "Birdies"?

Title: Re: - "Birdies"
Hi Maxime,
What you are talking about here is known as "spectral holes" (I think).
Spectral holes occur in modern audio encoders as a result of (over)quantization which sets all spectral components of some (scalefactor) bands to zero. This is directly related to the masking threashold calculated by the psychoacoustic model, so the threashold should be modified in bands that must not be zeroed.
Avoiding spectral holes is one of the top problems in designing good psychoacoustic models.
One of the methods for spectral hole avoidance is presented in 3GPP's AAC encoder specification: TS 26.403 (freely available at www.3gpp.org).
----- Original Message -----
Sent: Wednesday, March 15, 2006 8:34 PM
Subject: [AUDITORY] RE : - "Birdies"

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 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