From: syd@en.ecn.purdue.edu (Dennis P Hilgenberg)
Newsgroups: rec.audio
Subject: Re: Compression question
Keywords: compression algorithms: what's lost?
Date: 20 Dec 92 20:49:04 GMT
Organization: Purdue University Engineering Computer Network

Jim Pixton writes:

>I know this has probably been discussed before, but in light of
>the new MD threads I was hoping someone could explain how 
>music compression algorithmswork (in regards to MD and DCC)? 
>In particular, what's lost in the compression, do they cut
>out certain frequencies or just pieces of music.

This is bordering on "frequently asked," no?

The compression algorithms are based on the psychoacoustic model of the
ear's "critical bands," which are intervals of the audio spectrum in
which noise of a given power "sounds" the same regardless of its
spectral shape.  Also, in a given critical band, as an extension of
empirically obtained "masking" thresholds, noise is "inaudible" when
the measured signal to noise ratio is beneath a certain (computable)
threshold.

The basic scheme is to take the fast Fourier transform over a short
window, group the Fourier coefficients into their respective critical
bands, and quantize the coefficients of each critical band to the
coarsest possible resolution for which the quantization noise is
beneath the threshold for that band.  Note that these thresholds (and
hence the quantization resolution) are not the same for each critical
band, and also change over time.  I believe that the thresholds are
computed iteratively.

The "receiving" or playback then reverses this process.

Note that this information is as I have perceived it during a brief
research project unrelated to my usual work (I am in image processing)
and may not be exactly right; also, this information is borrowed from
several papers, none of which explicitly mentions MD or DCC; hence the
implementation may not be quite as I have described it.  However, to my
knowledge, the critical band model is indeed what is used in the
compression.

Someone (Bob Myers, maybe?) already posted a bibliography, which I
neglected to save, but it looked similar to the one in my write-up:

[1] J. D. Johnston.  "Transform Coding of Audio Signals Using
    Perceptual Noise Criteria."  IEEE Journal on Selected Areas
    in Communications, Vol. 6, No. 2, February 1988.

[2] M. R. Schroeder, B. S. Atal, and J. L. Hall.  "Optimizing
    Digital Speech Coders by Exploiting Masking Properties of
    the Human Ear."  Journal of the Acoustic Society of America,
    Vol. 66, No. 6, December 1979.

[3] D. D. Greenwood.  "Critical Bandwidth and the Frequency
    Coordinates of the Basilar Membrane."  Journal of the Acoustic
    Society of America, Vol. 33, No. 10, October 1961.

[4] E. Zwicker and U. T. Zwicker.  "Audio Engineering and 
    Psychoacoustics:  Matching Signals to the Final Receiver,
    the Human Auditory System."  Journal of the Audio Engineering
    Society, Vol. 39, No. 3, March 1991.

[5] R. P. Hellman.  "Asymmetry of Masking between Noise and Tone."
    Perception & Psychophysics, Vol. 11, No. 3, 1972.

[6] B. Scharf.  "Critical Bands."  Foundations of Modern Auditory
    Theory, J. V. Tobias, Ed., Chapter 5.  New York:  Academic Press,
    1970.

>Also, what does "5-lossy" mean.

Can't help you there.  Probably some advertising gimmick.

-- 
Dennis P. Hilgenberg
syd@ecn.purdue.edu