Re: [LAU] mp3 codec question

On Mon, Feb 11, 2013 at 11:35:41AM -0800, Aaron L. wrote:
 
> Is it "lossy" simply because it rounds-off stuff?
> Or does it actually drop samples (like various lossy video encoders?)

It does both in a sense. Not on the original samples that make
up a waveform, but on a spectral representation of the signal.

What happens (roughly) is that the signal is chopped up into small
fragments, and for each one the spectrum (fourier transform) is
computed. Transmitting or storing the spectra instead of the
signal would take exactly the same amount of information. But
the spectrum can be 'simplified' without affecting audio quality
quite easily. Depending on the relative levels, some frequencies
will 'hide' others - you don't hear the weak ones. So those can
be deleted from the spectrum ('dropping samples'). Also it doesn't
matter much if some frequency has a level of e.g. -35.4 or -35.3
dB, so these numbers require much less bits than a sample would
(rounding). The result is that the simplified spectrum needs much
less data than the original signal. The decoder just recomputes
the signal from the spectrum data.

Ciao,

-- 
FA
A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)
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