Re: podcasts?

Sean wrote: 
On 7/12/05 2:47 PM, "Curt Fischer"  typed:


Have you ever tried a side-by-side comparison?  The difference is
noticeable, although perhaps not with a 196 kbps mp3, which is what the
guy was talking before my top quoted response above.

My understanding of the mp3 encoding process is not detailed, but I
thought that more or less, Fourier transforms of the source are taken as
functions of time (which involves inevitable frequency cutoff due to the
finite time domain over which the FT is taken).  Next, peaks in the
frequency domain for a given time point are "sharpened" by removing
lower-intensity frequencies present around the peak frequency.  This
works because of some sort of masking phenomenon with human hearing that
says that if you have a say 50 dB sound at 440 Hz then no one can hear a
30dB sound at 441 Hz.  This sharpening provides most of the
compression, right, but is obviously a lossy process, right?



Of course I agree that it is "lossy." It's just not a simple frequency
filter or other straight-ahead waveform alteration. It's based on some
high-falutin' psychoacoustic stuff having to do with eliminating irrelevant
information.
There's a good explanation in Sound on Sound:
http://www.soundonsound.com/sos/may00/articles/mp3.htm


Thanks for an interesting link.

I don't know what you mean by straight-ahead "waveform alteration" that you keep mentioning. What is it and how would it help compress audio data? Also, I had a few details wrong but the gist spot on about the way the compression works: elements of the Fourier (or discrete cosine) spectrum are eliminated based on the psychoacoustic criterion you mentioned.

Maybe I will have to repeat the side-by-side comparison on a high kbps mp3, but I don't have my good headphones with me here in Japan.

--
Curt Fischer
.

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