RE: Hello and Help!

[Garrett Wollman <wollman>]

<<On Sun, 20 Jul 1997 10:01:17 -0400 (EDT), Rob Landry <umar@wcrb.com> said:

> Also, some people think that digital compression adversely affects the
> sound quality of music on FM.  They may think their station does not sound
> as loud or as good as stations playing CDs.  My experience is that what
> these people hear are *analog* problems.

Or, for stations using compressed digital STLs, it is possible for
multiple generations of compression artifacts to degrade the signal
even further.

And, as a point of fact, different people do, in fact, have different
senses of hearing.  Lossy compression schemes, like MPEG-audio (and
its visual equivalents like JPEG and MPEG-video) are based in part
upon a mathematical model of what sort of noise is least obtrusive to
the average listener/viewer.  They then strive to shape the noise
caused by the loss in the compression to meet that model.  As always,
some people and some receivers will have different characteristics,
and may be able to perceive the compression artifacts more clearly
than most other people.  These are usually a tiny minority.

Lossy compression schemes go back a long way.  A lossy /analog/
compression scheme that many readers are intimately familiar with is
the NTSC scheme for color television... there was no space in the
original TV spectrum for full-resolution color, so the color
information was squashed horizontally to fit in the available
interval.  As a result, NTSC color is only capable of representing 100
to 150 vertical lines, as compared to black-and-white which is about
2-1/2 times that.  (The horizontal resolution is fixed at 252.5
lines.)  If you've ever seen someone on TV who was wearing a shirt
with thin dark and white stripes, you've seen the principal artifact
of this system when their shirt turned pink-and-blue.  This is usually
not a problem, however, because people who regularly appear on
television know not to wear such shirts, and at the typical home
viewing distance of 10-20 feet, the typical home viewer can't see the
more subtle effects.  (Human color vision is substantially less acute
than black-and-white.)

- -GAWollman

- --
Garrett A. Wollman   | O Siem / We are all family / O Siem / We're all the same
wollman@lcs.mit.edu  | O Siem / The fires of freedom 
Opinions not those of| Dance in the burning flame
MIT, LCS, CRS, or NSA|                     - Susan Aglukark and Chad Irschick

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