From: Ed Kummel (tech_ed_at_yahoo.com)
Date: Sun May 04 2003 - 22:07:57 PDT
Not all music (or all sound) requires the same
bit-rate. Becuase MP3 is a Lossy format (it throws
away content that is formulaically determined to not
be needed) you can compare the music to a photo. If
you are taking a picture of the sky with little
discernable detail, then the compression rate can be
quite agressive, because the edge of a cloud in
relation to the blue of the sky is barely discernable.
The same for a picture that is only of a single color.
If that is compressed you won't notice it! Similar to
a lecture. The speaker's voice has little range, so
high compression is possible.
Now let's look at a picture that takes in a colorful
scene at a local Chinatown Chinese Newyear. The
straight lines and exact colors requires an exact
representation. This means that the areas that can be
compressed with little discernable loss are quite
small. Any edges or seperations between high contrast
colors will be noticed if compression is too high. The
same with music. Music that is vibrant and with a full
range of sound will suffer with high compression. The
exception here is that if there is loud sounds in the
music, they may drown out the compression artifacts.
This leads to the term artifacts. When the Lossy file
is decoded, the missing information is recreated
through a formula that best determines what was
removed. It's like taking a film of someone walking
from point A to point B in 15 frames. If the person is
walking in a straight line, then you can essentially
elimiate all the frames between the first and the last
and tell the decoder to fill in the inbetween frames
based on the two frame you give it. And as long as he
didn't do a little Irish jig on fram 10, you should
not notice any missing frames! But let's say that
somewhere the walker paused, and not every frame has
the same information on it. In a high compression
format, you may notice a "skip" from the original (if
you have the original available) These "mistakes" are
artifacts. These are represented in music as perhaps a
"slip" in the sound (kinda sounds like a sylable was
sped up for a fraction of a second) or represented in
a kind of "hall" effect, making it sound like the
music is coming from the bottom of a coffee can.
Now, this is a very broad representation and doesn't
work like this in real life. But you can determine if
a certain style of music needs high compression or
not. In fact, the music in a single song can vary as
to the compression level. In one part the obo solo can
withstand high compression, where-as a combined string
and percussion section may need more accurate
representation. This is where Variable Bit Rate
(VBR)compression is handy. You give a base level of
compression and let the music determine the level
required. I record all my music at VBR128. I get
compression levels from 96 to 160, and that works for
all the music I have ripped. But it is generally
accepted that classical music can handle more
compression than contemporary music. Because of it's
broad tonalities and rapid pace, speed-metal sounds
very tinny under normal compression, or even some
forms of Jazz.
But this is all moot if as you say, you can't tell the
difference...If that's the case, then great! you can
save on hard drive space...
Ed
web/gadget guru
come and see my Newt on the web (well, mostly) at:
http://65.84.243.167
--- Rick Ludwig <bensisko_at_mac.com> wrote:
<snip>
> off of a $6000 1 year old stereo system, and a 64k
> mp3 recorded from a
> CD and I couldn't tell the difference (the piece was
> Beetoveen's 5th
> symphony BTW).
> Okay, the question is (because I am def to this):
> What is the
> difference? Is the music dulled (i.e. you can't hear
> all the sections
> of the orcestra)? I'm just curious.
>
> -Rick
=====
"If the nation escalates to 'red alert', you will de assumed by athorities to be the enemy if you so much as venture outside your home."
- Sid Caspersen, NJ's director of the office of counter-terrorism
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