Perceptually, people have
different opinions about sound quality. When CDs were first released,
some thought they lacked the warmth and richness of analog LPs.
Furthermore, different genres of music and sound will cause different
perceptions, even when using the exact same method of recording/encoding.
The Centre for Communication
Interface Research (CCIR)
developed an "Impairment" scale for measuring qualitative
listening. Three international listening tests were given during
the MPEG I development, and listeners were asked to rate on the
following scale:
5.0 = transparent (same
as original signal)
4.0 = perceptible, but not annoying
3.0 = slightly annoying
2.0 = annoying
1.0 = very annoying
At 128 kbps (stereo),
MP3 scored from 3.6 to 3.8 (compare to MPEG I layer 2 which scored
from 2.1 to 2.6). An enterprising programmer, Roberto Amorim, created
a series of public listening
tests comparing different types of encoders and came up with
similar results.
For your own comparison,
consider the following:
Original (2.7MB
wav)
MP3
Encoded (500KB
mp3)
All sorts of errors can
creep into the encoding process including noise, distortion, and
roughness. Check out the page on artifacts for
more information.