23 Before? (MPEG-Audio)
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
23 Before? (MPEG-Audio)
Yes, if there is a significant (30 - 40dB ) shift in level.
The reason is believed to be that the brain needs some
processing time. Premasking is only about 2 to 5 ms. The
postmasking can be up till 100ms.
Other bit-reduction techniques involve considering tonal and
non-tonal components of the sound. For a stereo signal you
may have a lot of redundancy between channels. All MPEG
Layers may exploit these stereo effects by using a "joint-
stereo" mode, with a most flexible approach for Layer-3.
Furthermore, only Layer-3 further reduces the redundancy
by applying huffmann coding.
What are the downside? (MPEG-Audio)
The coder calculates masking effects by an iterative process
until it runs out of time. It is up to the implementor to
spend bits in the least obtrusive fashion.
For Layer 2 and Layer 3, the encoder works on 24 ms of sound
(with 1152 sample, and fs = 48 kHz) at a time. For some
material, the time-window can be a problem. This is
normally in a situation with transients where there are large
differences in sound level over the 24 ms. The masking is
calculated on the strongest sound and the weak parts will
drown in quantization noise. This is perceived as a "noise-
echo" by the ear. Layer 3 addresses this problem
specifically by using a smaller analysis window (4 ms), if
the encoder encounters an "attack" situation.
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