| Glossary |
Description |
Bitrate |
| |
Number of bits per second. The rate
at which the compressed bitstream is delivered
from the storage medium to the input of a decoder.
|
|
Bit Reservoir |
| |
Often, some passages of a musical
piece can not be coded to a given rate without
altering the musical quality. The MP3 then uses
then a short reservoir of bytes that acts as a
buffer by using capacity from passages that can
be coded to an inferior rate in the given flow.
|
|
Channel |
| |
A sequence of data representing
an audio signal intended to be reproduced at one
listening position. |
|
Codec |
| |
Acronym for compressor/decompressor.
|
|
db |
| |
Decibel
N db = 20 lg (Pn/P0)
N - number of db.
Pn - Level of sound pressure in pascals.
P0 = 2*105 pascals. |
|
JS |
| |
In the case of a stereophonic signal,
the MP3 format can then use a few more tools,
referred as Joint Stereo (JS) coding, to further
shrink the compressed file size.
In many mid-range Hi-fi sets , there is a unique
subwoofer. However you usually do not have the
feeling that the sound comes from this boomer,
but rather from satellite speakers. Indeed for
very low and very high frequencies, the human
ear is no longer able to locate the special origin
of sounds with full accuracy. The mp3 format can
therefore (optionally) revert to such a trick
by using what is called Intensity Stereo (IS).
Some frequencies are then recorded as a monophonic
signal followed by a few additional information
in order to restore a minimum of specialization.
The second joint stereo tool is called Mid/Side
(M/S) stereo. When the left and the right channels
are quite similar, then a middle (L+R) and a side
(L-R) channels are encoded instead of left and
right. This allows to reduce the final file size
by using less bits for the side channel. During
playback, the MP3 decoder will reconstruct the
left and right channels. |
|
MPEG |
| |
MPEG is the name of a working group
established under the joint direction of the International
Standards Organization/International Electrotechnical
Commission (ISO/IEC), that has for goal to create
standards for the digital video and the audiophonic
compression. More precisely, MPEG defines the
syntax of audio and video format needing low data
rates, as well as operations to be undertaken
by decoders.
Algorithms used by encoders are not defined by
MPEG. This authorizes continual encoder improvements
as well as their adaptation to specific applications,
without necessitating any redefinition of the
arrangement of data. Jointly to the audio and
video coding, MPEG defines also methods aiming
to test the conformity to standards of formats
and decoder, and publishes technical reports. |
|
MPEG Stages |
| |
The generations of standards defined
by the MPEG working group are referred to as stages.
These stages are normally noted in Arabic figures
(MPEG-1, MPEG-2, MPEG-4, MPEG-7 and MPEG-21). |
|
MPEG Audio Layers |
| |
MPEG-1 and MPEG-2 both have a three-layer
structure, each of which represents a family of
coding algorithms. These layers are noted in the
standards using Roman figures (Layer I, Layer
II, and Layer III). Note that for Internet audio
applications MPEG-1 Layer III audio has become
known as MP3. |
|
Profile |
| |
A defined sub-set of the syntax
of a specification. |
|
Psychoacoustic
Model |
| |
A mathematical model of the masking
behavior of the human auditory system. |
|
VBR |
| |
VBR means Variable Bitrate. Some
MP3 files can have variable bitrate, i.e. bitrate
of file can be changed during record and playback.
Lame codec V 3.91 supports VBR.
Operation where the bitrate varies with time during
the decoding of a compressed bitstream.
Although variable bit rate is acceptable for plain
linear playback, one important consideration not
to use variable bit rate is that reasonably quick
random access becomes nearly impossible.
There is no table of contents or index in MPEG.
The only tool the play back system has for approximating
the correct byte position is the requested play
back time stamp and the bit rate of the MPEG
stream. MPEG streams do not encode their play
back time. To approximate an intermediate position
in a variable bit rate stream, the play back
system must grope around near the end of the
stream to calculate the playback time, and assume
the stream is approximately constant bit rate.
The groping around for the correct position
can take several seconds.
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