Note: MPEG Version 2.5 was added lately to the MPEG 2 standard.
It is an extension used for very low bitrate files, allowing
the use of lower sampling frequencies. If your decoder does
not support this extension, it is recommended for you to use
12 bits for synchronization instead of 11 bits.

NOTES: All values are in kbps
V1 – MPEG Version 1
V2 – MPEG Version 2 and Version 2.5
L1 – Layer I
L2 – Layer II
L3 – Layer III

“free” means free format. The free bitrate must
remain constant, an must be lower than the maximum allowed
bitrate. Decoders are not required to support decoding of
free bitrate streams.
“bad” means that the value is unallowed.

MPEG files may feature variable bitrate (VBR). Each frame
may then be created with a different bitrate. It may be used
in all layers. Layer III decoders must support this method.
Layer I & II decoders may support it.

For Layer II there are some combinations of bitrate and mode
which are not allowed. Here is a list of allowed combinations.

Padding is used to exactly fit the bitrate.As an example: 128kbps
44.1kHz layer II uses a lot of 418 bytes and some of 417 bytes
long frames to get the exact 128k bitrate. For Layer I slot
is 32 bits long, for Layer II and Layer III slot is 8 bits long.

Note: Dual channel files are made of two independant mono channel.
Each one uses exactly half the bitrate of the file. Most decoders
output them as stereo, but it might not always be the case.
    One example of use would be some speech
in two different languages carried in the same bitstream, and
then an appropriate decoder would decode only the choosen language.

Mode extension is used to join informations that are of no
use for stereo effect, thus reducing needed bits. These bits
are dynamically determined by an encoder in Joint stereo mode,
and Joint Stereo can be changed from one frame to another,
or even switched on or off.

Complete frequency range of MPEG file is divided in subbands
There are 32 subbands. For Layer I & II these two bits determine
frequency range (bands) where intensity stereo is applied.
For Layer III these two bits determine which type of joint
stereo is used (intensity stereo or m/s stereo). Frequency
range is determined within decompression algorithm.

The copyright has the same meaning as the copyright bit on CDs
and DAT tapes, i.e. telling that it is illegal to copy the contents
if the bit is set.

The original bit indicates, if it is set, that the frame is
located on its original media.

The emphasis indication is here to tell the decoder that the
file must be de-emphasized, ie the decoder must ‘re-equalize’
the sound after a Dolby-like noise supression. It is rarely
used.