Chaque Trame (frame) contient les éléments suivants :
Header
|
Nb de bits
|
Nom
|
Détail
|
| 12 |
Synchro de trame |
0xFFF |
| 1 |
ID |
1=MPEG, 0=Reserved |
| 2 |
Layer |
11=Layer I, 10=Layer II, 01=Layer III, 00=Reserved |
| 1 |
Protection bit |
Equals '1' if no redundancy has been added, '0' if
redundancy has been added |
| 4 |
Bit Rate index |
| bit_rate_index |
Layer I |
Layer II |
Layer III |
| '0000' |
free format |
free format |
free format |
| '0001' |
32 kbit/s |
32 kbit/s |
32 kbit/s |
| '0010' |
64 kbit/s |
48 kbit/s |
40 kbit/s |
| '0011' |
96 kbit/s |
56 kbit/s |
48 kbit/s |
| '0100' |
128 kbit/s |
64 kbit/s |
56 kbit/s |
| '0101' |
160 kbit/s |
80 kbit/s |
64 kbit/s |
| '0110' |
192 kbit/s |
96 kbit/s |
80 kbit/s |
| '0111' |
224 kbit/s |
112 kbit/s |
96 kbit/s |
| '1000' |
256 kbit/s |
128 kbit/s |
112 kbit/s |
| '1001' |
288 kbit/s |
160 kbit/s |
128 kbit/s |
| '1010' |
320 kbit/s |
192 kbit/s |
160 kbit/s |
| '1011' |
352 kbit/s |
224 kbit/s |
192 kbit/s |
| '1100' |
384 kbit/s |
256 kbit/s |
224 kbit/s |
| '1101' |
416 kbit/s |
320 kbit/s |
256 kbit/s |
| '1110' |
448 kbit/s |
384 kbit/s |
320 kbit/s |
|
| 2 |
Sampling Frequency |
'00' 44.1 kHz ,'01' 48 kHz, '10' 32 kHz,
'11' reserved |
| 1 |
Padding byte |
if this bit equals '1' the frame contains an additional
slot to adjust the mean bitrate to the sampling frequency, otherwise
this bit will be '0'. Padding is only necessary with a sampling
frequency of 44.1kHz |
| 1 |
Private |
Non utilisé |
| 2 |
Mode |
'00' stereo,'01' joint_stereo (intensity_stereo
and/or ms_stereo) ,'10' dual_channel,'11' single_channel |
| 2 |
Mode Extension |
Layer I et II
'00' subbands 4-31 in intensity_stereo, bound==4,'01' subbands 8-31
in intensity_stereo, bound==8,'10' subbands 12-31 in intensity_stereo,
bound==12,'11' subbands 16-31 in intensity_stereo, bound==16
Layer III
bit0=ms_stereo(1 on; 0 off), bit 1=intensity_stereo(1 on, 0
off) |
| 1 |
CopyRight |
if this bit equals '0' there is no copyright on the
coded bitstream, '1' means copyright protected |
| 1 |
Original/Home |
this bit equals '0' if the bitstream is a copy, '1'
if it is an original |
| 2 |
Emphasis |
'00' no emphasis, '01' 50/15 microsec.
emphasis, '10' reserved, '11' CCITT J.17 |
| |
|
|
Error Check
|
Nb de bits
|
Nom
|
Détail
|
| 16 |
CRC Check |
Présent UNIQUEMENT si protection bit
dans le header est à 0 |
Audio Data
Dépend du Layer défini dans Layer dans le header
Layer I
Mode=Single Channel
|
Nb de bits
|
Nom
|
Détail
|
| 4*32 |
Allocation |
Allocation des 32 sous bandes
for (sb=0; sb<32; sb++)
allocation[sb]
0 si aucune donnée,
nb_bit-1 sinon, 0xF : invalid |
| 6*32 max |
Scale Factor |
Un scale factor est encodé uniquement
si allocation[i]>0
for (sb=0; sb<32; sb++)
if (allocation[sb]!=0)
scalefactor[sb] |
| [2..15]*32*12 |
Sample |
for (s=0; s<12;
s++)
for (sb=0; sb<32; sb++)
if (allocation[sb]!=0)
sample[sb][s] |
|