AC3 Header
This is just a quick reference to the frame header of an AC3 stream. There is no intention to fully explain AC3, for that you should read standard A/52a, available from ATSC.org (Advanced Television Systems Committee).
An AC3 frame has the following structure:
syncframe()
{
syncinfo();
bsi() ;
for(blk=0, blk<6, blk++)
{
audblk();
}
auxdata();
errorcheck() ;
} // end of syncframe
Syncinfo
| byte 0 | byte 1 | byte 2 | byte 3 | byte 4 | |||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Syncword - 0B77 | CRC1 | fscod | frmsizcod | ||||||||||||||||||||||||||||||||||||
CRC1 applies to the first 5/8 of the frame
fscod sampling rate values
| value | sampling rate |
|---|---|
| 00 | 48K |
| 01 | 44.1K |
| 10 | 32K |
| 11 | reserved |
frmsizcod frame size code
| value | nominal bit rate | 16-bit words per syncframe | ||
|---|---|---|---|---|
| Fs=32K | Fs=44.1K | Fs=48K | ||
| 000000 | 32Kbps | 96 | 69 | 64 |
| 000001 | 32Kbps | 96 | 70 | 64 |
| 000010 | 40Kbps | 120 | 87 | 80 |
| 000011 | 40Kbps | 120 | 88 | 80 |
| 000100 | 48Kbps | 144 | 104 | 96 |
| 000101 | 48Kbps | 144 | 105 | 96 |
| 000110 | 56Kbps | 168 | 121 | 112 |
| 000111 | 56Kbps | 168 | 122 | 112 |
| 001000 | 64Kbps | 192 | 139 | 128 |
| 001001 | 64Kbps | 192 | 140 | 128 |
| 001010 | 80Kbps | 240 | 174 | 160 |
| 001011 | 80Kbps | 240 | 175 | 160 |
| 001100 | 96Kbps | 288 | 208 | 192 |
| 001101 | 96Kbps | 288 | 209 | 192 |
| 001110 | 112Kbps | 336 | 243 | 224 |
| 001111 | 112Kbps | 336 | 244 | 224 |
| 010000 | 128Kbps | 384 | 278 | 256 |
| 010001 | 128Kbps | 384 | 279 | 256 |
| 010010 | 160Kbps | 480 | 348 | 320 |
| 010011 | 160Kbps | 480 | 349 | 320 |
| 010100 | 192Kbps | 576 | 417 | 384 |
| 010101 | 192Kbps | 576 | 418 | 384 |
| 010110 | 224Kbps | 672 | 487 | 448 |
| 010111 | 224Kbps | 672 | 488 | 448 |
| 011000 | 256Kbps | 768 | 557 | 512 |
| 011001 | 256Kbps | 768 | 558 | 512 |
| 011010 | 320Kbps | 960 | 696 | 640 |
| 011011 | 320Kbps | 960 | 697 | 640 |
| 011100 | 384Kbps | 1152 | 835 | 768 |
| 011101 | 384Kbps | 1152 | 836 | 768 |
| 011110 | 448Kbps | 1344 | 975 | 896 |
| 011111 | 448Kbps | 1344 | 976 | 896 |
| 100000 | 512Kbps | 1536 | 1114 | 1024 |
| 100001 | 512Kbps | 1536 | 1115 | 1024 |
| 100010 | 576Kbps | 1728 | 1253 | 1152 |
| 100011 | 576Kbps | 1728 | 1254 | 1152 |
| 100100 | 640Kbps | 1920 | 1393 | 1280 |
| 100101 | 640Kbps | 1920 | 1394 | 1280 |
| 100110 to 111111 | reserved | |||
BSI
The BSI contains many conditionally included fields, so there is no fixed alignment.
| standard (A52) variable | bit length | alternate (A52a) variable |
|---|---|---|
| bsid = 8 in this version | 5 | bsid = 6 in this version |
| bsmod | 3 | no change |
| acmod | 3 | : |
| if((acmod & 0x1) && (acmod != 0x1)) /* 3 front channels */ {cmixlev } | 2 | : |
| if(acmod & 0x4) /* if a surround channel exists */ {surmixlev } | 2 | : |
| if(acmod == 0x2) /* if in 2/0 mode */ {dsurmod } | 2 | : |
| lfeon | 1 | : |
| dialnorm | 5 | : |
| compre | 1 | : |
| if(compre) {compr } | 8 | : |
| lancode | 1 | : |
| if(langcode) {langcod } | 8 | : |
| audioprodie | 1 | : |
| if(audioprodie) {mixlevel } | 5 | : |
| if(audioprodie) {roomtyp } | 2 | : |
| if(acmod == 0) /* 1+1 mode, so some items need a second value */ | : | |
| { | : | |
| dialnorm2 | 5 | : |
| compr2e | 1 | : |
| if(compr2e) {compr2 } | 8 | : |
| lancod2e | 1 | : |
| if(langcod2e) {langcod2 } | 8 | : |
| audioprodi2e | 1 | : |
| if(audioprodi2e) {mixlevel2 } | 5 | : |
| if(audioprodi2e) {roomtyp2 } | 2 | : |
| } | : | |
| copyrightb | 1 | : |
| origbs | 1 | no change |
| timecod1e | 1 | xbsi1e |
| if(timecod1e) {timecod1 } | 14 | if(xbsi1e) { |
| dmixmod (2 bits) | ||
| ltrtcmixlev (3 bits) | ||
| ltrtsurmixlev (3 bits) | ||
| lorocmixlev (3 bits) | ||
| lorosurmixlev (3 bits) | ||
| } | ||
| timecod2e | 1 | xbsi2e |
| if(timecod2e) {timecod2 } | 14 | if(xbsi2e) { |
| dsurexmod (2 bits) | ||
| dheadphonmod (2 bits) | ||
| adconvtyp (1 bit) | ||
| xbsi2 (8 bits - reserved for future expansion) | ||
| encinfo (1 bit - reserved for encoder use) | ||
| } | ||
| addbsie | 1 | no change |
| if(addbsie) {addbsil } | 6 | : |
| if(addbsie) {addbsi } | 8*addbsil | no change |
bsmod bitstream mode
| bsmod | acmod | Type of Service |
|---|---|---|
| 000 | any | main audio service: complete main (CM) |
| 001 | any | main audio service: music and effects (ME) |
| 010 | any | associated service: visually impaired (VI) |
| 011 | any | associated service: hearing impaired (HI) |
| 100 | any | associated service: dialog (D) |
| 101 | any | associated service: commentary (C) |
| 110 | any | associated service: emergency (E) |
| 111 | 001 | associated service: voice over (VO) |
| 111 | 010 to 111 | main audio service: karaoke |
acmod audio coding mode
| acmod | audio coding mode | nfchans | Channel Array Order |
|---|---|---|---|
| 000 | 1+1 | 2 | Ch1, Ch2 |
| 001 | 1/0 | 1 | C |
| 010 | 2/0 | 2 | L, R |
| 011 | 3/0 | 3 | L, C, R |
| 100 | 2/1 | 3 | L, R, S |
| 101 | 3/1 | 4 | L, C, R, S |
| 110 | 2/2 | 4 | L, R, SL, SR |
| 111 | 3/2 | 5 | L, C, R, SL, SR |
cmixlev - center mix level (acmod 3, 5, 7)
| value | center mix level |
|---|---|
| 00 | 0.707 (-3.0 dB) |
| 01 | 0.595 (-4.5 dB) |
| 10 | 0.500 (-6.0 dB) |
| 11 | reserved |
surmixlev - surround mix level (acmod 4, 5, 6, 7)
| value | surround mix level |
|---|---|
| 00 | 0.707 (-3.0 dB) |
| 01 | 0.500 (-6.0 dB) |
| 10 | 0 |
| 11 | reserved |
dsurmod - dolby surround mode (acmod 2)
| value | indication |
|---|---|
| 00 | not indicated |
| 01 | Not Dolby Surround encoded |
| 10 | Dolby Surround encoded |
| 11 | reserved |
lfeon - Low frequency effects (LFE) on, 0=off, 1=on
dialnorm - Dialog normalization, average level, in -dB, of dialog channel with respect to digital 100%. The value of 0 is reserved, and decoders should use -31 dB
compre and compr - see section 7.7.2 of A52A
langcode and langcod - reserved
audproie - audio production information exists
mixlevel - mixing level, the absolute acoustic sound pressure level of an individual channel during the final mixing session. The peak mixing level is 80 + the 5-bit value dB SPL.
roomtyp - The type and calibration of the mixing room used for the final mixing session.
| roomtyp | Type of Mixing Room |
|---|---|
| 00 | not indicated |
| 01 | large room, X curve monitor |
| 10 | small room, flat monitor |
| 11 | reserved |
dialnorm2, compr2e, compr2, langcod2e, langcod2, audprodi2e, mixlevel2, roomtyp2 - same as above, but for Ch2 of 1+1 mode.
copyrightb - copyright bit, 0=not protected by copyright, 1=protected by copyright.
origbs - original bit stream, 1=original, 0=copy
timecod1e, timecod1 - most significant 14 bits of time code, as follows:
| 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| hours (0-23) | minutes (0-59) | seconds/8 (0-7) | |||||||||||
timecod2e, timecod2 - least significant 14 bits of time code, as follows:
| 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| seconds mod 8 (0-7) | frames (0-29) | 1/64 frames (0-63) | |||||||||||
addbsie - flag for presence of additional bit stream information.
addbsil - length in bytes of additional bit stream information.
addbsi - additional bit stream information, up to 64 bytes.
Fields of alternate (A52a) BSI
dmixmod - preferred stereo downmix mode
| dmixmod | Indication |
|---|---|
| 00 | not indicated |
| 01 | Lt/Rt downmix preferred |
| 10 | Lo/Ro downmix preferred |
| 11 | reserved |
ltrtcmixlev - Lt/Rt Center Mix Level
ltrtsurmixlev - Lt/Rt Surround Mix Level
lorocmixlev - Lo/Ro Center Mix Level
lorosurmixlev - Lo/Ro Surround Mix Level
| value | level |
|---|---|
| 000 | 1.414 (+3.0 dB) |
| 001 | 1.189 (+1.5 dB) |
| 010 | 1.000 (0.0 dB) |
| 011 | 0.841 (-1.5 dB) |
| 100 | 0.707 (-3.0 dB) |
| 101 | 0.595 (-4.5 dB) |
| 110 | 0.500 (-6.0 dB) |
| 111 | 0.000 (-inf dB) |
dsurexmod - Dolby Surround EX Mode
| dsurexmod | Indication |
|---|---|
| 00 | not indicated |
| 01 | Not Dolby Surround EX encoded |
| 10 | Dolby Surround EX encoded |
| 11 | reserved |
dheadphonmod - Dolby Headphone Mode
| dheadphonmod | Indication |
|---|---|
| 00 | not indicated |
| 01 | Not Dolby Headphone encoded |
| 10 | Dolby Headphone encoded |
| 11 | reserved |
adconvtyp - A/D Converter zType
| adconvtyp | Indication |
|---|---|
| 0 | Standard |
| 1 | HDCD |
Forward: http://rmworkshop.com/dvd_info/related_info/ac3hdr.html