AC3 (Dolby Digital) Headers

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/52b, 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

value	sampling rate
00	48K
01	44.1K
10	32K
11	reserved

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.

13	12	11	10	9	8	7	6	5	4	3	2	1	0
hours (0-23)	minutes (0-59)	seconds/8 (0-7)

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)

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

DVD-Video info home

roomtyp	Type of Mixing Room
00	not indicated
01	large room, X curve monitor
10	small room, flat monitor
11	reserved