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===
AC3
===
Specification taken from a_52a.pdf
I *think* that looking at the start of `syncinfo` should give me all the
information I need to (a) determine if this is AC3 and (b) determine if it is
"main" audio.
::
syncframe: -- synchronization frame
syncinfo: -- synchronization info
16: syncword -- 0x0B77
16: crc1
2: fscod -- [1]
6: frmsizecod
bsi: -- bit stream info
5: bsid -- 8 in *this* version of the standard [2]
3: bsmod -- [3]
3: acmod -- which main service channels are in use [4]
2: cmixlev -- center mix level (if 3 front channels)
2: surmixlev -- surround mix level (if surround sound channel)
2: dsurmod -- dolby surround mode (if in 2/0 mode) 2=surround
1: lfeon
5: dialnorm
1: compre
<etc>
for n in range(6):
audblk: -- coded audio block
(256 new audio samples per channel)
auxdata:
errorcheck: -- CRC for whole syncframe
1: crcsv
16: crc2
[1] fscod: sampling rate in kHz:
00 = 48
01 = 44.1
10 = 32
11 = reserved
[2] bsid: values less than 8 are for subsets of the standard. If the software
can decode data with bsid=8, it can also decode data with bsid<8.
[3] bit stream mode:
(the "full" column gives the full service flag for use in DVB's
AC-3_descriptor:AC-3_type [5])
bsmod acmod type of service full?
000 any main audio service: complete main (CM) 1
001 any main audio service: music & effects (ME) 0
010 any associated service: visually impaired (VI) X
011 any associated service: hearing impaired (HI) X
100 any associated service: dialogue (D) 0
101 any associated service: commentary (C) X
110 any associated service: emergency (E) 1
111 001 associated service: voice over (VO) 0
111 010-111 main audio service: karaoke 1
[4] audio coding mode:
bit meaning
0 center channel in use
1 <too complex to summarise here>
2 surround sound channels in use
audio full
coding bandwidth
acmod mode chans order
0 1+1 2 Ch1,Ch2 ("dual mono")
1 1/0 1 C
2 2/0 2 L,R
3 3/0 3 L,C,R
4 2/1 3 L,R,S
5 3/1 4 L,C,R,S
6 2/2 4 L,R,SL,SR
7 3/2 5 L,C,R,SL,SR
AC3 in TS
=========
AC sync frame contains 1536 audio samples. Its duration is::
48kHz -> 32ms
44.1kHz -> approx 34.83ms
32 kHz -> 48ms
For ATSC::
stream_type 0x81
stream_id 0xBD (private_stream_1) in PES header
registration_descriptor: (in PMT)
8: descriptor_tag -- 0x05
8: descriptor_length -- 0x04
32: format_identifier -- 0x41432D33 ("AC-3")
audio_stream_descriptor: (in PSI)
8: descriptor_tag -- 0x81
8: descriptor_length -- <number of bytes after this field>
3: sample_rate_code
5: bsid -- same as bsid above [2]
6: bit_rate_code
2: surround_mode
3: bsmod -- same as bsmod above [3]
4: num_channels
1: full_svc
---------------- further optional fields, depending on the above
ISO_639_language_code descriptor allows a stream to be tagged with
the 24-bit ISO 639 language code.
For DVB::
stream_type 0x06 (private_data)
stream_id 0xBD (private_stream_1) in PES header
AC-3_descriptor: (in PSI and PMT)
8: descriptor_tag -- 0x6A
8: descriptor_length -- <number of bytes after this field>
1: AC-3_type_flag
1: bsid_flag
1: mainid_flag
1: asvc_flag
4: <reserved bits, set to 0>
---------------- further fields present if their flag is set
8: AC-3_type -- [5]
8: bsid -- same as bsid above [2]
8: mainid -- 0-7 main audio service id
8: asvc -- associate service with main service
---------------- further info to the number of bytes indicated
n*8: additional info
[5] I *think* this is interpreted as follows:
Bits Meaning
0-2 0: mono
1: 1+1
2: 2 channel (stereo)
3: 2 channel Dolby surround encoded (stereo)
4: Multichannel audio (>2 channels)
Other values reserved
3-5 same as [3], bit stream mode
6 0: Use channel in combination with another
1: Full service channel, use alone
7 Must be 0
.. ***** BEGIN LICENSE BLOCK *****
License
-------
Version: MPL 1.1
The contents of this file are subject to the Mozilla Public License Version
1.1 (the "License"); you may not use this file except in compliance with
the License. You may obtain a copy of the License at
http://www.mozilla.org/MPL/
Software distributed under the License is distributed on an "AS IS" basis,
WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
for the specific language governing rights and limitations under the
License.
The Original Code is the MPEG TS, PS and ES tools.
The Initial Developer of the Original Code is Amino Communications Ltd.
Portions created by the Initial Developer are Copyright |copy| 2008
the Initial Developer. All Rights Reserved.
.. |copy| unicode:: 0xA9 .. copyright sign
Contributor(s):
Amino Communications Ltd, Swavesey, Cambridge UK
.. ***** END LICENSE BLOCK *****
.. -------------------------------------------------------------------------------
.. vim: set filetype=rst expandtab shiftwidth=2:
|