2008-04-14 04:09:29 +00:00
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===
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AC3
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===
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Specification taken from a_52a.pdf
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I *think* that looking at the start of `syncinfo` should give me all the
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information I need to (a) determine if this is AC3 and (b) determine if it is
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"main" audio.
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::
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syncframe: -- synchronization frame
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syncinfo: -- synchronization info
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16: syncword -- 0x0B77
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16: crc1
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2: fscod -- [1]
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6: frmsizecod
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bsi: -- bit stream info
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5: bsid -- 8 in *this* version of the standard [2]
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3: bsmod -- [3]
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3: acmod -- which main service channels are in use [4]
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2: cmixlev -- center mix level (if 3 front channels)
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2: surmixlev -- surround mix level (if surround sound channel)
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2: dsurmod -- dolby surround mode (if in 2/0 mode) 2=surround
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1: lfeon
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5: dialnorm
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1: compre
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<etc>
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for n in range(6):
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audblk: -- coded audio block
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(256 new audio samples per channel)
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auxdata:
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errorcheck: -- CRC for whole syncframe
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1: crcsv
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16: crc2
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[1] fscod: sampling rate in kHz:
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00 = 48
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01 = 44.1
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10 = 32
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11 = reserved
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[2] bsid: values less than 8 are for subsets of the standard. If the software
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can decode data with bsid=8, it can also decode data with bsid<8.
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[3] bit stream mode:
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(the "full" column gives the full service flag for use in DVB's
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AC-3_descriptor:AC-3_type [5])
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bsmod acmod type of service full?
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000 any main audio service: complete main (CM) 1
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001 any main audio service: music & effects (ME) 0
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010 any associated service: visually impaired (VI) X
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011 any associated service: hearing impaired (HI) X
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100 any associated service: dialogue (D) 0
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101 any associated service: commentary (C) X
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110 any associated service: emergency (E) 1
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111 001 associated service: voice over (VO) 0
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111 010-111 main audio service: karaoke 1
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[4] audio coding mode:
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bit meaning
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0 center channel in use
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1 <too complex to summarise here>
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2 surround sound channels in use
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audio full
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coding bandwidth
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acmod mode chans order
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0 1+1 2 Ch1,Ch2 ("dual mono")
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1 1/0 1 C
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2 2/0 2 L,R
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3 3/0 3 L,C,R
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4 2/1 3 L,R,S
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5 3/1 4 L,C,R,S
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6 2/2 4 L,R,SL,SR
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7 3/2 5 L,C,R,SL,SR
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AC3 in TS
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=========
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AC sync frame contains 1536 audio samples. Its duration is::
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48kHz -> 32ms
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44.1kHz -> approx 34.83ms
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32 kHz -> 48ms
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For ATSC::
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stream_type 0x81
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stream_id 0xBD (private_stream_1) in PES header
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registration_descriptor: (in PMT)
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8: descriptor_tag -- 0x05
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8: descriptor_length -- 0x04
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32: format_identifier -- 0x41432D33 ("AC-3")
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audio_stream_descriptor: (in PSI)
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8: descriptor_tag -- 0x81
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8: descriptor_length -- <number of bytes after this field>
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3: sample_rate_code
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5: bsid -- same as bsid above [2]
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6: bit_rate_code
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2: surround_mode
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3: bsmod -- same as bsmod above [3]
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4: num_channels
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1: full_svc
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---------------- further optional fields, depending on the above
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ISO_639_language_code descriptor allows a stream to be tagged with
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the 24-bit ISO 639 language code.
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For DVB::
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stream_type 0x06 (private_data)
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stream_id 0xBD (private_stream_1) in PES header
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AC-3_descriptor: (in PSI and PMT)
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8: descriptor_tag -- 0x6A
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8: descriptor_length -- <number of bytes after this field>
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1: AC-3_type_flag
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1: bsid_flag
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1: mainid_flag
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1: asvc_flag
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4: <reserved bits, set to 0>
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---------------- further fields present if their flag is set
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8: AC-3_type -- [5]
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8: bsid -- same as bsid above [2]
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8: mainid -- 0-7 main audio service id
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8: asvc -- associate service with main service
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---------------- further info to the number of bytes indicated
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n*8: additional info
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[5] I *think* this is interpreted as follows:
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Bits Meaning
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0-2 0: mono
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1: 1+1
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2: 2 channel (stereo)
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3: 2 channel Dolby surround encoded (stereo)
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4: Multichannel audio (>2 channels)
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Other values reserved
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3-5 same as [3], bit stream mode
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6 0: Use channel in combination with another
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1: Full service channel, use alone
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7 Must be 0
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.. ***** BEGIN LICENSE BLOCK *****
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License
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-------
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Version: MPL 1.1
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The contents of this file are subject to the Mozilla Public License Version
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1.1 (the "License"); you may not use this file except in compliance with
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the License. You may obtain a copy of the License at
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http://www.mozilla.org/MPL/
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Software distributed under the License is distributed on an "AS IS" basis,
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WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
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for the specific language governing rights and limitations under the
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License.
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The Original Code is the MPEG TS, PS and ES tools.
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The Initial Developer of the Original Code is Amino Communications Ltd.
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Portions created by the Initial Developer are Copyright |copy| 2008
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the Initial Developer. All Rights Reserved.
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.. |copy| unicode:: 0xA9 .. copyright sign
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Contributor(s):
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Amino Communications Ltd, Swavesey, Cambridge UK
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.. ***** END LICENSE BLOCK *****
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2008-06-14 16:42:19 +00:00
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.. -------------------------------------------------------------------------------
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.. vim: set filetype=rst expandtab shiftwidth=2:
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