===
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

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