Error in streamed MPEG4 handling - Movie.pm or Audio::Scan error ?

**bpa** · 2022-01-02, 10:03

Looking at feasibility of an Accuradio plugins and was checking the audio streams.
I've having a problem playing some MPEG-4 streaming URLs. Tests are on a Ubuntu system LMS 8.3 (2Jan)

It looks like a bug in either LMS MPEG-4 Handling of InitialAudioBlock or in Audio::Scan

Example URL.
https://d1qg6pckcqcdk0.cloudfront.ne...atormundii.m4a

When downloaded as a file, the file plays OK on both an Receiver (i.e. faad MP4->Flc transcode) and a DAC32 (i.e. MP4->aac)

When played as an URL
- no output on Receiver - faad (-q removed) produces an error - Duration, speed and channel all are wrong.

Code:

Streamed input format  samplerate 22050 channels 0.
- file info:
ADTS, 1.000 sec, 16 kbps, 44100 Hz

Error: Bitstream value not allowed by specification

- DAC32 error following error is logged

Code:

[22-01-02 16:58:25.3427] Slim::Player::Squeezebox2::statHandler (156) Error: 7c:9e:bd:2b:52:94: Decoder does not support file format, code 0

On examination an extra 4 bytes of zeroes had been inserted by LMS into the created ADTS audio stream from MPEG-4 file.
It seems there is padding in the m4a file and so the value of "seek offset" to start of audio and length of MPEG4 header are not the same.

In Movie::findFrameBoundaries - Audio ::Scan is called on initial Audio block of m4a stream.
Data:

ump::dump of variable $info (returned from Audio::Scan)

Code:

[22-01-02 16:38:20.1771] Slim::Formats::Movie::findFrameBoundaries (198)  Dump of info from AudioScan do {
  require MIME::Base64;
  {
    audio_offset      => 22_092,
    audio_size        => 880_370,
    avg_bitrate       => 62_768,
    compatible_brands => ["M4A ", "mp42", "isom", "\0\0\0\0"],
    dlna_profile      => "AAC_ISO_192",
    file_size         => 22_116,
    major_brand       => "mp42",
    minor_version     => 0,
    mv_timescale      => 90_000,
    samplerate        => 22_050,
    seek_header       => MIME::Base64::decode("
    .
    .(deleted)
    .
    "),
    seek_offset       => 22_108,
    song_length_ms    => 224_351,
    tracks            => [
                           {
                             audio_object_type => 2,
                             audio_type        => 64,
                             bits_per_sample   => 16,
                             channels          => 1,
                             duration          => 224_351,
                             encoding          => "mp4a",
                             handler_name      => "Sound Media Handler",
                             handler_type      => "soun",
                             id                => 1,
                             max_bitrate       => 37_608,
                             samplerate        => 22_050,
                           },
                           { duration => 224_351, id => 1 },
                         ],
  };

Audio::Scan - the length of $info->{seek_header} is 22,100
Audio::Scan - audio_offset is at 22,092 but seek_offset to start of audio is 22,108

LMS seems to be streaming from end of MPEG4 header when it should be streaming from seek_offset which is start of audio.
Not sure if the error is in Audio::Scan or Movie.pm.

I'll try to come up with a solution but any suggestions welcome. I need to check Audio::Scan documentation to see if the returned values are correct and expected.

**philippe_44** · 2022-01-02, 11:06

Originally Posted by bpa

Looking at feasibility of an Accuradio plugins and was checking the audio streams.
I've having a problem playing some MPEG-4 streaming URLs. Tests are on a Ubuntu system LMS 8.3 (2Jan)

It looks like a bug in either LMS MPEG-4 Handling of InitialAudioBlock or in Audio::Scan

Example URL.
https://d1qg6pckcqcdk0.cloudfront.ne...atormundii.m4a

When downloaded as a file, the file plays OK on both an Receiver (i.e. faad MP4->Flc transcode) and a DAC32 (i.e. MP4->aac)

When played as an URL
- no output on Receiver - faad (-q removed) produces an error - Duration, speed and channel all are wrong.

Code:

Streamed input format  samplerate 22050 channels 0.
- file info:
ADTS, 1.000 sec, 16 kbps, 44100 Hz

Error: Bitstream value not allowed by specification

- DAC32 error following error is logged

Code:

[22-01-02 16:58:25.3427] Slim::Player::Squeezebox2::statHandler (156) Error: 7c:9e:bd:2b:52:94: Decoder does not support file format, code 0

On examination an extra 4 bytes of zeroes had been inserted by LMS into the created ADTS audio stream from MPEG-4 file.
It seems there is padding in the m4a file and so the value of "seek offset" to start of audio and length of MPEG4 header are not the same.

In Movie::findFrameBoundaries - Audio ::Scan is called on initial Audio block of m4a stream.
Data:

ump::dump of variable $info (returned from Audio::Scan)

Code:

[22-01-02 16:38:20.1771] Slim::Formats::Movie::findFrameBoundaries (198)  Dump of info from AudioScan do {
  require MIME::Base64;
  {
    audio_offset      =&gt; 22_092,
    audio_size        =&gt; 880_370,
    avg_bitrate       =&gt; 62_768,
    compatible_brands =&gt; ["M4A ", "mp42", "isom", "\0\0\0\0"],
    dlna_profile      =&gt; "AAC_ISO_192",
    file_size         =&gt; 22_116,
    major_brand       =&gt; "mp42",
    minor_version     =&gt; 0,
    mv_timescale      =&gt; 90_000,
    samplerate        =&gt; 22_050,
    seek_header       =&gt; MIME::Base64::decode("
    .
    .(deleted)
    .
    "),
    seek_offset       =&gt; 22_108,
    song_length_ms    =&gt; 224_351,
    tracks            =&gt; [
                           {
                             audio_object_type =&gt; 2,
                             audio_type        =&gt; 64,
                             bits_per_sample   =&gt; 16,
                             channels          =&gt; 1,
                             duration          =&gt; 224_351,
                             encoding          =&gt; "mp4a",
                             handler_name      =&gt; "Sound Media Handler",
                             handler_type      =&gt; "soun",
                             id                =&gt; 1,
                             max_bitrate       =&gt; 37_608,
                             samplerate        =&gt; 22_050,
                           },
                           { duration =&gt; 224_351, id =&gt; 1 },
                         ],
  };

Audio::Scan - the length of $info->{seek_header} is 22,100
Audio::Scan - audio_offset is at 22,092 but seek_offset to start of audio is 22,108

LMS seems to be streaming from end of MPEG4 header when it should be streaming from seek_offset which is start of audio.
Not sure if the error is in Audio::Scan or Movie.pm.

I'll try to come up with a solution but any suggestions welcome. I need to check Audio::Scan documentation to see if the returned values are correct and expected.

The error is probably in movie.pm where I calculate the offset. From memory, I don’t rely on Audio::Scan when header is at the end of the file and I need to do an HTTP range to reach it. I assume I made a mistake and some files have an extended mdat header (or a similar issue) and I can’t just add 8 to the mdat position.

**bpa** · 2022-01-02, 11:39

Originally Posted by philippe_44

The error is probably in movie.pm where I calculate the offset. From memory, I don’t rely on Audio::Scan when header is at the end of the file and I need to do an HTTP range to reach it. I assume I made a mistake and some files have an extended mdat header (or a similar issue) and I can’t just add 8 to the mdat position.

The URL is streamable and so the header is at the start but there is padding at end (maybe a "free" atom) which is throwing things off.

After looking at AudioScan soc, it says "seek_header - A rewritten MP4 header that can be prepended to the audio data" - I interpret "rewritten" to means "not the same as source" (e.g. "free" atom removed) and so I think the problem lies in Movie.pm.

I now need to remember all the MPEG4 and InitialAudioBlock stuff to create a fix.

**philippe_44** · 2022-01-02, 13:13

Originally Posted by bpa

The URL is streamable and so the header is at the start but there is padding at end (maybe a "free" atom) which is throwing things off.

After looking at AudioScan soc, it says "seek_header - A rewritten MP4 header that can be prepended to the audio data" - I interpret "rewritten" to means "not the same as source" (e.g. "free" atom removed) and so I think the problem lies in Movie.pm.

I now need to remember all the MPEG4 and InitialAudioBlock stuff to create a fix.

If you are not too much in a hurry, I can have a look today or tomorrow

**philippe_44** · 2022-01-02, 13:48

Originally Posted by philippe_44

If you are not too much in a hurry, I can have a look today or tomorrow

I think the "mdat" box has an extended size over 8 bytes and not 4, that's what I'm missing; I'll re-read mp4 standard to be sure

**bpa** · 2022-01-02, 14:18

Originally Posted by philippe_44

I think the "mdat" box has an extended size over 8 bytes and not 4, that's what I'm missing; I'll re-read mp4 standard to be sure

It's a background project so no urgency at all. I'll have a look at the standard again as I'm sure I've forgotten lots.

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