[FFmpeg-devel] [PATCH] avcodec: fix wrong duration of packets (dvd, bluray)

[Александр Слободенюк]

Fixing very little bug for blu ray pcm in .mts files:

The nature of bug: duration of blu ray packets is little bit wrong.
How to reproduce:
./ffprobe -i bluray-pcm-file.mts -show_packets -select_streams a:0 | head -n 50
...
Stream #0:1[0x1100]: Audio: pcm_bluray (HDMV / 0x564D4448), 48000 Hz, stereo, s16, 1536 kb/s
...
[PACKET]
codec_type=audio
stream_index=1
pts=118350
pts_time=1.315000
dts=118350
dts_time=1.315000
duration=451
duration_time=0.005011
convergence_duration=N/A
convergence_duration_time=N/A
size=964
pos=1035456
flags=K_
[SIDE_DATA]
side_data_type=MPEGTS Stream ID
[/SIDE_DATA]
[/PACKET]
[PACKET]
codec_type=audio
stream_index=1
pts=118800
pts_time=1.320000
dts=118800
dts_time=1.320000
duration=451
duration_time=0.005011
convergence_duration=N/A
convergence_duration_time=N/A
size=964
pos=1048704
flags=K_
[SIDE_DATA]
side_data_type=MPEGTS Stream ID
[/SIDE_DATA]
[/PACKET]


Between the packets you can see that dts increments by "450", not "451":

dts=118350 
=> 
dts=118800

We can compute the duration by ourself from the size: 
size = 964 .
 BDPCM Header length is 4 bytes.
 => samples size = 960
 size of one sample = 16 bits/8 * 2 channels = 4
 then number of samples = 960/4 = 240 samples
 
 duration = 240 * 90000 / 48000 = 450
 
 so, the duration in packet
 "duration=451"
 is incorrect
 
 ============
 Following the code in avcodec:utilc.c get_audio_frame_duration(),
 we can see, that header sizes were not excluded from the frame size.
 ============
 P.S. The bug is the same for dvd pcm, but it doesn't affect on anything. 
 
 How to make sure, that "magic constants" 3 and 4 for the streams are constant for all cases:
 see the decoding code:
 
 pcm-bluray.c : 144
 pcm-dvd.c : 250
 s302m.c : 108 (yes, s302m case is not modified)
 
 P.P.S.
 1. It looks like for s302m the exclusion of 4 bytes happens (in a hackish way, splitted with protection from division by zero)
 2. You can see that code around the patch contains 3 different ways of protection from dividing by zero. 
 And actually for all this streams we can check bps > 8 (isn't it?).
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