[Libav-user] avcodec_open2 is changing the channels and channel_layout... why?
Michael Bradshaw
mbradshaw at sorensonmedia.com
Fri Oct 12 00:25:16 CEST 2012
On Thu, Oct 11, 2012 at 4:06 PM, Carl Eugen Hoyos <cehoyos at ag.or.at> wrote:
> Michael Bradshaw <mbradshaw at ...> writes:
>
>> I've just been hunting down a bug in one of our programs, and I
>> tracked it down to the number of channels reported by ffmpeg. When I
>> first open the AVFormatContext and do avformat_find_stream_info(), I
>> can see that my audio stream's AVCodecContext has a reported
>> channel_layout of 3 (stereo) and channels == 2. However, when I call
>> avcodec_open2() to start decoding, it changes the streams'
>> AVCodecContext channel info so that channel_layout is 4 (mono) and
>> channels == 1! This is causing a crash in a later part of the program,
>> as one part of the program expects two channels and the other part
>> expects 1 channel.
>>
>> Two questions:
>>
>> 1) Is this "expected behavior" in any case, or is it a bug?
>
> While I cannot really answer the question, I wonder
> what you expect when the number of channels really change:
> At least for AC-3 and AAC, this is not only allowed,
> but happens regularly (for example on DVB).
Really? That's useful to know. I suppose I should rely on
AVFrame.channels, correct?
While that's useful to know, I don't think it this AAC stream changes
channels. I just used ffprobe to look at this file. You'll see that
when it dumps the format (dump_format), it shows the aac stream as
being stereo, but the [STREAM] info from ffprobe says channels=1.
./ffprobe -show_streams -select_streams 0 ~/MP4-Apple264-AACPlus.mp4
ffprobe version N-45341-g3632f35 Copyright (c) 2007-2012 the FFmpeg developers
built on Oct 11 2012 16:10:23 with gcc 4.2.1 (GCC) (Apple Inc. build
5666) (dot 3)
configuration: --enable-libopenjpeg --enable-libx264 --enable-gpl
libavutil 51. 74.100 / 51. 74.100
libavcodec 54. 65.100 / 54. 65.100
libavformat 54. 32.100 / 54. 32.100
libavdevice 54. 3.100 / 54. 3.100
libavfilter 3. 19.102 / 3. 19.102
libswscale 2. 1.101 / 2. 1.101
libswresample 0. 16.100 / 0. 16.100
libpostproc 52. 1.100 / 52. 1.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from
'/Users/mjbshaw/MP4-Apple264-AACPlus.mp4':
Metadata:
major_brand : mp42
minor_version : 0
compatible_brands: mp42isomavc1
creation_time : 2012-01-16 21:01:41
encoder : Sorenson Squeeze
Duration: 00:00:11.19, start: 0.000000, bitrate: 789 kb/s
Stream #0:0(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz,
stereo, s16, 32 kb/s
Metadata:
creation_time : 2012-01-16 21:01:41
handler_name : soun
Stream #0:1(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p,
480x360, 765 kb/s, 23.98 fps, 23.98 tbr, 23976 tbn, 47952 tbc
Metadata:
creation_time : 2012-01-16 21:01:41
handler_name : vide
Stream #0:2(und): Data: none (mp4s / 0x7334706D)
Metadata:
creation_time : 2012-01-16 21:01:57
handler_name : sdsm
Stream #0:3(und): Data: none (mp4s / 0x7334706D)
Metadata:
creation_time : 2012-01-16 21:01:57
handler_name : odsm
Unsupported codec with id 0 for input stream 2
Unsupported codec with id 0 for input stream 3
[STREAM]
index=0
codec_name=aac
codec_long_name=AAC (Advanced Audio Coding)
profile=unknown
codec_type=audio
codec_time_base=1/22050
codec_tag_string=mp4a
codec_tag=0x6134706d
sample_fmt=s16
sample_rate=44100
channels=1
bits_per_sample=0
id=N/A
r_frame_rate=0/0
avg_frame_rate=0/0
time_base=1/44100
start_pts=0
start_time=0.000000
duration_ts=493568
duration=11.192018
bit_rate=32000
nb_frames=241
nb_read_frames=N/A
nb_read_packets=N/A
DISPOSITION:default=0
DISPOSITION:dub=0
DISPOSITION:original=0
DISPOSITION:comment=0
DISPOSITION:lyrics=0
DISPOSITION:karaoke=0
DISPOSITION:forced=0
DISPOSITION:hearing_impaired=0
DISPOSITION:visual_impaired=0
DISPOSITION:clean_effects=0
DISPOSITION:attached_pic=0
TAG:creation_time=2012-01-16 21:01:41
TAG:language=und
TAG:handler_name=soun
[/STREAM]
But all of the frames have channels=2:
$ ./ffprobe -show_frames -select_streams 0 ~/MP4-Apple264-AACPlus.mp4
ffprobe version N-45341-g3632f35 Copyright (c) 2007-2012 the FFmpeg developers
built on Oct 11 2012 16:10:23 with gcc 4.2.1 (GCC) (Apple Inc. build
5666) (dot 3)
configuration: --enable-libopenjpeg --enable-libx264 --enable-gpl
libavutil 51. 74.100 / 51. 74.100
libavcodec 54. 65.100 / 54. 65.100
libavformat 54. 32.100 / 54. 32.100
libavdevice 54. 3.100 / 54. 3.100
libavfilter 3. 19.102 / 3. 19.102
libswscale 2. 1.101 / 2. 1.101
libswresample 0. 16.100 / 0. 16.100
libpostproc 52. 1.100 / 52. 1.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from
'/Users/mjbshaw/MP4-Apple264-AACPlus.mp4':
Metadata:
major_brand : mp42
minor_version : 0
compatible_brands: mp42isomavc1
creation_time : 2012-01-16 21:01:41
encoder : Sorenson Squeeze
Duration: 00:00:11.19, start: 0.000000, bitrate: 789 kb/s
Stream #0:0(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz,
stereo, s16, 32 kb/s
Metadata:
creation_time : 2012-01-16 21:01:41
handler_name : soun
Stream #0:1(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p,
480x360, 765 kb/s, 23.98 fps, 23.98 tbr, 23976 tbn, 47952 tbc
Metadata:
creation_time : 2012-01-16 21:01:41
handler_name : vide
Stream #0:2(und): Data: none (mp4s / 0x7334706D)
Metadata:
creation_time : 2012-01-16 21:01:57
handler_name : sdsm
Stream #0:3(und): Data: none (mp4s / 0x7334706D)
Metadata:
creation_time : 2012-01-16 21:01:57
handler_name : odsm
Unsupported codec with id 0 for input stream 2
Unsupported codec with id 0 for input stream 3
[FRAME]
media_type=audio
key_frame=1
pkt_pts=0
pkt_pts_time=0.000000
pkt_dts=0
pkt_dts_time=0.000000
pkt_duration=2048
pkt_duration_time=0.046440
pkt_pos=6211
sample_fmt=s16
nb_samples=2048
channels=2
channel_layout=stereo
[/FRAME]
[FRAME]
media_type=audio
key_frame=1
pkt_pts=2048
pkt_pts_time=0.046440
pkt_dts=2048
pkt_dts_time=0.046440
pkt_duration=2048
pkt_duration_time=0.046440
pkt_pos=24505
sample_fmt=s16
nb_samples=2048
channels=2
channel_layout=stereo
[/FRAME]
[...and so on; every frame has channels=2...]
Any thoughts?
Thanks,
Michael
More information about the Libav-user
mailing list