我正在进行一个项目，我只需要将FFMEPG用作解复用器复用器，以便接收带有音频和视频流的MPEG源，并分别输出不同的流。我还需要它来引入尽可能小的延迟。为了做到这一点，我有关于输入的所有必要信息：使用的容器、音频和视频编解码器、音频和音频速率以及音频通道，所以我想向FFMPEG提供这些信息，并避免验证阶段。然而，如果不指定一个大问题，我就无法让FFMPEG工作，这会导致延迟，并且根据输入，会导致FFMPEG由于无法获得所需信息（我事先有）而失败。
我一直在研究FFMPEG提供的不同选项，我发现以下选项可以帮助我：
-
-codec:a和-codec:v以提供关于输入的音频和视频编解码器的信息。 -
-f以提供关于所用容器的信息。 -
-r:v以提供关于视频帧速率的信息。（我也尝试过-framerate，但似乎只有少数解复用器支持，mpeg不是其中之一）。 -
-probesize以避免花费时间探测输入，因为我可以提供信息。 -
-copy_unknown只需复制流，而不是在无法识别的情况下失败。
对于所有这些，我使用以下命令：./ffmpeg -hide_banner -loglevel debug -codec:v h264 -codec:a pcm_alaw -flags low_delay -probesize 32 -analyzeduration 0 -r:v 15 -f mpeg -i udp://127.0.0.1:41071?listen -y -map 0:a:0? -acodec copy -copy_unknown -ar 8000 -payload_type 8 -f rtp udp://127.0.0.1:33605?pkt_size=1200 -map 0:v:0? -vcodec copy -copy_unknown -payload_type 96 -f rtp udp://127.0.0.1:48527?pkt_size=1200
Splitting the commandline.
Reading option '-hide_banner' ... matched as option 'hide_banner' (do not show program banner) with argument '1'.
Reading option '-loglevel' ... matched as option 'loglevel' (set logging level) with argument 'debug'.
Reading option '-codec:v' ... matched as option 'codec' (codec name) with argument 'h264'.
Reading option '-codec:a' ... matched as option 'codec' (codec name) with argument 'pcm_alaw'.
Reading option '-flags' ... matched as AVOption 'flags' with argument 'low_delay'.
Reading option '-probesize' ... matched as AVOption 'probesize' with argument '32'.
Reading option '-analyzeduration' ... matched as AVOption 'analyzeduration' with argument '0'.
Reading option '-r:v' ... matched as option 'r' (set frame rate (Hz value, fraction or abbreviation)) with argument '15'.
Reading option '-f' ... matched as option 'f' (force format) with argument 'mpeg'.
Reading option '-i' ... matched as input url with argument 'udp://127.0.0.1:41071?listen'.
Reading option '-y' ... matched as option 'y' (overwrite output files) with argument '1'.
Reading option '-map' ... matched as option 'map' (set input stream mapping) with argument '0:a:0?'.
Reading option '-acodec' ... matched as option 'acodec' (force audio codec ('copy' to copy stream)) with argument 'copy'.
Reading option '-copy_unknown' ... matched as option 'copy_unknown' (Copy unknown stream types) with argument '1'.
Reading option '-ar' ... matched as option 'ar' (set audio sampling rate (in Hz)) with argument '8000'.
Reading option '-payload_type' ... matched as AVOption 'payload_type' with argument '8'.
Reading option '-f' ... matched as option 'f' (force format) with argument 'rtp'.
Reading option 'udp://127.0.0.1:33605?pkt_size=1200' ... matched as output url.
Reading option '-map' ... matched as option 'map' (set input stream mapping) with argument '0:v:0?'.
Reading option '-vcodec' ... matched as option 'vcodec' (force video codec ('copy' to copy stream)) with argument 'copy'.
Reading option '-copy_unknown' ... matched as option 'copy_unknown' (Copy unknown stream types) with argument '1'.
Reading option '-payload_type' ... matched as AVOption 'payload_type' with argument '96'.
Reading option '-f' ... matched as option 'f' (force format) with argument 'rtp'.
Reading option 'udp://127.0.0.1:48527?pkt_size=1200' ... matched as output url.
Finished splitting the commandline.
Parsing a group of options: global .
Applying option hide_banner (do not show program banner) with argument 1.
Applying option loglevel (set logging level) with argument debug.
Applying option y (overwrite output files) with argument 1.
Applying option copy_unknown (Copy unknown stream types) with argument 1.
Last message repeated 1 times
Successfully parsed a group of options.
Parsing a group of options: input url udp://127.0.0.1:41071?listen.
Applying option codec:v (codec name) with argument h264.
Applying option codec:a (codec name) with argument pcm_alaw.
Applying option r:v (set frame rate (Hz value, fraction or abbreviation)) with argument 15.
Applying option f (force format) with argument mpeg.
Successfully parsed a group of options.
Opening an input file: udp://127.0.0.1:41071?listen.
[mpeg @ 0x55a769cc3d40] Opening 'udp://127.0.0.1:41071?listen' for reading
[udp @ 0x55a769cc4b00] No default whitelist set
[udp @ 0x55a769cc4b00] end receive buffer size reported is 425984
[mpeg @ 0x55a769cc3d40] Before avformat_find_stream_info() pos: 6 bytes read:40 seeks:0 nb_streams:0
[mpeg @ 0x55a769cc3d40] probing stream 1 pp:2500
[mpeg @ 0x55a769cc3d40] probed stream 1
[mpeg @ 0x55a769cc3d40] parser not found for codec pcm_alaw, packets or times may be invalid.
[mpeg @ 0x55a769cc3d40] Probe buffer size limit of 32 bytes reached
[mpeg @ 0x55a769cc3d40] Stream #0: not enough frames to estimate rate; consider increasing probesize
[mpeg @ 0x55a769cc3d40] Could not find codec parameters for stream 0 (Video: h264, 1 reference frame, none): unspecified size
Consider increasing the value for the 'analyzeduration' (0) and 'probesize' (32) options
[mpeg @ 0x55a769cc3d40] After avformat_find_stream_info() pos: 87804 bytes read:88476 seeks:0 frames:1
Input #0, mpeg, from 'udp://127.0.0.1:41071?listen':
Duration: N/A, start: 50436.772978, bitrate: 64 kb/s
Stream #0:0[0x1e0], 0, 1/90000: Video: h264, 1 reference frame, none, 90k tbr, 90k tbn
Stream #0:1[0x1c0], 1, 1/90000: Audio: pcm_alaw, 8000 Hz, mono, s16, 64 kb/s
Successfully opened the file.
Parsing a group of options: output url udp://127.0.0.1:33605?pkt_size=1200.
Applying option map (set input stream mapping) with argument 0:a:0?.
Applying option acodec (force audio codec ('copy' to copy stream)) with argument copy.
Applying option ar (set audio sampling rate (in Hz)) with argument 8000.
Applying option f (force format) with argument rtp.
Successfully parsed a group of options.
Opening an output file: udp://127.0.0.1:33605?pkt_size=1200.
[udp @ 0x55a769cf6500] No default whitelist set
Successfully opened the file.
Parsing a group of options: output url udp://127.0.0.1:48527?pkt_size=1200.
Applying option map (set input stream mapping) with argument 0:v:0?.
Applying option vcodec (force video codec ('copy' to copy stream)) with argument copy.
Applying option f (force format) with argument rtp.
Successfully parsed a group of options.
Opening an output file: udp://127.0.0.1:48527?pkt_size=1200.
[udp @ 0x55a769d08fc0] No default whitelist set
Successfully opened the file.
Output #0, rtp, to 'udp://127.0.0.1:33605?pkt_size=1200':
Metadata:
encoder : Lavf59.16.100
Stream #0:0, 0, 1/8000: Audio: pcm_alaw, 8000 Hz, mono, s16, 64 kb/s
[rtp @ 0x55a769d06f40] dimensions not set
Could not write header for output file #1 (incorrect codec parameters ?): Invalid argument
Error initializing output stream 1:0 --
Stream mapping:
Stream #0:1 -> #0:0 (copy)
Stream #0:0 -> #1:0 (copy)
Last message repeated 1 times
[AVIOContext @ 0x55a769d06d40] Statistics: 0 bytes written, 0 seeks, 0 writeouts
[AVIOContext @ 0x55a769d19800] Statistics: 0 bytes written, 0 seeks, 0 writeouts
[AVIOContext @ 0x55a769cd4fc0] Statistics: 88476 bytes read, 0 seeks
我使用的是FFMPEG 5.0.2，这是一个相当更新的版本。
正如您所看到的，-copy_unknown选项没有效果，因为FFMPEG在尝试识别输入时仍然会崩溃（我也尝试过在输入上指定它，但结果是一样的）。从日志中，我认为主要问题是FFMPEG没有正确解释帧速率，这阻止了它对视频进行解复用。我一直在寻找另一种方法来指定它，但找不到。
这是一个bug吗？或者可能是遗漏了什么？有人知道我如何实现目标吗？