On Thu, Jan 5, 2023 at 7:04 PM Jayson Larose wrote:
> How is the `aresample` filter supposed to work? I'm recording video
> from a v4l2 capture device, audio from jack, like such:
>
> ffmpeg -vsync cfr -timestamps mono2abs -copyts -framerate 59.73 \
> -f v4l2 -thread_queue_size 2048 -i /dev/video10 -f jack \
> -thread_queue_size 2048 -i capture_out -map 0:v:0 -map 1:a:0 \
> -c:v hevc_nvenc -filter:v setpts=PTS-STARTPTS -c:a:0 aac \
> -b:a:0 128k -filter:a:0 aresample=async=1000 -f matroska \
> -f matroska output.mkv
>
> The end recording has the following stream durations, as reported by
> ffprobe:
>
> Input #0, matroska,webm, from 'recording__2023-01-04 02_13_52.mkv':
>Metadata:
> ENCODER : Lavf59.25.100
>Duration: 02:34:51.14, start: 0.00, bitrate: 1574 kb/s
>Stream #0:0: Video: hevc (Main), yuv420p(tv, progressive), 1120x1008
> [SAR 1:1 DAR 10:9], 59.73 fps, 59.73 tbr, 1k tbn
> Metadata:
>ENCODER : Lavc59.33.100 hevc_nvenc
>DURATION: 02:34:51.13800
>Stream #0:1: Audio: aac (LC), 96000 Hz, stereo, fltp
> Metadata:
>ENCODER : Lavc59.33.100 aac
>DURATION: 02:34:50.87000
>
> This is .268 seconds of difference, but more importantly, if I
> actually analyze the content of the recorded file, the beginning of
> the video starts out with audio and video reasonably well
> synchronized (audio is ahead by ~4 frames), but at the end of the
> video, there's nearly a half of a second of desynch between the two
> (audio is behind by ~23 frames).
>
> I was led to believe that using `aresample=async=96` would
> stretch the audio stream up to 96 samples per second in order to
> keep audio and video in sync, but it doesn't seem to be doing
> anything at all. The documentation is super vague about how it goes
> about doing this, just talking about "timestamps". So I checked to
> make sure that the PTS timestamps coming from my audio and video
> streams were close to in sync:
>
> % ffprobe -timestamps mono2abs -i /dev/video10 -of compact \
> -show_packets 2>/dev/null | head -n 1 & \
> ffprobe -timestamps mono2abs -i capture_out -f jack -of compact \
> -show_packets 2>/dev/null | head -n 1
>
>
> packet|codec_type=video|stream_index=0|pts=1672940948329853|pts_time=1672940948.329853|dts=1672940948329853|dts_time=1672940948.329853|duration=16742|duration_time=0.016742|size=1693440|pos=N/A|flags=K_
>
> packet|codec_type=audio|stream_index=0|pts=1672940948367972|pts_time=1672940948.367972|dts=1672940948367972|dts_time=1672940948.367972|duration=10666|duration_time=0.010666|size=8192|pos=N/A|flags=K_
>
> and they look to be pretty much in sync. If I don't supply
> `-timestamps mono2abs` the video packets come in monotonic time
> and the audio packets come in wallclock time.
>
> This is driving me up the wall, because it's a very manual and
> tedious procedure to fix the audio for every file I record.
>
>
async option make sense only when there are pts gaps between audio.
pts gaps means that frame's pts + frame'duration is < than next frame's pts.
--Jays
>
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