Background: I am using FFMPEG to combine an audio and video stream into a combined MPEGTS stream over the network. Video comes encoded in h264 from the raspberry pi camera, audio comes ogg vorbis encoded from a sound card over a local TCP stream. A single FFMPEG process takes care encoding the audio as AAC and combining audio/video.
raspivid --nopreview --ev 10 -ih -t 0 -rot 180 -w 720 -h 480 -fps 30 -b $BITRATE -g $KEYFRAME_PERIOD -pf baseline -o - | ffmpeg -r 30 -i - -i tcp://127.0.0.1:3000 -vcodec copy -acodec libfdk_aac -b:a 64k -ac 2 -filter:a "atempo=0.9945" -f mpegts tcp://192.168.42.2:6000
Problem: I had a problem with audio and video de-synchronizing over time. I fixed the initial time difference by tweaking the sound card driver myself, however over time the streams can still very slowly run out of sync: the audio plays slightly faster than the video. Letting it run on eventually causes playback buffering to re-sample the audio, resulting in video latency increasing.
The reason for this tempo offset is probably that the sound card runs on its own clock slightly different than that of the raspberry pi. I have managed to reduce this issue using the atempo audio filter. However, considering this may run for hours to days, it is inadequate as the crystal clock speeds will vary.
Question: Considering the audio comes in as an audio stream, I was wondering if there is a way to automatically adjust the tempo to keep the buffered audio sample time constant? Something that makes FFMPEG maintain a steady audio stream, keeping x seconds in the buffer at all times, slowing down playback if catching up on the buffer. The tempo is the key here: if tempo is not changed it will result in problems during playback when audio and video are combined.
I can not use standard synchronization methods because the h264 stream has no timestamps, and the ogg stream timestamps have a tempo problem.
Any hints towards resolving this are appreciated.