(I know that there probably is actually an audio track generated by Blender, but it's just silence as I don't add any audio tracks to the simple titles.) It's only 10 seconds long. It actually has no audio, as it's just two 'slides' of text.

The title file I created in Blender, and tried to duplicate the long file's format: 720x480, 127 audio bitrate, MP4 containing H264 and AAC. I captured the video from a Sony Handycam Video8 tape using a Hauppauge Live-2 USB converter, using the VLC CLI to generate the original. Before concatenating the title file, it is in perfect a/v sync at all times. ts file and I use ffmpeg to encode it into the mp4 format. The long file is an MP4 file using H264 and AAC. I have about 20 old 2-hour family video cassettes that I am digitizing. I'm using Ubuntu Linux 16.04 and ffmpeg 2.8.6-1ubuntu2, which I think is the latest ffmpeg version. I'm a digital video semi-noob and have faced a steep learning curve, but I am learning.

If necessary, I can re-encode the title file because it's just white text on a black background. I want to avoid re-encoding the long file to maintain quality. I can understand why the concat might break the synchronization, but I don't understand why it would cause the synchronization to become progressively worse.

However, when I use ffmpeg -f concat -i mylist.txt -c copy outfile.mp4 to concatenate the title file to the long file, the audio in the long file gradually goes out of sync, progressively becoming worse until at the end of 2 hours, the audio is a full second or two behind the video. The long file has perfect a/v sync from start to finish. I have one 2-hour video file and want to add a 10-second title sequence to its start.

Ffmpeg concat audio out of sync