audio synchronization problem using AviSynth, videos recorded by Pixel XL

[poscogrubb]

I'm using AviSynth to edit a sequence of video clips that were recorded by my Pixel XL smartphone camera. The files are .MP4:
video codec is avc1, variable framerate, NTSC standard
audio format is AAC LC, sampling rate 48.0 kHz, frame rate 46.875 FPS

Each of the videos is a slightly different frame rate, close to 29.9 FPS. How do I join clips of these multiple videos together? Audio synchronization is very important (this is a video recording of a classical music performance).

    a = FFmpegSource2("filename-a.mp4", atrack=-1).ChangeFPS(preset="ntsc_video")
    b = FFmpegSource2("filename-b.mp4", atrack=-1).ChangeFPS(preset="ntsc_video")
    a.Trim(u, v) + a.Trim(w, x) + b.Trim(y, z)

I used VirtualDub2 to output this script to a new .mp4 using 8-bit H264 (2 passes) and AAC compression. The resulting video shows the picture at a slightly faster rate than the audio, so that by the end of the 5 minute video, the audio is behind the picture by almost 2 seconds.

[jagabo]

For VFR use fpsnum and fpsden in ffmpegsource2.