I have 2 DVDs of the same movie but in different language, I want to add to my rip of the first DVD the audio stream from the second DVD. The problem is
with the added audio stream(from second DVD) the longer the movie plays the more out of sync is the added stream.
I use ffmpeg to extract, convert(ac3 to mp3) and merge audio.
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Are the two movies the exact same running time? mp3 is not even a valid DVD audio.
They are not, the movie is 3 seconds longer than the audio, but I cut the last 3 seconds off the movie before merging and result seems to be the same.
I see, you're creating a rip. Does the video have variable frame rate?
Cutting the last 3 seconds will not sync the audio since the audio is out of sync from the start,you will either need to stretch the audio to match and if that doesn't work then you need to edit the audio to see if there is different cut scene times that need to be adjusted.I think,therefore i am a hamster.
It's possible that the two versions have slight editing discrepancies, in which case it is necessary to examine them both side-by-side in a non-linear editor and adjust the video or audio or both accordingly, painstakingly, all the way through the end (only a few frames missing or added here and there can result in a noticeable audio delay). I had to do that twice, it's quite a PITA (in some instances I had to cut-and-paste segments of the non-dubbed audio to fill “holes” in the dubbed audio, and other tricks like that). If the discrepancy is only in the introduction credits it shouldn't be too difficult (in the case exposed in the thread linked earlier the introduction was edited very differently between the two versions, so I completely replaced the audio from the introduction by that of the non-dubbed version, in much better quality anyway, then joined that segment with the dubbed audio, before the beginning of the dialogues, almost seamlessly, with a crossfade).
MP3 is now an antiquated codec, and Xvid + MP3 DVD rips (especially the “700MB” ones which still continue to pop up in 2020 although noone in their right mind keeps storing movies on CD-R which was the sole justification for that particular size) should now be banned by federal laws in the whole world, every offender should be whipped on the crotch with urtica for 24 hours straight.
The current standard is MP4 or MKV container with H.264 video (or H.265 but it's more geared toward higher resolutions) and AAC audio. For best quality AAC encoding, qaac is recommended.
In any case, if you have to process / edit the audio, convert to WAV first to avoid an unnecessary re-compression.
Standard framerate for most movies is 23.976 FPS. Movies on PAL DVDs are slightly sped up to reach 25 FPS. Movies on NTSC DVDs are processed with a method called “telecine” to reach 29.97 FPS, which involves partial interlacing and duplication of frames. If both DVDs are in 25 FPS, no special processing should be necessary (although it's probably better to restore the original framerate and playback speed, I'm not sure what the recommended guidelines are in that regard). If one of them is in 29.97 FPS, then it becomes more complicated.