Hello! I'm new to forums, so I don't know where my question should be put. As the title says I'm experiencing lots of troubles trying to add a PAL audio track to a NTFS video. Usually the FPS difference was reflected in a minor length of the PAL track, in such a way that I just had to increase its time by 4,096% (typically with Audacity) to convert it to NTFS without altering the pitch. After that I was always able to mux (by means of Mkvtoolnix) audio and video tracks with a perfect synchronization. However, the problem I have is that the two files have the same length (about 2h14m, with minimal seconds' difference) in spite of the fact that the audio one (extracted from a DVD rip) has a frequency of 25fps, while the video one (a MKV) has a frequency of 24fps (as it is reported by MediaInfo: I usually deal with 23.976fps videos). I've tried over and over to mux the tracks, but every time the audio becomes progressively desynchronized, even trying to set delays or other similar tweaks. I read that a software like Avisynth can change the frame rate without modifying the duration of the track, but I'm not familiar with scripts and I wouldn't know how to use the program correctly. My questions are: 1) Is there a way simpler than this to make the fps match in order to synchronize the tracks? 2) Could someone provide me an explanation of the phenomenon (same length with different frames-per-second)? Thanks in advance.
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The audio track sampling rate is independent of the video track frame rate. There is no such thing as an audio frequency of 24p or 25p.
You should be able to mux any compatible audio track (compatible in the sense of bitrate, compression format and sampling rate).
If it does not fit and you need the track, do not mess around with the frame rate but simply speed up or slow down the audio either with or without frequency changes.
Last edited by newpball; 27th Jul 2015 at 18:15.
Yes, of course, what I wrote makes no sense! I should have referred the 25fps to the video track of the DVD from which the audio was extracted. Still, if I extract the audio track (AC3) and apply it to the NTSC MKV the audio goes progressively out of synchronization, even if the duration is the same. So could the issue be related to incompatibilites of the bitrate or the sampling rate? Sorry if I keep asking but I'm pretty much incompetent at this subject.
I read only now the second part of your answer. I'll try to work on the audio speed, then. Thanks!
Audio doesn't have to start at the same time as the video, nor does it have to end at the same time. Video releases in different countries can have different cuts of the video with sections added or removed. Sometimes the video is sped up or slowed down arbitrarily. So having audio and video tracks with the same or different running times doesn't mean anything.
If the two version of the video doen't suffer from cuts or additions you just need to find two sync points, one near the start of the video, one near the end. Note the difference in time of the two events in the video, and in the audio. Stretch or shrink the audio to match the video. Then mux with whatever audio advance/delay is necessary to sync the first point.
Chances are the audio and video won't match up from start to finish, but if you're using MKVMergeGUI you could try adjusting the video frame rate a little. MKVMergeGUI has an option to stretch the audio when muxing (normally you'd need to re-encode it to alter the speed/duration) but the audio can play back with glitches if you stretch it. It depends on the type of audio and how much you stretch, but I'd leave the audio alone and try syncing it to the video by adjusting the video frame rate up or down a little instead (whatever's required). That shouldn't cause playback problems and there's no re-encoding of the video required.
Of course if you're keeping the original audio too, changing the frame rate will put it out of sync with the video, so that won't be an option, but if you're replacing the audio with the new version it won't matter.