Differences

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+====== VBR MP3 ======
+===== Background =====
+Most of the time, Avidemux computes the offset and length of audio by doing size=time*bitrate. That works for all CBR streams.
+For VBR, it does not work as bitrate is not constant.
+===== Audio time map =====
+To compute the start offset/size, Avidemux has to know how the bitrate changes. For that use Audio->Build VBR Time Map.
+Avidemux will then decompress the whole stream one time and build a time map to know how to handle the stream. You will need libmad installed, else it will do nothing.
+When the time map is built, Avidemux will have a conversion table between time and offset, and you will be able to split or save the AVI in a VBR way.
+If you don't do it, you will end up with a badly desynced (potentially with too short audio) AVI. Even if you just load and save it.
+You **must** do it when dealing with VBR MP3.
+===== Limitation =====
+When **saving** a dual audio AVI, the second audio stream cannot be VBR.

Avidemux