Why convert PAL AC3 to NTSC AC3 ?

[TheJez]

I am authoring a couple of PAL DVD's, but want to make NTSC versions of them aswell. For this, I use Procoder to convert my AVI's to PAL and NTSC MPEG2. This also produces wav-files for both versions, but they are identical, so I throw one away. The resulting wav-file is then converted to AC3 using SoftEncode.

Then, during authoring, I simply use the PAL video file, with the AC3-file to create the PAL DVD, and the NTSC video file with the same AC3-file to create the NTSC DVD. Both discs play fine on my pc and stand-alone (PAL/NTSC) player (on PAL tv).

So why do I see remarks in many threads regarding PAL<->NTSC conversion, that the AC3-file needs to be converted aswell? I realy don't get this. I'd think a certain AC3-file produces audio of a certain length, regardless of the video format that is being played in parallel ?

What am I missing here ???

Thanks,

The Jez

[Tuning]

I think your AVI -> MPEG2 converter has in-built support for that task.So it may have done it already.To find out the real try demuxing ac3 files from both DVDs and put in a playlist of any player capable of ac3 playback.Then you can see the difference.(May be several seconds,but there will be...)

[SomeJoe]

TheJez is quite correct. AC3 is AC3. There is no such thing as "PAL AC3" or "NTSC AC3".

The bottom line is the length of the AC3 file and video files in hours:minutes:seconds.fractional seconds. (Note: Not hours:minutes:seconds:frames. PAL frames last longer than NTSC frames, so you can't compare time using a frame count). If you're doing a straight conversion of the video from PAL to NTSC or vice versa where your video conversion process results in a video file of the other format that is the exact same length, there is absolutely no need to do anything to the original AC3. It can be muxed back to a new DVD with no changes and the audio will sync perfectly.

Now, if your video conversion process results in a video file that is a different length than the original (such as when taking a 23.976 fps NTSC file and speeding it up 4% to make it a 25 fps PAL file), this is a different story. The resulting video will be shorter in time. The audio will have to be converted to WAV and shortened as well, and then reencoded.

[TheJez]

Tuning, SomeJoe,

Thanks for your responses. I guess we all three are right ;-)
If the video-encoder slightly changes the speed of the movie during conversion, the AC3 has to be changed accordingly. Fortunately Procoder produces the same length video for both PAL and NTSC mpeg video. Well, almost. A PAL movie lenght is always a multitude of 1/25 seconds (40ms), while the NTSC version will be a multitude of 1/29.97 seconds (33.37ms).

I am just glad I don't have to make NTSC-AC3 :-)

Best wishes,

The Jez