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Print at Jun 2, 2020 5:54:20 AM

Posted by atteSmythe at Apr 28, 2004 2:51:02 AM
Re: How to Create Videos
LongJohnGrey wrote: 
What do people recommend for making a movie? Realistically, I'd have to record in Mpeg 2 (real time), and convert to Mpeg 4 (divx) afterwards.

Don't convert.

It's not actually a conversion - it'll compress the video, uncompress it, and recompress it again. What you end up getting is all sorts of transcoding errors. MPEG audio codecs, I know, can be pretty bad about these cascading artifacts. I imagine that the video is the same or worse.

Some things that might help you in your searches:
MPEG 2 - a collection of standards
MPEG 2, Layer 2 - Audio codec defined by the standard, used widely in radio for sending voice over ISDN, as well as other applications
MPEG 2, Layer 3 - MP3 audio codec, also called MPEG Layer 3.
MPEG 4 - A new collection of standards. They skipped 3 to avoid confusion over the popular MP3
MPEG 4, AAC - Audio codec defined in the MPEG4 standard.

Layer 2 and Layer 3 are perceptional audio codecs - they rely on the psychoacoustic principles to remove information from the audio that you literally cannot hear (the shape of the ears and ear canal filter certain frequency and time components before the sound wave even hits your eardrum). This makes the audio easier to compress, since there's less data to worry about.

How it does this is by taking a sample of the audio over some period of time, working on that, and storing it in a frame. Since each frame is worked on seperately, there is an inaudible but mathmatically important boundary between the frames. When you reencode the audio, the sample of audio that the second codec takes will most likely not be exactly lined up frame-by-frame. Since the sound before and after the boundary has been treated differently, it's much more difficult to compress the second time through, which leads to audible artifacts.

Though transcoding over MPEG 4 AAC is believed to be much better than over MP2 Layer 2 or Layer 3, no formal testing has been done to prove it.

atteSmythe,
finally with a post that even LJG should find thorough. ;)
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