process of avi conversion

[ABAB]

Can you clear this up for me, when a conversion from dvd to avi is done the file is shrunk to 1/6 the size....I always thought that some frames got dropped. Is this correct or have I got it totally wrong.
Did a search but far too many topics to trawl and I'm sure one of my learned friends here will have no problem helping me out.

Thx

[bugster]

Its not normal for frames to be dropped. The reduction in size is usually down to using a much more efficient compression algorithm. DVD is mpeg-2, most movie conversions use divx or xvis. These are mpeg-4 codecs which allo for much greater compression. Some loss of quality also occurs, how much depends on how much compression is used.

[ABAB]

Understand that bit but am trying to find out why there is a drop in quality when you convert back from DivX/XviD to a DVD compliant file. As I mentioned, I always thought is was due to dropped frames which is obviously not the case.
Gonna do a google session for some info I think.

Thx for your reply

[bugster]

ANY conversion will cause a drop in quality.

One of the biggest problems with DVD->Divx->DVD is resize. Say you have an NTSC DVD that is 4:3 aspect ratio (AR). On the DVD this is 720 * 480 pixels. This is displayed as 4:3 because TV's do not use square pixels. A PC display does have square pixels, so hwn converted to divx for PC use this would become 640*480 (for instance, lost of other choices here). Resizing like this causes loss of info that cannot be recovered. When converting back to DVD it would once again become 720*480, another drop in quality due to the resize.

Of course, simply doing a format conversion causes some quality loss on its own, resize just adds to it. Conversion also often involve changes in colourspace (RGB, YUV etc) and other issues that all add up.