Why it´s not recommended to convert frame rates?

[guilhalm]

i´m doins vcd´s from 23,97 NTSC-FILM movies.

it is said that it´s not recommended to convert the framerates when encoding to the MPG file (for example, from 23,97 to NTCS standard 29,97).

can someone tell me why? is it just a matter of time to encode? or quality is also affected? and video/sound syncronization?

[bugster]

because its difficult and time consuming to get good results.

Converting from 23.97 to 29.97 should not ne necessary.

If mpeg-1, leave as 23.97

If mpeg-2, leave as 23.97 and apply 3:2 pulldown (use pulldown.exe from tools section)

[hustbaer]

right.
but.

up-converting is normally no problem, only when the ratio is completely unsquare - like 27->30 frames, but 27 is no "realistic" framerate, so no problem here.
but like bugster implied - it does not make sense too.

the downside:
*) using CBR you will waste very MUCH bitrate
*) using VBR you will waste some bitrate too, but usually much less than with CBR

divX VBR should be absolutely no problem

down-converting however can be a real pain-in-the-ass (sorry).
there are 2 possible situations, one as bad as the other:
let's assume your target-frame rate is 25 full frames (progressive).
(25 frames/50 fields interlaced would make the situation far worse!)

1) the source is a real 30-frames NTSC progressive tv-series
so the original frame-rate is 30.
when you down-convert, you have to
1.a) drop every X frames
or
1.b) blend two frames together for those frames that
so - 1.a) will give you a little skipping.
and 1.b) will give you those nasty blurring of motion scenes.

2) the source was FILM (24fps) and converted to 30 fps.
here you have to "guess" the correct frames to drop.
that works well as long as the source-material has no dropped frames
if it HAS dropped frames, the frame-dropping will get "out-of-sync" with the up-converting process done before.
and that gives very much poor results

so - just leave it as it is.

one exception:

converting 50/60 fps (interlaced) to 1/2 of the frame rate, means 25/30 fps.
no problem there, just drop each 2nd frame (field).

hope that helps understand the framerate-stuff a little better...

btw: sorry for my bad english - i hope it was "understandable"

bye,
--hustbaer