adding black bars above/below widescreen for 4.3 ratio

[anorakgrease]

i am trying to convert a series of widescreen avi files into 4.3 ratio movies with the subtitles below the films. This is much easier to read than having the text over the film.
Im not sure how to add the back bars without streching the film ratio.
If i select a dvd preset this happens automatically and i get a nice 4.3 screen ratio with the subs floating below the film. But i then have to reconvert to avi or what ever...

any pointers please.
Thanks

[manono]

What are you doing? Taking an AVI and reencoding it, also to AVI, and adding black below, and burning the subs into the black?

If so, use the AviSynth AddBorders command to add, say, 100 pixels of black below. The AR won't change.

AddBorders(0,0,0,100)

http://www.avisynth.org.ru/docs/english/corefilters/addborders.htm

If this is for DVD, then use FitCD to get the script, and if it doesn't give you enough black below, then shift some from the top. For example, if your source AVI is 640x352, and you're going for an NTSC 4:3 DVD, FitCD gives me (with ITU resize checked):

LanczosResize(720,352,0,0,640,352)
AddBorders(0,64,0,64)

You can adjust that to give yourself 100 pixels below the movie like so:

LanczosResize(720,352,0,0,640,352)
AddBorders(0,28,0,100)

[anorakgrease]

thanks manono but im working on mac os x so none of that software will work for me and im trying to do this using ffmpegx if possible.
the secondary encoding will be for two or more films on one dvd or even simply re-encoded as avi +subs allows me to view on tv with the subs comfortably below the movie. there are a number of ways i could find this useful.

as opposed to this