ac3 overhead calculation on 2 CD xvid encode

[kaitsuburi]

Greetings!

On doing a 2CD XviD (qpel, chroma, b-frames 3-100-150) encode of The Rock (R2 NTSC) with original ac3 track (3.2 448Kb delay 6ms iirc) muxed with the latest avimux-gui, I got a file oversized by about 35 Mb. For filesize calculation I had used Gknot 0.27, checking 1x ac3 (1 frames interval=42ms) and fed it the exact size of the ac3 file too. XviD hit the 2nd pass size it was given quite correctly, so I suspect the ac3 overhead calculation must have been wrong.

I've been wondering about the "frames [#]" and "interval [ms]" settings under "Interleaving and AVI overhead" in Gknot. Nic's minicalc that comes with XviD (that I admit embarrassedly I haven't used yet :o but I will when I get home) has a similar pair of settings. Could someone enlighten me what those mean and what they should be set to given i am muxing the ac3 track with avimux-gui.

TIA

-kaitsuburi

ps. Searching the forums, I got the following thread but I didn't fully understand alexnoe's explanation :confused:

http://forum.doom9.org/showthread.php?s=&threadid=44612&highlight=ac3+overhead

[alexnoe]

Causing an overhead of 35 MB is hardly possible; this would require something like a 3 hours movie with 3 languages in MP3, or with 8 languages in AC3...so there must've been more sources for errors.

AVI-Mux GUI uses one chunk per 64 ms of AC3 audio.

BTW, does that overhead size calculator distinguish between standard and opendml AVI? That's a difference...ps. Searching the forums, I got the following thread but I didn't fully understand alexnoe's explanation Unfortunately, the exact calculation of AVI overhead is not easy :( I've never used any of these calculators...

Maybe I should set up an explanation and give some insides into the AVI file format...