AC3 VS vbrMP3

[GreatKent]

In the Bitrate Tab of GKnot, when I change the audio from '1x ac3' to '1x vbr-mp3', the the average Bitrate doesn't rise up, but drops slightly. That really surprises me, since I do think AC3 should take much more space than VBR-MP3 as it consists of 6 channels. Could any1 please tell the reason?
Thanks!

[jggimi]

That's an overhead calculation, for the interleaving of audio and video. The storage consumed by the audio
track(s) will be found to the left, in the Audio A (and optionally Audio B) settings area.

[GreatKent]

Well, I wonder if the ac3 audio tracks are encoded into MP3 format, as I don't see BeSweet running whenever I choose ac3 audio.

[GreatKent]

By the way, would you mind telling me what the Frames(next to '1x ac3' in 'Interleaving & AVI-Overhead') means? And what Frames number is better? higher or lower?
Thanks!

[hakko504]

When you choose AC3 audio it is only muxed, not converted into MP3. And lower settings means more overhead, i.e. lower bitrate available for video, but it also means better audio synch! Usually a setting of 2 or 3 frames is optimal for AC3.