How is bitrate distributed in AC3 multichannel streams?

[zambelli]

How is bitrate distributed between channels in AC3 audio? For example, in a 4.1 channel AC3 stream at total 640 kbps, is every channel allocated 128 kbps? Or does the LFE channel get smaller bandwidth? Is there such a concept as variable bitrate allocation between channels?

[Gabriel_Bouvigne]

There is a variable allocation between channels, and channels coupling.

[Skelsgard]

I believe (might be wrong) that is something like 5 1/3 (5x for each full range channel plus 1/3x for LFE). So: 448 is 84 x 5 + [1/3 84] x 1 = 420 + 28, OR 60 x 5 + [1/3 60] x 1 = 320.
I believe it is:
40 --> 224
48 --> 256
60 --> 320
72 --> 384
84 --> 448
96 --> 512
108 --> 576
120 --> 640