budwhite said:

Damn, I've always though the DD 5.1 on LD had 640 kbit/s.

But really, just 384. Lower bitrate than DVD, that's surprising.

I also read that the bitrate in cinemas for DD is just 320(!!!) That's shockingly low

On 35mm film, the AC3 bitrate has to remain low because there is very little physical room for the digital information to be printed onto the film stock, whilst also being very prone to damage (mainly because the AC3 track is printed between the sprocket holes) so a lot of the available data bandwidth must also be used for error correction. I'm guessing AC3 soundtracks stored on Laserdiscs have to contend with similar issues, due to LD being an analogue storage format like 35mm, hence the low AC3 bitrate.

This was (one of the reasons) why DTS was invented for 35mm film use; there is only a sync pulse track on the actual film stock which allows the DTS audio tracks to be contained on data CDs which are synced to the film. This negates the problems of data bandwidth limits and physical damage necessitating high error correction.

Technically (because AC3 soundtracks on 35mm film and DVD all use the same compression algorithms) if the AC3 soundtracks are both compressed from the same master, your DVD could have a higher fidelity soundtrack on it than the film projected in cinemas.