Hallo,

I read about AC3 meta data which should "tell the decoder" to play parts of the music / soundtrack very loud (used for explosions in movies, for example). Is it true, do they exist? And if so, why? I always thought AC3 had enough dynamics.

Regards, fileman.

Yes, there is many meta-data in AC3, if you want to know what is stored all in the meta-data, read ATSC A/52A: http://www.atsc.org/standards/a_52a.pdf
The thing you mean is called "dymnamic range compression", if you activate it load parts are not as load and silent parts are not that silent. This is very useful, because in a cinema the dynamic range can be much higher than on a normal television speaker. That allows you to use the same file/bitstream in cinemas and in home systems.
But the other way round, meta-data to extend dynamic range, I haven´t heard about that. But of course there is something in the stream to tell the decoder how load to play something in the movie. Otherwiese everythning would have exactly the same volume. I don´t know how it works, probably a global gain for one frame (1536 samples) like in mp3.