[MEncoder-users] size of AC3

[Trent Piepho]

On Tue, 20 Jun 2006 Abu Zaher wrote:
>
> Oh, I forgot. I have seen 384 kbps ac3 too, if only a few though.
>
> But all the ac3 we find these days are CBR, isn't that right?

AC3 can not be VBR the way something like mpeg video is.  For a given audio
sampling rate (of which AC3 supports 32, 44.1, 48 kHz only) there is a set
list of allowable bitrates.

Each AC3 frame is made up a constant number of audio samples, and is a fixed
sized based on the sampling rate and bitrate.  If the audio happens to need
less space to compress, the frame is padded so that it is exactly the size it
is supposed to be.

So in this sense, AC3 can only be perfectly constant bit rate.  Something like
CBR MPEG-1 isn't perfectly constant, each frame is not exactly the same size
like in AC3.

It's not that simple however.  Each AC3 frame (about 1-2 KB) has a header that
specifies the channels, sampling rate, bit rate and so on.  You could have an
AC3 stream that is normally 192 kbps 2.0 audio and then switches to 384 kbps
5.1 audio for a few seconds at a time for each surround effect.  No one would
actually do that, but they could.  What has been done is to change from one
bit rate to another in a single stream.  Though you will probably never see
this done on a DVD.