Why ac3 file so small?

I have encoded a bunch of AC3 2.0 files and they average in size at about 184 megs. This is for about a 2 hour video. Data rate was at 192 kbps. I have a 2 hour file that is AC3 5.1 encoded and creating the AC3 file came out to 206 megs. It has a data rate of 224 kbps. I would have thought this file should have been 4 or 5 hundred megs no. Using Sonic Soft Encoder.

FileSize = BitRate * Time If you increase the BitRate from 192 to 224 New_FileSize = (224/192) * Old_FileSize = 1.17 * Old_FileSize Your files are ok. If you increase the channels (2.0 -> 5.1) you must increase the BitRate also, at least 384 Kb/s is recommended.

Thanks for the info. Figured the extra channels in and of themselves would increase it. Is this necessary to increase the bitrate that much or just recommended? In Sonic Soft Encoder, I switched the setting from 2.0 to 5.1 and it chose 224 kbps itself.

Habitual values for ac3 5.1 are 384 and 448 Kb/s, but I don't know what kind of upmix you make. I don't understand for what you convert a 2.0 to a 5.1, afaik, Sonic Foundry Soft Encoder only can fill the rest of channels with silence.

Misunderstanding. I am not trying to convert from 2.0 to 5.1. I had Soft Encoder set to 2.0 for some wave files and it was set to 192 kbps. I now have a 5.1 wave file so went into Soft Encoder and selected 3,2 in the encoder options. It automatically set the bitrate to 224.

Then use at last 384 Kb/s and recommended 448 Kb/s.

Thanks for all the info. I'll try 448 since that's what the source seems to be at as long as I do not end up reducing my video size tremendously because of it.