Newbie AC3 issue

[soundboy]

Hi,

I'm editing a concert video and I'm taking my first steps into 5.1 audio. I got a good quality stereo track from the concert and upmixed it to 5.1 by following the steps in several guides I found on this board, especially this one (http://forum.doom9.org/showthread.php?s=&threadid=59949)

Now, after filtering and separating the different frequencies from my audio, I ended up with my 6 mono WAV files. So far so good. I encoded the 6 wav files into a single AC3 file, using BeSweet with the BeSweet GUI (following the directions in SurroundBoy's guide). Now, what puzzles me is that the mono WAVs are about 560 MB each, and the new encoded AC3 is less than 140 MB. (at 384 kbps, 48kHz)... is that right? Now, I'm kinda screwed because I don't have a proper 5.1 setup in my house to test the sound, but through my PC's 2.1 speakers and my TV's normal stereo speakers, the stereo track sounds much better than the 5.1 AC3 track. I just want to know, is this all normal?

[Gilron]

You didn't specify the total duration of the recording or what kind of mono samples we are talking about, but if we assume you are talking about 16-bit mono WAVs at 48kHz, they sum up to 4608 kbit/s (6 channels), which is exactly 12 times the bit rate of your produced AC3 file. Now if we compare file sizes your WAVs are ~24 times the size of your AC3 file.

My conclusion would be something is wrong if you have 16 bit WAVs, but if they are 8 bit instead it looks ok.

[soundboy]

Don't you mean that it would be OK if they were 32 bit instead of 8? because they are 32-bit. (I have no idea why I encoded them at 32 bits, I think I overlooked that).

So, is it normal for a 5.1 AC3 track to sound lousy (much worse than the stereo track) when played through a stereo setup?

[Gilron]

No, I meant 16 bit PCM format. 32-bit samples are very rare, high-end equipment use 24-bit for example. After you know in which PCM format your WAVs are recorded you can calculate the total bit rate and compare it to your AC3 bitrate and file sizes to see if the equation matches. To simplify: the ratio of the total bit rates should be equal to ratio of file sizes if the duration is the same.

It's hard to say anything about the quality, but in general the original stereo soundtrack should be better. How much, that is the question. There are many factors which have an effect - how the upmixing is done and how the downmixing back to stereo is done... etc