MUXing an MP2 file with AC3 audio in TMPGEnc-No size change?

[ButchNapkins]

Hi there.. I've been converting the audio for several of my MP2s to AC3 Dolby Digital, with the intent to cut down on filespace while still preserving the overall quality. I had originally started out with my three MP2s totalling around 6.5 gb, and since I want them all to fit on a DVD, I shrank each one from 8000 kbps to 5000 using TMPGEnc - This brought the size of the three to 4.04 gb (note - I did keep my original 6.5 gb MP2 files though, just to compare later). Decided then that I'd convert the audio streams to AC3 to save even more space.. Well, I just finished MUXing my first MP2 file back together with the new DD audio I created using it's original WAV.. It's the exact same size as the MP2 that I'd shrunk from 8000kbps to 5000, the one with uncompressed audio!! Here's the full process I went through, using my smallest MP2 as an example. Originally this file was 184 mb, but after the shrink, it was 120 mb. The goal was to make it even smaller through AC3 converting:


First, I ran the original (184 mb) file through Virtualdub, and saved the resulting audio stream to a 35+ mb WAV file. Then after having considerable trouble with Besweet and Soft Encode I was able to use Scenarist to transfer the WAV file to a 8.41 mb AC3 file. So far, so good! Okay, so I load the AC3 and the original (184 mb) MP2 into TMPGEnc.. I used the original MP2 because I figured that since I was using the same settings from when I first resized the files, best to just do it over with the original file - Everything was set the same as far as I could see, except now I was using an AC3 track to combine with the video. The resulting file was 120 mb - same as the file I got when I initially resized the clip from 184 mb.. Shouldn't it be less? The AC3 was 1/3 the size of the initial WAV that I scanned from the file. I thought perhaps TMPGEnc might somehow be combining the audio or something (yes I was reaching!), so I loaded the original (184 mb) MP2 into M2-Edit Pro.. I saved only the video from the file (175 mb, which was odd in itself - what, the audio was only 9 mb uncompressed?), then took that and combined it with the AC3 in TMPGEnc using the same settings... 120 mb!! What!?


Can anyone shed a little light on this? I don't understand why the file sizes are the same when one file has compressed audio and the other has uncompressed. Please advise if possible, and thanks in advance.


Butch

[DJRumpy]

I've never seen an appreciable difference between AC3 and MP2 compression for audio. The audio portion of a DVD normally takes up such a small amount of space that it becomes irrelevant to the entire DVD size.

I had originally started out with my three MP2s totalling around 6.5 gb, and since I want them all to fit on a DVD, I shrank each one from 8000 kbps to 5000 using TMPGEnc

These numbers sound like video encoding bitrate. Can you verify? Your audio should be between 128 & 384 Kbs, not 5000-8000 kbps.

[ButchNapkins]

I've never seen an appreciable difference between AC3 and MP2 compression for audio. The audio portion of a DVD normally takes up such a small amount of space that it becomes irrelevant to the entire DVD size.

I had originally started out with my three MP2s totalling around 6.5 gb, and since I want them all to fit on a DVD, I shrank each one from 8000 kbps to 5000 using TMPGEnc

These numbers sound like video encoding bitrate. Can you verify? Your audio should be between 128 & 384 Kbs, not 5000-8000 kbps.

That's true, those numbers are the video encoding bitrate.. The audio was at 384 kbs, 48000Hz. Am I wrong in thinking that the WAV's I rip from the MP2's in Virtualdub are identical in size to the uncompressed audio in the Mp2's? 'Cos if so, I see an appreciable difference between files - My largest MP2 is 2.98 gb, the wav ripped from it was 860mb, and the ac3 file I encoded from that is 214 mb.. Am I looking at this correctly?


Butch

[DJRumpy]

That makes a bit more sense, but there's something wrong here too:

My largest MP2 is 2.98 gb, the wav ripped from it was 860mb, and the ac3 file I encoded from that is 214 mb.. Am I looking at this correctly?

Your MP2 should have been about the same size as your AC3 file. The WAV you extracted should have been the big one (2.98GB). I think you may have them confused.

For example, if your MP2 is 200MB, and you extract it to a WAV file, the wave should have been about 1-2GB (I don't know the compression ratio of the top of my head).

That WAV file, once it was re-compressed to AC3 should have been back around 200MB (give or take).

What your describing is completely backward from this.

[ButchNapkins]

Your MP2 should have been about the same size as your AC3 file. The WAV you extracted should have been the big one (2.98GB). I think you may have them confused.

Hi again.. I used the guide from this site to convert the audio from my mp2's to WAV - https://www.videohelp.com/virtualdubaudio.htm.. I followed it exactly, except I do change the sampling rate to 48000Hz (the guide says to use 44100Hz). I haven't messed with any other settings, and the resulting WAV file I get from doing this with my 2.98 gb MP2 file is always 860 mb (I've done this several times).. Once I convert it to ac3 it's down to 214 mb. If what you say is true about how the WAV size should surpass the size of it's source MP2, then Virtualdub must somehow be compressing the heck out of the WAV. To support your info, however, at one point I decided to change one of the ac3 files I created BACK to wav (using ac3decoder) and the file size was enormous. What am I missing here?


Butch

[ButchNapkins]

I've never seen an appreciable difference between AC3 and MP2 compression for audio. The audio portion of a DVD normally takes up such a small amount of space that it becomes irrelevant to the entire DVD size.

Wait a minute here! I think I might have figured it out - An MP2 file already has compressed audio, correct? If that's the case, then THAT is why my end files are of similiar size to the intial mp2s.. But I'd thought that when you capture, your intial files would contain uncompressed audio unless you compress it yourself? I read that somewhere anyway..


Butch

[DJRumpy]

MP2, and AC3 are compressed. WAV can also be compressed, but again, most people don't bother. I suspect that your MP2 was either corrupt, or using a bitrate that far exceeded the needs of the audio. Your MP2's, and AC3's should be smaller than any WAV file. Wav compression, assuming everything else is equal as far as frequency, and bits, pales in comparison. When it comes to compression, WAV sucks, not to put too fine a point on it.

What bitrate is your MP2 using? You should be able to use 'MpegProperties' from the tools section to the left to see the bitrate. I would suggest you extract it to WAV (uncompressed is fine). Use BeSweet, and the BeSweet GUI (also in the tools section) to re-compress it to MP2 (use the standard 224Kb/sec for bitrate), and check the filesize. I would bet it will be around the same size as your AC3 file.