Transcoding AC3s as part of Big 3 DVD Backup

[AndyP]

Hi

I was recently trying to backup SPR using the big 3 (inc. NuMenu). All worked and with numenu I could increase the bitrate for the main movie from about 2300 to 2500, but it was still pretty poor quailty (even with CCE). It then struck me that the AC3 was about 550MB (448kbps 5.1 AC3). I redid it with besweet to a 224kbps 5.1 AC3 sized about 260MB, which pushed the bitrate for the movie up to about 2800. This looks much better.

I have done a search and can find very little about doing this. It seems to have worked well, so why are more people not doing it?? I know there are some problems with the ac3enc in besweet but am I missing something dreadfully obvious??

Cheers
Andy

[hendrix]

have you actually listened to 5.1 @ 225kbps? i would think it wouldn't sound too good...the sound range for 5.1 @ 448kbps is 20hz -20khz which is broadcast quality...the sonic range for 5.1 @ 225kbps is 9khz which may sound more annoying than a bad AM radio. i've done some mastering with 2.0 dolby digital surround @ 192kbps (20hz-20khz) anything higher would be wasting space - i do my mixing with ProTools and prefer to use SoftEncode and Surcode to produce my AC3 and DTS tracks.

i prefer good sound on my system but if you prefer to downsample your .ac3 files then thats your choice. it's all about preference :D

[AndyP]

Hi

This was the first one, I admit, but listening on my PC (4.1 creative speaker system) I personally couldn't hear a difference. Appreciate the thoughts though. What is the minimum rate you would recommend for 5.1??

Cheers
Andy

[hendrix]

for 5.1 i would encode at 448 although ive seen dvds encoded at 384, i dont know why because at 384 gives you an audio bandwidth of 18.0khz

for 2.0 i would encode at 196 anything over is just wasting space

i heard of some compatibily problems with Besweet, i never used that program so i cant verify

[KpeX]

@hendrix

Where are you getting your audio bandwidth numbers?

@AndyP

It is true that BeSweet's ac3enc is considered buggy and extremely unstable, and if you do some searching you'll find that DSPguru recommends it only for experimental use. This means that if you do use it and have problems with your DVD, you have no right to post any problems or ask for any fixes regarding ac3enc. However you'll find certain people (me ;)) that haven't experienced any problems with ac3enc's output. If you want to be totally safe your best bet is a commercial AC3 encoder. Please see the AC3 FAQ (http://forum.doom9.org/showthread.php?s=&threadid=18318) #9 for more info on minimum bitrates and reencoding techniques.

[hendrix]

@KpeX
im getting my numbers from Sonic Foundry's SoftEncode and Vegas 4 - is there something im missing :confused:

[KpeX]

Originally posted by hendrix
@KpeX
im getting my numbers from Sonic Foundry's SoftEncode and Vegas 4 - is there something im missing :confused:
No, I was just curious. I don't have a lot of experience with transcoding AC3, but when I have, the quality loss has not been very high, not as high as it would seem if your numbers were accurate. But I haven't experimented with either of those programs.

[hendrix]

i will try some tests tonight - i will do some 5.1 .ac3 encoding @ 224 to and hear how it sounds and give you my opionion.