AC3->AAC transcode, how do you know what the delay should be set to?

[colema19]

I'm using AACmachine to try and convert an AC3 track to AAC.
When I had auto-detect max gain checked, I could see BeSweet open and run for about 10 seconds as it scanned the file and reported the db level, but nothing else seemed to happen.

So I unchecked that and clicked the start button again and the AACmachine app is now eating 100% CPU, but I can't tell if it's actually doing anything. Is it hung or do I need to wait more?

Thanks!

[colema19]

Ok NM, I think I got it. I updated my BeSweet and I think I was missing BeSlice.

[colema19]

Ok so I'm following the AACMachine guide to try and get an AAC encoded stream into my DVD rip. I noticed that the guide example shows a -111 msec delay. How do you know what this value should be?

Thanks!

[KpeX]

@colema19

I merged your two threads. As far as finding the delay value, it depends on how you demuxed your AC3. If you used DVD2AVI, which is the most common way, the delay is in the filename. If you are using besweet in vob input mode, you just need to use -ota ( -d auto ) in your command line.

[colema19]

Ok thanks! Yeah I finally noticed that when I expanded my column width, so I feel stupid now. :)

Uh next stupid incoming that feels like being bashed with a rock. Is an AC3 stream AAC encoded already? Is there any point in transcoding AC3 to AAC? I've started to confuse myself. :)

Thanks!

[KpeX]

No, AC3 (aka Audio Coding 3 aka Dolby Digital aka A-52) and AAC (aka Advanced Audio Coding) are two completely different audio formats. AAC is not supported in the DVD specs, AC3 is supported and is the most common format used on DVDs. The main reason to transcode to AAC would be to reduce the size. You will never improve quality by transcoding. However you could in many cases transcode to AAC without a noticeable quality loss and a significant decrease in size.

AAC is seen by many as the successor to AC3. AC3 for the most part is not a high quality codec, not because it is poorly engineered, but poorly implemented, most AC3 tracks are CBR whereas AAC implementations such as Nero take advantage of features like VBR and SBR. For these reasons when reducing bitrate AAC is much more efficient than AC3.

[colema19]

I guess I'm not understanding something else then. This is why I got confused. If I rencode the AC3 stream to AAC on the normal setting using AACMachine, and preserve 5.1 it shows that the stream size would be 440kbps, which isn't really that much better then AC3@448kbps. Does that mean I can get away with one of the smaller settings with minimal perceivable loss of quality?

Oh, and I forgot to ask, the Azid downmix, in particular the LFE to LR channels, does that only occur if a downmix is happening? If I reencode from 5.1 to 5.1 should I change the LFE downmix on the LRs to 0 from the default -3db?


Thanks!

[KpeX]

Originally posted by colema19
I guess I'm not understanding something else then. This is why I got confused. If I rencode the AC3 stream to AAC on the normal setting using AACMachine, and preserve 5.1 it shows that the stream size would be 440kbps, which isn't really that much better then AC3@448kbps. Does that mean I can get away with one of the smaller settings with minimal perceivable loss of quality?
Possibly, yes. You're right that there's probably not much point in transcoding to normal @ 5.1 Try one of the lower settings and let your ears tell you the answer.

Originally posted by colema19
Oh, and I forgot to ask, the Azid downmix, in particular the LFE to LR channels, does that only occur if a downmix is happening? If I reencode from 5.1 to 5.1 should I change the LFE downmix on the LRs to 0 from the default -3db? I would think these settings only have precedence if a downmix occurs, but I haven't used AACMachine for a while. Check your besweet logfile to be sure.

[mfluder]

Originally posted by colema19
I guess I'm not understanding something else then. This is why I got confused. If I rencode the AC3 stream to AAC on the normal setting using AACMachine, and preserve 5.1 it shows that the stream size would be 440kbps, which isn't really that much better then AC3@448kbps. Does that mean I can get away with one of the smaller settings with minimal perceivable loss of quality?
I suggest you use DSPguru's OagMachine from http://dspguru.notrace.dk and use Nero's AAC encoder dll's. Basically Nero's AAC encoder is more efficient in 5.1 encoding and should give you better quality at any bitrate compared to psytel or faac encoders. Actually developers who were working on these encoders (Ivan and Menno) are now working on the Nero one. You can find more information about using BeSweet and Nero's dll's in this (http://forum.doom9.org/showthread.php?threadid=62672) thread.

Oh, and I forgot to ask, the Azid downmix, in particular the LFE to LR channels, does that only occur if a downmix is happening? If I reencode from 5.1 to 5.1 should I change the LFE downmix on the LRs to 0 from the default -3db?
When doing 5.1 to 5.1 transcoding LFE downmix is not required because you are encoding LFE channel as one separate channel. Just turn that option off.

If you have some questions along the way feel free to ask them.

mfluder