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AC3 5.1 to AAC 5.1 Suitable Mode/Profile/Bitrate?
Makaveli7184
post Jun 21 2012, 15:25
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I have a large number of 5.1 AC3 audio file encoded at CBR 384 kbps that i want to transcode to AAC. Channel mapping is not a problem. Bytes are scarce, so my goal is to minimize bitrate while preserving audio transparency.
Now i've read in various online posts that AAC standards recommend around 260 kbps for 5.1 audio. I'm using a certain application (i'm not interested in suggestions of using another one or another encoder for that matter) that uses up-to-date AAC codecs. This application gives me the option of encoding in CBR AAC (bitrate ranging from 4 to 448 kbps in increments of 4, and profiles HE2/HE/LC are completely user selected) and VBR AAC (where i can choose a quality preset from 1 to 5, 5 being highest quality and largest filesize and where profile is strictly automatic, HE-AAC 2 kicks in when using Q1, HE-AAC when using Q2, and from Q3 to Q5, format profile is LC).

Giving all the above, and knowing that audio transparency and quality is usually subjective, what would be a good option?
My initial preference is using VBR Q3/LC which gives me 5.1 audio files encoded at around 250 ~ 300 kbps. Do i need to go higher?? Can i go lower without losing perceptive quality?? Should i consider lower CBR with HE profile??

Waiting for some suggestions. Thank you.
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john33
post Jun 26 2012, 17:34
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When decoding a lossy audio file, the content that was discarded by the encoder is replaced by noise (it may not be audible, but it's there) to pad out the data. Whatever encoder you use to re-encode that wave data will not 'know' what is real audio data and what is the added noise. To go from ac3 to ac3, you will need to increase the bitrate to reduce the quality loss, but whatever bitrate you choose, there will still be further degradation in quality. To go from one lossy format to another often appears to work better as the psychoacoustic models will differ. However, it is impossible to suggest a suitable bitrate in the example given as it will be content and audience dependent. You may get away with using a similar bitrate but it is unlikely that a bitrate much lower will be acceptable.

As has already been said, ac3 5.1 at 384kbps is already pushing the limits of what ac3 can achieve and transcoding to any other format at a lower bitrate with any level of transparency will be a challenge. Only you can determine whether the result will be acceptable to you. Most people, I would suggest, would leave the audio alone as the bitrate is not excessive by any stretch and any savings in size achieved are likely to be minimal.


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Makaveli7184
post Jun 26 2012, 18:35
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QUOTE (onkl @ Jun 26 2012, 15:14) *
Lossy to lossy is ugly and not so advanced formats like AC3 at very low bitrates probably suffer most. So when you re-encode your AC3 file at the same bitrate, quality will be worse. If it's still good enough for your ears, only you can tell.
Simple cutting and adding silence could be done without re-encoding.


QUOTE (john33 @ Jun 26 2012, 16:34) *
When decoding a lossy audio file, the content that was discarded by the encoder is replaced by noise (it may not be audible, but it's there) to pad out the data. Whatever encoder you use to re-encode that wave data will not 'know' what is real audio data and what is the added noise. To go from ac3 to ac3, you will need to increase the bitrate to reduce the quality loss, but whatever bitrate you choose, there will still be further degradation in quality. To go from one lossy format to another often appears to work better as the psychoacoustic models will differ. However, it is impossible to suggest a suitable bitrate in the example given as it will be content and audience dependent. You may get away with using a similar bitrate but it is unlikely that a bitrate much lower will be acceptable.

As has already been said, ac3 5.1 at 384kbps is already pushing the limits of what ac3 can achieve and transcoding to any other format at a lower bitrate with any level of transparency will be a challenge. Only you can determine whether the result will be acceptable to you. Most people, I would suggest, would leave the audio alone as the bitrate is not excessive by any stretch and any savings in size achieved are likely to be minimal.

Thank you for these informative and constructive responses.
DelayCut will be of great help in some DVD authoring/backup projects.

In my current case though, audio editing might include fading in/out in addition to some cutting, so re-encoding can't be avoided. Also, i want to re-encode in AAC for HW playback reasons.
The AC3 to AC3 re-encoding question came as a benchmark comparative mean for me to choose my AAC encoding bitrate, since i now trust that AAC vbr quality preset 3 for 5.1 audio is pretty much equivalent to AC3 @ 384 kbps. However, your additional explanation of the lossy to lossy encoding flaws convinced me of taking the safe route (for me that is) and re-encoding in AAC vbr quality preset 4 (which yields on average 360~400 kbps in 5.1 audio).

This post has been edited by Makaveli7184: Jun 26 2012, 18:38
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Posts in this topic
- Makaveli7184   AC3 5.1 to AAC 5.1 Suitable Mode/Profile/Bitrate?   Jun 21 2012, 15:25
- - probedb   Do some tests and find out. You're going from ...   Jun 21 2012, 16:02
- - Makaveli7184   Well, i appreciate your input, but that's not ...   Jun 21 2012, 16:39
- - onkl   5.1 AC3 with only 384 kbps is already quite low. I...   Jun 21 2012, 22:00
- - Makaveli7184   Well, let's say that i wanted to convert an or...   Jun 25 2012, 20:29
|- - saratoga   QUOTE (Makaveli7184 @ Jun 25 2012, 15:29)...   Jun 26 2012, 02:44
|- - Makaveli7184   QUOTE (saratoga @ Jun 26 2012, 01:44) QUO...   Jun 26 2012, 03:05
|- - probedb   QUOTE (Makaveli7184 @ Jun 26 2012, 03:05)...   Jun 26 2012, 08:03
- - saratoga   If you're just doing it for fun, might as well...   Jun 26 2012, 04:05
- - Makaveli7184   Wow... The hostility on this forum is crazy.. Seco...   Jun 26 2012, 15:55
- - onkl   Lossy to lossy is ugly and not so advanced formats...   Jun 26 2012, 16:14
|- - jetpower   QUOTE (onkl @ Jun 26 2012, 17:14) Simple ...   Jun 26 2012, 17:30
|- - john33   QUOTE (jetpower @ Jun 26 2012, 17:30) QUO...   Jun 26 2012, 17:40
- - john33   When decoding a lossy audio file, the content that...   Jun 26 2012, 17:34
|- - Makaveli7184   QUOTE (onkl @ Jun 26 2012, 15:14) Lossy t...   Jun 26 2012, 18:35
- - Makaveli7184   Also, is there a decent "DelayCut" equiv...   Jun 26 2012, 18:39
- - onkl   MP3directcut which can also do fading, so somethin...   Jun 26 2012, 18:45
|- - Makaveli7184   QUOTE (onkl @ Jun 26 2012, 17:45) MP3dire...   Jun 26 2012, 19:04
|- - Dynamic   QUOTE (Makaveli7184 @ Jun 26 2012, 18:04)...   Jun 28 2012, 07:03
- - Makaveli7184   Thanks for everyone who contributed in this thread...   Jul 10 2012, 20:03


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