AC3 5.1 to AAC 5.1 Suitable Mode/Profile/Bitrate?

[Makaveli84]

Hello,
First of all, let me say that this is my first post here, and i really don't appreciate the 5 days waiting period between registration and posting :confused:... I mean nowadays, waiting periods for buying guns are less than that in many states in the U.S :D !!

Second, 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 posts online 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.

[Atak_Snajpera]

Second, 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.
5.1 AC3 384kbps is already strongly compressed! Do not touch it. Conversion to AAC only makes sense if your source is lossless (TrueHD/DTS-MS/PCM)

[Makaveli84]

Well, that's not really helpful. AAC transcoding is a must for me. Now based on that, i refer you to what i'm asking for in my initial post.
So, what to do?

[tebasuna51]

Always recommended AAC LC VBR

For AC3 5.1 384 Kb/s is enough a low setting, I don't know your AAC encoder (maybe is Fraunhofer).

With CLI recommended encoders:

NeroAacEnc -q 0.35 (Nero encoder)

qaac -V 45 (QuickTime encoder)

fhgaacenc --vbr 3 (Fraunhofer-Winamp encoder)

[shon3i]

If you bring about filesezie, only way to go is HE-AAC around 160kbps. In this case you will get smaller size and keep maximum possible quality. With LC-AAC you need simmilar bitrate as source and that lossy format change is not recommend and as Atak_Snajpera says there is no reason to reencode if source is not lossless.

Also try tebasuna51 recommendation.

[Makaveli84]

Always recommended AAC LC VBR

For AC3 5.1 384 Kb/s is enough a low setting, I don't know your AAC encoder (maybe is Fraunhofer).

With CLI recommended encoders:

NeroAacEnc -q 0.35 (Nero encoder)

qaac -V 45 (QuickTime encoder)

fhgaacenc --vbr 3 (Fraunhofer-Winamp encoder)

Indeed, i am using the Winamp Fraunhofer encoder, and indeed, my initial preference was using VBR 3 which yields an LC stream. I didn't want to get into these specifics because most people would then recommend different encoder and such.




If you bring about filesezie, only way to go is HE-AAC around 160kbps. In this case you will get smaller size and keep maximum possible quality. With LC-AAC you need simmilar bitrate as source and that lossy format change is not recommend and as Atak_Snajpera says there is no reason to reencode if source is not lossless.

Also try tebasuna51 recommendation.
One, i need to reencode to AAC, so again keeping the original AC3 tracks is not an option.
Two, tebasuna's recommended LC-AAC at a higher bitrate (-vbr 3 yields tracks in the range of 250~300 kbps) whilst you recommended HE-AAC at a lower bitrate (160 kbps). These are two contradictory suggestions.

What i'm looking for is the best trade off in quality and size. So what i'm asking is which is better in quality: HE at around 160 or LC at around 280??

[Atak_Snajpera]

lc 280 kbps.

why do you insist on re-encoding ac3 to aac? it is like converting jpeg to jpeg-xr aka hdphoto. no mather how much bitrate you use source ac3 384kbps will always sound better. did you calculate how many mb you will save on hdd?

[Makaveli84]

Thx for your input Atak.. AAC is for specific hardware and storage is not on HDD, so that's not the problem.
BTW, according to all the reading i've been doing, and to make a long story short, although there will be loss of quality in transcoding in general, converting AC3 5.1 @ 384 kbps to AAC 5.1 @ vbr 256 kbps yields the same quality as transcoding the original AC3 384kbps to another AC3 384 kbps (which means it's absolute minimal).
Now i know i don't have to go above AAC vbr 256 kbps.

[turab]

Thx for your input Atak.. AAC is for specific hardware and storage is not on HDD, so that's not the problem.
BTW, according to all the reading i've been doing, and to make a long story short, although there will be loss of quality in transcoding in general, converting AC3 5.1 @ 384 kbps to AAC 5.1 @ vbr 256 kbps yields the same quality as transcoding the original AC3 384kbps to another AC3 384 kbps (which means it's absolute minimal).
Now i know i don't have to go above AAC vbr 256 kbps.
Although AC3 384 kbps is roughly equivalent to AAC 256 kbps, your logic isn't correct. Generation loss is unavoidable, even when you're encoding at the same bitrate as the original. So, it isn't true that you don't have to go above 256 kbps with AAC. It depends on the listener and their equipment.

As for HE vs LC, the former is used when encoding at very low bitrates. One good use could be for recording speech. I can guarantee that HE @ 160 kbps will be inferior to LC @ 280 kbps, but whether there's a noticeable difference is your call. If there's isn't, you might as well go with the lower bitrate to save space. I suppose HE would be appropriate.