Wrong AAC bitrate

[daWsOn_s]

Hello
I used to encode my stereo AC3 tv series to AAC 128kbit CBR with Megui and I've found that the final size (audio + video CBR) calculated before with bitrate calculator was always 1 MB bigger but I haven't take it seriously since today.

Now I'm encoding a 106 minutes movie to be stored in a DVD 4,38GB with other 2 movies so 1493 MB per movie and here the filesize is very important.
I found the aac 5.1 track set at 384 kbit CBR has a real bitrate at 414kbit, same thing the other track 128 kbit that is become 131 kbit. So I have a final MKV/MP4 size 15MB bigger. This means I can't put 3 movies on a DVD if the bitrate is not respected at all!

Why this, is that normal, what can I do to have a precise audio size like MP3?

[daWsOn_s]

I controlled the file is 30-40MB bigger :(:(

I search on the Internet and I read that it is a particularity of AAC to compress -/+ 5kbit range of the chosen bitrate. This is really a bad notice for me cause in this way I can never have a real CBR bitrate = fixed file size.

But, why is it called CBR? are ABR/CBR the same thing? :scared:

[Dark Shikari]

I controlled the file is 30-40MB bigger :(:(

I search on the Internet and I read that it is a particularity of AAC to compress -/+ 5kbit range of the chosen bitrate. This is really a bad notice for me cause in this way I can never have a real CBR bitrate = fixed file size.

But, why is it called CBR? are ABR/CBR the same thing? :scared:
CBR = constant bitrate = bitrate never changes
ABR = average bitrate = bitrate varies over the course of the audio clip, with the average bitrate being the value you specify. More complex portions get more bitrate.

ABR is inherently better quality, for obvious reasons.

AAC is inherently variable bitrate; even CBR AAC, I believe, varies over the course of a "buffer size", where each section of audio of length "buffer size" averages the same bitrate.

[daWsOn_s]

So if there's no change to have a desired file size I can't see the usefulness of CBR.

[Skelsgard]

Have you tried with ABR 2-passes in neroaacenc?

Later

[daWsOn_s]

Is that possibile with megui?

[Sharktooth]

no it's not possible. i found it weird though, since i did a lot of encodes with nero AAC and the encoder rate control never failed...
what version of neroaacenc are you using?

[daWsOn_s]

1.0.7.0

I guess it's not the last version.

Anyway how do you convert?

[Sharktooth]

get the latest version first, it will probably solve your problem.

[daWsOn_s]

get the latest version first, it will probably solve your problem.

Done, same thing. :angry:

[Dark Shikari]

Wait.

Are you confusing kilobits (1000 bits) with kilobits (1024 bits)? that would explain the discrepency.

[daWsOn_s]

I'm considering 1024 bits

[Sharktooth]

you should consider 1000bits... not 1024, coz 1kbit is exactly 1000bits, while 1kbyte = 1024 bytes. also... try also using the normal encoder and not the SSE one (IIRC it's buggy)...

[daWsOn_s]

I'm using the SEE one, I will try.

Anyway I don't get the point.

If I chose 128 CBR and I get 131kbit what 1000bits has to do with that?

[Dark Shikari]

I'm using the SEE one, I will try.

Anyway I don't get the point.

If I chose 128 CBR and I get 131kbit what 1000bits has to do with that?128 * 1024 = 131072

[daWsOn_s]

128 * 1024 = 131072

Sharktooth said before that 128kbit is 128000 so:

384 * 1024 is not 414000 and if I use ABR I get something near 128kbit but always a bigger file.

MP3 codec uses 128kbit correctly.


I'm really confused:o

[Sharktooth]

yes but when you look at the filesize you should consider the 1024 (2^8) factor.
For example:
1 MByte = 1,024 KByte = 1,048,576 Bytes = 8,388,608 bits = 8,388.608 Kbits = 8.389 Mbits (rounded)
or
1 Mbit = 1,000 Kbits = 1,000,000 bits = 125,000 Bytes = 122.071 KBytes (rounded) = 0.119 MBytes (rounded)

so a file size of 132.934 KB is 129 MB (rounded with no decimals)

If you took into consideration all of that and you still think the filesize is wrong, then there should be a problem in the encoder ratecontrol. Have you tried using the non-SSE version?

[daWsOn_s]

I do consider those values.

Anyway I found the TWO causes of my calculations:

- The buggy SSE encoder = now I get the correct final size
- The buggy megui's bitrate calculator = the final audio size is wrong. It says that a 106,11 minutes audio file at 320kbit will be 243MB instead of 248,67MB. 97,3MB instead of 99,4MB...that's why setting all the bitrates the final muxed file was bigger of 30MB (considering the buggy SSE encoder too). Or maybe the calculator considers 128kbit like 125kbit (128000 bit) and 320 like 312.

:eek: If so I can't see why

[daWsOn_s]

Oh my God I'm getting nuts.

Same audio file in mp3 will be exactly 97,3MB like Megui says.

Am I right that MP3 considers 1kbit like 1024bits and ACC 1kbit like 1000bits? :scared:ù


AAC CBR 320kbit comes out like 345kbit

[Atak_Snajpera]

Did you try CT-AAC instead of nero? CT uses normal formula. 131072 bps = 128 kbps

[daWsOn_s]

Did you try CT-AAC instead of nero? CT uses normal formula. 131072 bps = 128 kbps

No, where can I get it?

[Sharktooth]

get a free version of winamp and install it.
copy enc_aacplus.dll and nscrt.dll from the winamp_installation_folder\plugins into megui_installation_folder\tools\enc_aacplus
dont forget to use a CT profile...

[daWsOn_s]

It works!

Anyway what's the difference between Nero e CT AAC?

Maybe, if megui used the real bitrate for Nero AAC instead of the classic 128=125kbit of all the codecs (i guess video ones too) there would not be mistakes :rolleyes:

[Atak_Snajpera]

Anyway what's the difference between Nero e CT AAC?

In terms of quality probably very small (VBR vs ABR in CT)
CT in other side is alot faster during encoding than nero
http://forum.doom9.org/showthread.php?p=1066876#post1066876