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Topic: qaac in CBR mode (Read 6876 times) previous topic - next topic
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qaac in CBR mode

Hi guys,

is it normal for AAC to have small variance (+/- some kbps) with the bitrate when using CBR mode? Never observed this with LAME. Maybe it's different whit other AAC encoders.

I was testing different encoded AAC files (320 kbps CBR) and remember some of them not playing correctly. Later i remembered that some of them had their Album Art not removed, so this was probably the only problem why this happened.  But i won't be able to test it until this weekend. Tested on a JVC audio player.

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I would even prefer just a variable bitrate ~256 kbps ("-v256 -q2"). But than again i don't know if the decoder will support bitrates above 320 kbps, which occur with this settings. Guess i will have to test it out.

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MediaInfo output:
Code: [Select]
General
Writing application                      : qaac 2.47, CoreAudioToolbox 7.9.9.4, AAC-LC Encoder, CBR 320kbps, Quality 96

Audio
ID                                       : 1
Format                                   : AAC
Format/Info                              : Advanced Audio Codec
Format profile                           : LC
Codec ID                                 : 40
Duration                                 : 3mn 29s
Bit rate mode                            : Variable
Bit rate                                 : 320 Kbps
Maximum bit rate                         : 330 Kbps
Channel(s)                               : 2 channels
Channel positions                        : Front: L R
Sampling rate                            : 44.1 KHz
Compression mode                         : Lossy

Thanks
Mr. Gelpke

qaac in CBR mode

Reply #1 –
Don't mix up MP3 and AAC - they're different codecs. LAME is MP3. QAAC is AAC.

So-called CBR in iTunes/QAAC is more like Constrained VBR (CVBR). Bitrate tends to increase for transients and similar situations, but the overall bitrate is fairly consistent. AAC decoders should be OK with >320kbps.

MP3 is different and has fixed choices of frame size, so LAME can produce true CBR (although in practice use of bit reservoir acts a bit like CVBR). Compliant MP3, unlike AAC, doesn't support >320kbps frame sizes.
Dynamic – the artist formerly known as DickD

qaac in CBR mode

Reply #2 –
Thanks for the explanation, this makes sense now. I am pretty sure that the player just doesn't like album art. I meant MP3 when i mentioned LAME, just forgot to make it clear.

qaac in CBR mode

Reply #3 –
Quote from: Dynamic on
So-called CBR in iTunes/QAAC is more like Constrained VBR (CVBR).

So-called CBR in Apple/qaac is CBR.

https://github.com/nu774/qaac/wiki/Command-Line-Options

Code: [Select]
-a, --abr <bitrate>    AAC ABR mode / bitrate
-V, --tvbr <n>         AAC True VBR mode / quality [0-127]
-v, --cvbr <bitrate>   AAC Constrained VBR mode / bitrate
-c, --cbr <bitrate>    AAC CBR mode / bitrate

qaac in CBR mode

Reply #4 –
Hi eahm,

"--ignorelength -s --no-optimize -c 320 -o %d -"
This is the command line i used in foobar2000.

Even if foobar2000 says that the file is CBR, it still is not the same as CBR in MP3.

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If i add "--stat" to the command line, this bitrate statistics are created:
Big Barn Bed.stat.txt

I guess it shows the bitrate per frame.

qaac in CBR mode

Reply #5 –
Quote from: rudolf.gelpke on
Even if foobar2000 says that the file is CBR, it still is not the same as CBR in MP3.

Of course is not, one is MP3 and the other is MP4. Is this what you're trying to say? I don't know the specifics of the encoding and I don't really care, you have to ask some expert for that but I am pretty sure that -c with qaac creates a CBR AAC file.

qaac in CBR mode

Reply #6 –
I don't doubt either that qaac creates correct AAC CBR files.

Quote from: eahm on
Quote from: Dynamic on
So-called CBR in iTunes/QAAC is more like Constrained VBR (CVBR).

So-called CBR in Apple/qaac is CBR.

I just answered because compared to MP3, CBR in qaac is a little like Constrained VBR with +- some kbps. Like Dynamic explained. So yes, i was just comparing the two, and of course they are not the same. I thought you meant that it would produce CBR like in MP3, so thats why i posted the pastebin. Yeah, just some misunderstandings from me.