[ffmpeg] why using AAC codec the bitrate is alwais VBR? I need CBR

[marcorocchini]

Hi, soory but I'm a ***

why using ffmpeg with

-c:a aac

the AAC audio encoder, the resultant bitrate is alwais Variable Bit Rate ?

I need to encode using AAC and absolutly CBR, not VBR. How is possible that? thanks

[amaipaipai]

Why use a low quality aac encoder when you can use a high quality one?
https://wiki.hydrogenaud.io/index.php?title=AAC_encoders

Use Apple's, the best one available.
Download iTunes and extract with this script (it comes with MeGUI)

@echo off
setlocal disabledelayedexpansion

for /F "tokens=1,2*" %%i in ('reg query HKLM\Software\7-Zip /v Path') do (
    set "PATH=%%k;%PATH%"
)

7z >NUL
if not %errorlevel% == 0 (
    echo 7z.exe is required
    goto end
)

if not "%~1" == "" (
    set "installer=%~1"
) else if exist iTunes6464Setup.exe (
    set installer=iTunes6464Setup.exe
) else if exist iTunes64Setup.exe (
    set installer=iTunes64Setup.exe
) else if exist iTunesSetup.exe (
    set installer=iTunesSetup.exe
) else if exist QuickTimeInstaller.exe (
    set installer=QuickTimeInstaller.exe
) else (
    echo installer executable not found
    goto end
)

7z e -y "%installer%" AppleApplicationSupport.msi
7z e -y "%installer%" AppleApplicationSupport64.msi

if exist AppleApplicationSupport.msi (
    call :extract QTfiles AppleApplicationSupport.msi
) else (
    echo cannot extract AppleApplicationSupport.msi from installer
    goto end
)
if exist AppleApplicationSupport64.msi (
    call :extract QTfiles64 AppleApplicationSupport64.msi
)
goto end

:extract
mkdir %1

7z l -i!CoreAudioToolbox.* %2 | findstr CoreAudioToolbox

if errorlevel 1 (
    7z e -y -o%1 ^
             -i!*AppleApplicationSupport_ASL.dll ^
             -i!*AppleApplicationSupport_CoreAudioToolbox.dll ^
             -i!*AppleApplicationSupport_CoreFoundation.dll ^
             -i!*AppleApplicationSupport_icudt*.dll ^
             -i!*AppleApplicationSupport_libdispatch.dll ^
             -i!*AppleApplicationSupport_libicu*.dll ^
             -i!*AppleApplicationSupport_objc.dll ^
             -i!F_CENTRAL_msvc?100* ^
             %2

    pushd %1
    for %%f in (AppleApplicationSupport_*.dll) do (
        for /F "tokens=1* delims=_ " %%t in ("%%f") do move /Y %%f %%u
    )
    for %%f in (x64_AppleApplicationSupport_*.dll) do (
        for /F "tokens=2* delims=_ " %%t in ("%%f") do move /Y %%f %%u
    )

    for %%f in (F_CENTRAL_msvcr100*) do move /Y %%f msvcr100.dll
    for %%f in (F_CENTRAL_msvcp100*) do move /Y %%f msvcp100.dll
    popd
) else (
    7z e -y -o%1 ^
             -i!ASL.dll ^
             -i!CoreAudioToolbox.dll ^
             -i!CoreFoundation.dll ^
             -i!*icu*.dll ^
             -i!libdispatch.dll ^
             -i!objc.dll ^
             -i!pthreadVC2.dll ^
             %2

    mkdir %1\Microsoft.VC80.CRT

    7z e -y -oQTfiles\Microsoft.VC80.CRT ^
            -i!msvcp80.dll.* ^
            -i!msvcr80.dll.* ^
            -i!manifest.* ^
            AppleApplicationSupport.msi

    pushd %1\Microsoft.VC80.CRT

    rem strip assembly version number from filenames of msvc runtime dlls
    for %%f in (msvcr80.dll.*) do move /Y %%f msvcr80.dll
    for %%f in (msvcp80.dll.*) do move /Y %%f msvcp80.dll

    rem find needless one out of the two manifests and remove it
    for /F "delims=:" %%t in ('findstr win32-policy manifest.*') do del %%t

    rem rename manifest
    for %%f in (manifest.*) do move /Y %%f Microsoft.VC80.CRT.manifest

    popd
)
exit /b

:end
if exist AppleApplicationSupport.msi del AppleApplicationSupport.msi
if exist AppleApplicationSupport64.msi del AppleApplicationSupport64.msi
endlocal

Use qaac to encode audio with it. Make sure you have 7-zip installed.

[pandy]

Hi, soory but I'm a ***

why using ffmpeg with

-c:a aac

the AAC audio encoder, the resultant bitrate is alwais Variable Bit Rate ?

I need to encode using AAC and absolutly CBR, not VBR. How is possible that? thanks

Seem you discovered bug - after quick test aac is indeed VBR not CBR even if bitrate is reported as constant by mediainfo:

General
Complete name                  : E:\Media\FF\WeaveSample_aac.mp4
Format                         : MPEG-4
Format profile                 : Base Media
Codec ID                       : isom (isom/iso2/mp41)
File size                      : 748 KiB
Duration                       : 46 s 998 ms
Overall bit rate mode          : Constant
Overall bit rate               : 130 kb/s
Writing application            : Lavf58.22.100

Audio
ID                             : 1
Format                         : AAC LC
Format/Info                    : Advanced Audio Codec Low Complexity
Codec ID                       : mp4a-40-2
Duration                       : 46 s 998 ms
Bit rate mode                  : Constant
Bit rate                       : 129 kb/s
Channel(s)                     : 2 channels
Channel layout                 : L R
Sampling rate                  : 48.0 kHz
Frame rate                     : 46.875 FPS (1024 SPF)
Compression mode               : Lossy
Stream size                    : 738 KiB (99%)
Language                       : English
Default                        : Yes
Alternate group                : 1

[Attachment 47307 - Click to enlarge]

libfdk_aac seem also produce variable bitrate... perhaps this may be related to aac profile or something else...

[marcorocchini]

so is there a way to encode in CBR mode with ffmpeg?

[pandy]

so is there a way to encode in CBR mode with ffmpeg?

No clue but issue may be related to different format (profile) of aac - perhaps try LC MPEG-2 and LC MPEG-4? Maybe true CBR aac don't exist and all codecs by CBR mean ABR mode?
If you check this: https://hydrogenaud.io/index.php/topic,82865.0.html then answer is quite explicit:

MPEG-2 nbc (aka aac) does not make a specification of packet sizes, but instead specifies an Average bitrate that should be achieved (ABR).

[hello_hello]

Hi, soory but I'm a ***

why using ffmpeg with

-c:a aac

the AAC audio encoder, the resultant bitrate is alwais Variable Bit Rate ?

I need to encode using AAC and absolutly CBR, not VBR. How is possible that? thanks

I might be missing something, but don't you have to specify a bitrate in order to get constant bitrate? Without it, it probably gives you the default VBR quality.

https://trac.ffmpeg.org/wiki/Encode/AAC

[pandy]

I might be missing something, but don't you have to specify a bitrate in order to get constant bitrate? Without it, it probably gives you the default VBR quality.

https://trac.ffmpeg.org/wiki/Encode/AAC

https://ffmpeg.org/ffmpeg-codecs.html#Options-5

Set bit rate in bits/s. Setting this automatically activates constant bit rate (CBR) mode. If this option is unspecified it is set to 128kbps.

[hello_hello]

I guess I got that the wrong way around. I'm sure it works differently for mp3 encoding, where if you don't specify a bitrate you get VBR, specifying a bitrate on it's own gives you CBR, and specific a bitrate with -abr gives you an average bitrate encode.

Anyway, I tried four different AAC encoders in CBR mode (ffmpeg, QAAC, FDKAAC and NeroAAC) and they all resulted in bitrates that varied a little, so I guess there's no such thing as truly CBR AAC.

To add to the fun through, QAAC and FDK AAC both added CBR to the written encoder information, and (coincidentally?) MediaInfo declared them both to be variable bitrate.

Writing application                      : fdkaac 0.6.3, libfdk-aac 3.4.22, CBR 128kbps

Audio
ID                                       : 1
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : mp4a-40-2
Duration                                 : 3 min 56 s
Bit rate mode                            : Variable
Bit rate                                 : 128 kb/s
Maximum bit rate                         : 163 kb/s
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 44.1 kHz
Frame rate                               : 43.066 FPS (1024 SPF)
Compression mode                         : Lossy
Stream size                              : 3.61 MiB (99%)
Encoded date                             : UTC 2018-12-02 03:50:36
Tagged date                              : UTC 2018-12-02 03:50:36

Writing application                      : qaac 2.67, CoreAudioToolbox 7.10.9.0, AAC-LC Encoder, CBR 128kbps, Quality 96
Encoding Params                          : (Binary)

Audio
ID                                       : 1
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : mp4a-40-2
Duration                                 : 3 min 56 s
Bit rate mode                            : Variable
Bit rate                                 : 128 kb/s
Maximum bit rate                         : 136 kb/s
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 44.1 kHz
Frame rate                               : 43.066 FPS (1024 SPF)
Compression mode                         : Lossy
Stream size                              : 3.61 MiB (99%)
Encoded date                             : UTC 2018-12-02 00:43:48
Tagged date                              : UTC 2018-12-02 00:43:54

[poisondeathray]

Are you determining if it's "VBR" or "CBR" with "mediainfo" ? How accurate is that ?

I'd put more weight in the results from a plot like the one in pandy's post. At least it looks a bit more authentic. But mediainfo said it was CBR there too....

A long time ago, there was CT-AAC , and it was known for CBR AAC . I don't know if it's true, and I don't know of a good way to measure an audio bitrate plot

[hello_hello]

Yeah, with MediaInfo.
I only mentioned it as I thought it was a little funny there were two encoders that included CBR along with their writing application details, and they were the two MediaInfo show as being VBR.

I assume they're all a little VBR. Probably due to the bit reservoir system, and I remember there being a "fill" element for AAC encoders to use to keep the bitrate somewhat constant.
I opened an encode from each encoder with MP3DirectCut and it sees them all as ABR, and I have foobar2000 configured to display the bitrate in real time, and for all four encoders it changes a little as the audio plays. No doubt they all confirm to the AAC specification in respect to CBR, but obviously it's not absolutely constant.

[poisondeathray]

What about CBR vs VBR MP3 , or AC3 - can you distinguish between those in foobar2000 ?

[hello_hello]

For CBR MP3 it always jumps to something a tad higher than the constant bitrate when it begins playing the MP3, but it goes straight back to the MP3's constant bitrate and never changes after that. It shows CBR for AC3. I'm not sure I've met any VBR AC3 audio. I guess I could make one....

For MP3 there could be more information to work with, which might help. There's info in the Xing header I'm not sure how to interpret yet, although it would appear there's bitrate information written to every MP3 frame. For a 128k CBR MP3 it's always 128. For VBR MP3 it varies.

There's even less I understand about what MediaInfo shows for AAC, but once you drill down past the tag and element info, what it displays seems to relate to bitrate. The second two screenshots are the info MediaInfo displays close to the end of two NeroAAC encodes for a track that faded to silence. The first encode was 128k CBR. The second was encoded with Nero's default VBR setting, which resulted in a higher average bitrate, but towards the end of the track you can see the difference. If the byte values relate directly to bitrate then CBR AAC isn't always constant.

1. MP3 128k CBR frame.
2. AAC 128k CBR
3. AAC VBR

[drake7]

"mediainfo" reads Bit rate mode: Variable with -c:a aac as well as
-c:a aac -b:a 128k but
Constant with 96k as well as 192k.

However, with 265k and 384k I also get Variable.

(I've tested that with -c:v libx264 and .mp4)

[Cornucopia]

To mention the elephant in the room, what I'd like to know is why you think you MUST have cbr?
Is there a service you are attempting to provide to that is expecting that? If so, do they really mean that it must, or is that just what they say to get people to encode properly? Or are you just assuming that's what they need?
Or is it an app that is rejecting your vbr/abr encodes? I find that very unlikely.


Scott