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piedro
Asked: 2011-01-05 04:20:34 +0800 CST

ffmpeg options: -acodec libfaac -ab 192k produce 150kbit files?

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When I use ffmpeg to convert an audiofile with the option 

-acodec libfaac -ab 192k

and use 

ffmpeg -i

on that file afterwards to get the audio file information, it tells me 

bitrate: 152 kb/s

Why ist this? Do I miss something here? If I want to convert a file with a bitrate of 192kb it should give me 192 kbit after the conversion, shouldn't it?

Or: How do I get the 192 kbit rate then?

ffmpeg

3 Answers

  1. I can't provide any further explanation, but several places (e.g. [1]) mention:

    Note that the maximum average bit-rate (what -ab specifies in the context of libfaac) is 152 kbit/s. Setting this higher will be ignored and 152 kbit/s will be used.

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  2. Best Answer

    There are now much better aac encoders for Linux such as NeroAacEnc or fdkaac but if you are still keen on using faac there is a patch that means faac can encode with a bitrate of 192k, and a great deal higher as well. Handbrake formerly used this patch and hosted it on their site but the developers have dropped faac support now, I now host the patch here:

    http://www.andrews-corner.org/patches/A00-bitrates.patch

    Patch Faac and then compile FFmpeg with --enable-libfaac and you can have high bitrate aac files with Faac. Here is a sample encode with the patched faac, encoding directly with faac:

    andrew@skamandros~/media$ faac -b 192 -o test.m4a test.wav 
Freeware Advanced Audio Coder
FAAC 1.28

Average bitrate: 192 kbps
Quantization quality: 100
Bandwidth: 20872 Hz
Object type: Low Complexity(MPEG-4) + M/S
Container format: MPEG-4 File Format (MP4)
Encoding test.wav to test.m4a
   frame          | bitrate | elapsed/estim | play/CPU | ETA
11720/11720 (100%)|  192.0  |   21.7/21.7   |   12.52x | 0.0  

andrew@skamandros~/media$

    And it works as well with FFmpeg. But there are definitely better aac encoders out now...

    References:

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  3. For high and low bitrates you have to specify a different cutoff frequency. without cutoff frequency this example results in a max bitrate of 166kbit/s (+ 2kbit/s muxing overhead):

    ffmpeg.exe -y -i input.ts -vn -c:a libfaac -ar 48k -b:a 210k out.mp4

size=     308kB time=00:00:15.01 bitrate= 168.0kbits/s

    When I specify the cutoff frequency to half of the samplerate, I can go up to 210kbit/s

    ffmpeg.exe -y -i input.ts -vn -c:a libfaac -ar 48k -b:a 210k -cutoff 24k out.mp4

size=     388kB time=00:00:15.01 bitrate= 211.7kbits/s

    Likewise, the default will not go below 64.5kbit/s:

    ffmpeg.exe -y -i input.ts -vn -c:a libfaac -ar 48k -b:a 20k out.mp4

size=     122kB time=00:00:15.01 bitrate=  66.4kbits/s

    But if I set the cutoff frequency very low, I can get 20kbit/s:

    ffmpeg.exe -y -i input.ts -vn -c:a libfaac -ar 48k -b:a 20k -cutoff 1k out.mp4

size=      40kB time=00:00:15.01 bitrate=  21.9kbits/s

    In these examples I used a samplerate of 48kHz. When using a lower samplerate, the bitrate will/can also be lower.

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