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Thread: FDK AAC Questions on Constant Bit Rate and CBR Low Pass filtration

  1. #1

    Join Date
    May 2008
    Posts
    7

    FDK AAC Questions on Constant Bit Rate and CBR Low Pass filtration

    Greetings -
    Just started using the m4a AAC Encoder (FDK) Release 7 with Music Converter. The VBR Q5, and CBR 256 and 320Kb settings sound great... to my ears all sound reasonably equivalent to FLAC.

    1) I noticed that when I select CBR the files are not truly Constant Bit Rate like I'm used to with MP3 CBR. My player shows the bit rate varying between frames around the selected bit rate, almost like an Average Bit Rate. Is this normal? Is the encoder incapable of being truly Constant Bit Rate, or are these just the default settings passed to the encoder by Music Converter?

    2) I noticed when looking at the spectral analysis of the resulting M4A files, the VBR Q5 files appear to contain the full spectrum of sound up to ~20KHz, whereas the CBR 256 and 320Kbps files appear to have had a low-pass filter applied, cutting off the at 17-18KHz. It's true that most people can't hear frequencies above this range - I'm just curious if this is a "feature" of using CBR, or if the low-pass filtration is again the result of parameters being sent to the encoder by Music Converter?

    3) Can the detailed settings / parameters passed to the encoder by Music Converter be changed / customized? If so, how?

    Any info or pointers would be helpful... thanks!
    Regards,
    mikebo

  2. #2
    Administrator
    Join Date
    Apr 2002
    Posts
    38,481

    Re: FDK AAC Questions on Constant Bit Rate and CBR Low Pass filtration

    1. fdk is not capable of true CBR I believe.
    2. We do not specify the cut-off, so it is decided internally in the codec to use this
    3. We do not offer the customization of the commands

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