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Topic: AC3 encoder options (Read 4000 times) previous topic - next topic

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  • B7k
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AC3 encoder options
I'm curious about ac3's bandwidth and the difference between "fixed adaptive bandwidth", "variable adaptive bandwidth", and just setting it at 60(99% of full bandwidth) How would each one of these settings effect the finished file?

  • saratoga
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AC3 encoder options
Reply #1
AC3 is an MDCT codec like vorbis or AAC, so I'm not sure what that setting would even mean.  You should probably consult the manual for whatever encoder offered it to you.  However as with MP3, AAC, Vorbis, etc I would not try to tweak how the encoder handles files more then I absolutely had to, as the programmer usually knows best . . .

  • B7k
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AC3 encoder options
Reply #2
Sorry I should have told the forum I was using WAV to AC3 Encoder 4.5 It uses the Aften encoder. After some searching I found the help doc
but I still need help here is the definition from the help file.

Bandwidth
                      The bandwidth setting corresponds to the high-frequency
                      cutoff.  Specifically, it sets the highest frequency bin
                      which is encoded.  The AC-3 format uses a 512-point MDCT
                      which gives 256 frequency levels from 0 to 1/2 of the
                      samplerate.  The formula to give the number of coded
                      frequency bins from bandwidth setting is:
                      (w * 3) + 73, which gives a range of 73 to 253
                      I hope to replace this setting with one where the user
                      specifies the actual cutoff frequency rather than the
                      bandwidth code.
                      There are 2 special values, -1 and -2.
                      When -1 is used, Aften automatically selects what
                      it thinks is an appropriate bandwidth.  This is the
                      default setting.
                      When -2 is used, a bandwidth is chosen for each frame
                      based on CBR frame size and a target quality of 240.
                      Variable bandwidth cannot be used with VBR mode.


So value -1 being "fixed adaptive bandwidth" the encoder choose the correct cutoff frequencies for me?
and value -2 being "variable adaptive bandwidth", the user specfies the setting from Min/Max cutoff frequencies?

The default value -1 is good for movies but lets say I'm making a music video and knowing music is allot more complicated then just some speech and sound effects would their be a need for the user too use Value -2 and set a specific Min/Max value for cutoff frequencies?
  • Last Edit: 21 May, 2010, 12:56:44 AM by B7k

  • saratoga
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AC3 encoder options
Reply #3
The default value -1 is good for movies but lets say I'm making a music video and knowing music is allot more complicated then just some speech and sound effects would their be a need for the user too use Value -2 and set a specific Min/Max value for cutoff frequencies?


This tweak concerns bandwidth, not complexity of music, so I would use the same setting in either case.  That said, unless the encoder is completely worthless, it should be able to pick the correct setting.  If automatic works more poorly then some manual setting, I would just find a new codec . . .

  • B7k
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AC3 encoder options
Reply #4
Thanks saratoga for youre help I will look more into ac3 and other codecs so mutch I want to learn.