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AC3 encoder options, dvd encoding
B7k
post May 21 2010, 02:23
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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?
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saratoga
post May 21 2010, 03:04
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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 . . .
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B7k
post May 21 2010, 05:55
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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?

This post has been edited by B7k: May 21 2010, 05:56
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saratoga
post May 21 2010, 07:18
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QUOTE (B7k @ May 21 2010, 00:55) *
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 . . .
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B7k
post May 21 2010, 08:01
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Thanks saratoga for youre help I will look more into ac3 and other codecs so mutch I want to learn.
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