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131 lines
4.5 KiB
131 lines
4.5 KiB
export_mp2 Documentation
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========================
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Last update 2004-08-03, Simone Karin Lehmann < simone at lisanet dot de >
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General:
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--------
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Some time ago I spend a lot of time to convert short movie clips to
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vcd compliant mpeg files. And with some of them I had the need to convert them
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from NTSC 23.976 fps to PAL 25 fps. I decided to do it the same way as it's usualy
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done with native film material, meaning the movie is just played faster. No
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additional frames should be generated. So I began to hack the transcode sources and
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wrote this module.
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For more information on changing the speed of the video stream please take a
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look at section - Changing video speed - at the end of this documentation.
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I've coded a new audio export module instead of a filter plugin because it was
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easier to implement this audio speed changing stuff by using pipes. And pipes are
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easy to implement in export modules.
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IMO this isn't a drawback, because most needs to change audio speed will be
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in cases mentioned above where native film material (24 fps) should be converted
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to PAL (25 fps) or from PAL to NTSC (23.976 fps, 29.97 pulldown) and be played
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on hardware DVD players. Therefor MPEG 1 Layer 2 was choosen as audio codec.
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This module processes audio streams only. There are no compile time
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depencies. It uses the ffmpeg encoder to process audio and therefor it's very fast.
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For speed changing issues you need the sox binary.
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Depencies:
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----------
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At run time ffmpeg and sox must be present.
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Usage:
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------
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To use this module just do
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-y <video_module>,mp2
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Additionaly, if you want to change the speed of the audio, you can append a factor
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to the module. Use the module the following way
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-y <video_module>,mp2=speed=N
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where N is an unsigned float. N < 1.0 will slow down audio (meaning playing
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time will increase) whereas N > 1.0 will speed up audio (meaning playing time
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will decrease) The pitch of the audio is not corrected. E.g. to speed up
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audio by 25/23.976 (this is used to convert NTSC film material to PAL) use
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-y <video_module>,mp2=speed=1.0427093
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Changing video speed:
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---------------------
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Well, the basic idea is to bypass the internal transcode calculations and settings
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of the export frame rate. The bypass has to be done because otherwise transcode will
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call some frame rate modifying filters to generated new frames or drop existing frames.
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The -f option is used by transcode to overwrite the autoprobed value of the input stream.
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The --export_fps, if not given on the command line, will be set to the same value as -f
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and handed over to the export module and therefor to the encoder.
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Now if you specify a --export_fps value which differs from -f (or autoprobe) transcode
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just calls the fps filter plugin automaticly. This is the same as using transocde
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with "-J fps" or the like. And this will generate or drop some frames.
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But just remeber, we want to change the speed, meaning the total playing time, and not
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the amount of frames.
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With mpeg videos this can simply be done by setting a flag in headers of the output
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video stream. For the ffmpeg and mpeg2enc export module this i decribed now.
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export_ffmpeg:
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Use somewhere in your ffmpeg.cfg (or ~/.ffmpeg.cfg) config file the option
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fps_code = VALUE
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where VALUE has the same range and meaning as transcode's frame rate code (-f 0,N)
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VALUE means
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0 use transcodes fps settings, this is the default
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1 23.976 fps
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2 24
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3 25
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4 29.970
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5 30
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6 50
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7 59.940
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8 60
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If this option is set, all transcode frame rate settings (-f, --export_fps or
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autoprobed values) are ignored. The encoder frame rate is set to the
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corresponding value shown above. This is useful, if you want to "convert" from
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PAL to NTSC or vice versa by speeding up or slowing down the video. No frame
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will be dropped or added, only the frame rate entries in the headers are set
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to the new value.
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To "convert" from 23.976 fps to 25 fps put
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fps_code = 2
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or from 25 to 23.976 fps
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fps_code = 1
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expot_mpeg2enc:
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The mpeg2enc binyry which is driven by this modules know about an option called to
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set the frame rate in the resulting output stream. You can specify this option by adding
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a string to transcode's -F option. E.g. if you are using mpeg2enc to encode to standard
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dvd compliant streams do
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-y mpeg2enc,<audio_module> -F 8,"-F 1"
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But please don't mix them up. -F 8 is recognized by the transcode export module and
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"-F 1" is passed to the mpeg2enc binary itself. Just take a look at the manpages of
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transcode and mpeg2enc.
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... hope you enjoy.
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Simone Karin Lehmann
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