when I encode 720p HD sources with 29,97 to BD compilant 720p sources I usually use changefps(59.94) in Avisynth and make a 720p59.94 x264 encode.
This time time I have a 25fps 720p HD source so I want to go for the 720p50 specification but somehow changefps(50.00) doesn't work. I get an error and don't understand why this is a wrong divisor?
There is also the possibility to not use the avisynth fps command and just go for a
720p25, which (according to http://www.x264bluray.com/home/720p-encoding) doubles the framerate with the "--pulldown double preset".
The encode works fine but plays with 50% more speed which is totally senseless.
Any advice is appreciated
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Always post your complete AviSynth script, x264 command, logs and error messages.
Note that pulldown may not work correctly with all muxers/authoring softwares (tsMuxer may have problems)
In general you should prefer either a framerate preset string ("pal_double") or a pair of numerator and denominator (50, 1) over a floating point number.
And to exactly duplicate frames, there may be more reliable functions than ChangeFPS. I would guess: Interleave(last, last).AssumeFPS(50, 1)
My avisynth script was
--bluray-compat --level 4.1 --keyint 50 --sar 1:1 --slices 4 --vbv-bufsize 30000 --vbv-maxrate 40000 --b-pyramid none --weightp 0 --colorprim bt709 --transfer bt709 --colormatrix bt709 --merange 24
causes the error: don't know what pal_double means
x264 offer possibility to use pulldown 2:2 - no penalty on bitrate.