[mythtv-users] is it really 29.97 fps or 24 fps?
Greg Oliver
oliver.greg at gmail.com
Fri Sep 3 03:01:14 UTC 2010
On Thu, Sep 2, 2010 at 9:55 PM, Brian J. Murrell <brian at interlinx.bc.ca> wrote:
> When I play a recording of a given television show (a drama, so filmed
> on real film, not video tape) recorded from clearqam with mplayer, it
> says about it:
>
> VIDEO: MPEG2 720x480 (aspect 2) 29.970 fps 15000.0 kbps (1875.0 kbyte/s)
> [V] filefmt:29 fourcc:0x10000002 size:720x480 fps:29.970 ftime:=0.0334
>
> Yet if I step through it a frame at a time with `.' I can see quite
> clearly that it's (a) not interlaced but progressive and (b) that there
> are 24 frames in every second played.
>
> So that seems to make it quite obvious that the MPEG that my cableco
> sends is the original 24fps progressive frames from film. I guess the
> expectation is that whatever is displaying that to a CRT TV will
> telecine it. But I digress.
>
> What I find interesting is that mythfrontend plays this content as 29.97
> fps, even to my plasma display which is quite capable of a 24 fps
> refresh rate.
>
> Indeed, given another file which mplayer reports as:
>
> VIDEO: [XVID] 624x352 24bpp 23.976 fps 912.1 kbps (111.3 kbyte/s)
> Auto-selected AVI audio ID = 1
> [V] filefmt:3 fourcc:0x44495658 size:624x352 fps:23.976 ftime:=0.0417
>
> and I can step through and see is progressive and count 24 fps,
> mythfrontend does change the refresh rate to 24 fps on my display and
> does display 24.
>
> So what's the deal with the clearqam recorded MPEGs? Why can mplayer
> figure out they are 24fps (despite the header saying otherwise) and yet
> mythfrontend cannot?
I have seen this sort of thing as well.. The only explanation I have
drawn - and subsequentally lived with - is that the programs are
making frame rate assumptions based on container alone. Once they
start playing them, they use the proper frames and get it right
though. I have never really fussed about it since they seem to get it
right (well, they have to if they are not to break spec :) ) once
playback starts...
I may be way out in the barn here though !
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