[vdr] Re: How is the length of a recording calculated

[Patrick Cernko <errror@ps.uni-sb.de>]

Also sprach Sameer Khosla zu "13.06.2004 06:33" Anno Domini:
> ----Original Message-----
> Sameer Khosla wrote:
>
> > Hi, I don't know if this question has come up or not, or even if the PAL
> > users experience (since I am guessing this issue only affects NTSC users),
> > but I am wondering how the length of a recorded file is recorded.
> >
> > The reason I ask is that with the NTSC sources that I record, the reported
> > time for the recording is *always* longer.
> >
> > For example, if I set a timer to record from 8pm to 9pm, it does.  However
> > it reports the length of the recording to be something like 1:10:35.
> >
> > It's is not always predictable, sometimes on an hour long recording it
> will
>
> > show as 7 minutes longer, and sometimes 15 minutes longer, and sometimes
> 2.5
>
> > minutes longer.
> >
> > But it is always longer.  My thoughts (which could be wrong, this is just
> a
>
> > guess) is that could this possibly have something to do with calculating
> the
>
> > length based on number of frames (where the fps is different for PAL and
> > NTSC).
> >
> > Is there any easy way to fix this?
>
> VDR/recording.h:
>
> #define FRAMESPERSEC 25
>
> Set this to 30 (which I believe is the frame rate of NTSC).
>
> Klaus
>
>
>
> Actually, I'm not sure, but I believe the framerate for NTSC is actually
> 29.7 (going from memory, will check).  I am sure that it isn't a whole
> number.  Is that constant an integer or float? 
Interesting! I thought all of you knew it already but obviously most don't:

NTSC _was_ broadcasted with 30 fps a very long time ago. Then, they 
invented _COLOR_-Video-Broadcasts which led to the problem, that the 
color modulation into the signal interfered with the b&w video signal. 
Instead of chosing another modulation type, they decided to simple drop 
each 1000th frame (quiet easy :-) ). This yielted in a framerate of 
exaclty 29,97 (30 - 30/1000). Also no problem, there used analog deviced 
which had no problems with foating point values! I want to thank you all 
these famous and ultra-intelligent NTSC developers from here for 
choosing such a very _EASY_ computable framerate!

Source: Phillip Susallek: Lecture on Multimedia
http://graphics.cs.uni-sb.de/Courses/ws0102/Multimedia/folien/Video-Dateien/v3_document.htm
(see slides titled "Video standards")

I am so happy to live in Germany using this _crappy_ 25 fps integer 
value in the PAL standard! :-)

CU
--
Patrick Cernko | mailto:errror@errror.de | http://www.errror.de
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