Subject: Re: [linux-audio-user] [ANNOUNCE] rtc based MTC generator
From: Dan Mills (dmills_AT_spamblock.demon.co.uk)
Date: Sat Dec 18 2004 - 21:58:16 EET
On Saturday 18 December 2004 05:24, Reuben Martin wrote:
> >Great. Btw: does anyone know exactly what framerate non drop 30 fps
> >MTC has? Should i really send with 29.97 fps?
>
> For NTSC, yes. The frame rate has to do with the frequency used by the AC
> power grid. In the US, the frequency is a little under 60HZ (59.94HZ). TVs
> were designed so that each scan would coincide with the AC cycle. Since the
> TV signal is interlaced, it scans twice for each frame. That's how we get
> the frame rate of 29.97. In Europe, the power grid configuration is so
> much better, the power output is more consistant and the frequency drifts
> less. It also runs at 50HZ, hence the PAL framerate of 25fps.
>
> Just a bit of useless trivia for you. :)
Actually this is not quite correct.....
The US power grid runs at a nominal 60Hz and so does American BLACK & WHITE TV
(field rate - frame rate is half of this), howether in in order to get the
relationship between the colour subcarrier and both the line and field timing
right (so that the carrier phase would do the right thing at the start of
each frame), the frame rate for colour telly under the NTSC system had to be
changed slightly hence 27.97 (approx).
Non drop 30 (also all 24, & 25 FPS) should run at their nominal rates, I
cannot remember exactly what drop frame does but I THINK it also runs at a
nominal 30FPS with some butchery to its frame count (anyone have a good
reference for the details? It is in "Control Systems for Live Entertainment",
but I cannot find my copy.....).
Note that even drop frame TC drifts slightly compared to a real video frame
count.... It a mess.
BTW:
While I like our (EU) 230/400V distribution - it makes lighting cable much
lighter and is generally les of a PITA for big amplifiers, there are a few
"issues".
For example the UK is changing the colour codes for installed wiring from
Red/Yellow/Blue - Phases
Black - Neutral
to
Brown/Gray/Black - phases
Blue - Neutral.....
I forsee smoke!
Regards, Dan.
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