[mythtv-users] OT: NTSC Video synchonization

[Brian Wood]

Patrick Doyle wrote:
> I realize that this off topic for the purpose of this list, so I would
> appreciate any pointers folks would care to give me regarding a
> list/forum/textbook where questions such as these are the
> bread-and-butter of the list and not off topic.
> 
> I'm curious how the mpeg-2 video stream gets synchronized to the
> display.  From the textbooks I read, NTSC video operates at a frame
> rate of 29.97 frames/second, whereas it seems to me that most monitors
> operate at a frame rate of 30 frames/second.  If I capture (analog)
> broadcast NTSC video and encode it with the hardware encoder on my
> video capture card, I presume that it is encoded at the 29.97
> frames/second.  If I later decide to play that video on my display, I
> presume that it is pulled off the disk at a rate of 30 frames/second.
> 
> How is this resolved?  Is the X server configured for 29.97 frames/second?

You are confusing the frame rate of the NTSC video with the parameters
of the video sent from the computer to the monitor. Essentially the
"transition" occurs by storing the frames in a buffer, reading them into
the buffer at NTSC rate and reading them out at whatever rate is
appropriate for your display. In addition, NTSC is interlaced, and most
computer displays these days are not, so that transition has to be
accounted for as well.

As long as the "write" sync is NTSC, and the "read" sync whatever your
display needs, you are OK. X handles the signal as simply another window.

Depending on your setup that "buffer" might be in your system RAM, or on
your video card.

It's actually a fairly complex topic to understand completely, but not
really difficult to implement. I'm not sure if you are looking for a
complete technical description, or just enough information to get things
working.

> 
> On what list could I hang out where questions such as this are
> considered run-of-the-mill?

I've passed this question on to some friends of mine who are still
working in the TV broadcast fiend (I'm retired from it). I'm not aware
of any internet forums/lists etc., when I learned all that stuff we
basically had to go to school (often vendor's training classes), read
books and just BS among ourselves, the "internet" not having grown
"mature" at that point.

If any of my friends have any suggestions I'll pass them along.

Material on basic Video theory is available from Tektronix, Snell and
Wilcox, Sony, Ikegami and various other vendors. Unfortunately, such
material can be hard for the general public to obtain.

S+W's "Video Standards - Signals, Formats and Interfaces" (ISBN 1900739
07 0), although a decade old now, is a good starting point. It explains
a lot of basic video as well as MPEG compression. It used to be
available from Snell and Wilcox for $25, not sure how you would get it,
or its modern equivalent, today.

Tektronix has a lot of publications, generally available from their
sales people for free, but only if they think they can sell you some
equipment, which you are unlikely to be able to afford (a typical old
analog Tek TV demodulator was around $30,000).

Tandberg makes a lot of commercial video compression gear.

Depending on where you live you might also try stopping by a local TV
station and asking their engineers if they have any old copies of manuals.

Hope this helps, at least a little.

Feel free to mail me privately, as this is a bit OT :-)

BEWW
beww at beww.org