On Sat, Feb 16, 2002 at 06:02:51PM -0600, patachon wrote:
> >
> > The "appearing" number of frames on NTSC television is 30 per second. The
> > number 60 comes from the fact that an NTSC signal scans twice for each frame,
> > projecting first one half of the frame data, then the other half. I am still
> > trying to figure out whether this data is interleaved, or whether you get the
> > top half, then the bottom half. I suspect the former.
> >
The term is "interlaced"--alternate frames contain alternate scan
lines. A top/bottom scan at 30 fps would have very visible flicker.
Interlacing is a simple compression method that could be implemented
with the analog technology of the original television. It doesn't
work for finely-detailed still images that get close examination;
interlace on a computer monitor leads to distracting flicker.
>
> I always learned the scanning of the frame was in one direction, in Europe.
> but actually it's also possible an ultrafast scanning must happen when the
> beam goes back to the start of the next frame. It's supposedly then that
> complementary info, eventually other data not relevant for the frame
> fabrication is transmitted to the software of the receptor.
>
That's correct; there is a "retrace" time that allows for the magnetic
fields which direct the beam to be reversed. At the beginning of the
scan, in the "black" area at the top of the screen, additional
information can be transmitted. A lot of information can be stuffed
into the retrace; the basic bandwidth of a television channel is 6
Mhz, and even a few scan lines can contain quite a few bits.
Randolph
> >
> > The "appearing" number of frames on NTSC television is 30 per second. The
> > number 60 comes from the fact that an NTSC signal scans twice for each frame,
> > projecting first one half of the frame data, then the other half. I am still
> > trying to figure out whether this data is interleaved, or whether you get the
> > top half, then the bottom half. I suspect the former.
> >
The term is "interlaced"--alternate frames contain alternate scan
lines. A top/bottom scan at 30 fps would have very visible flicker.
Interlacing is a simple compression method that could be implemented
with the analog technology of the original television. It doesn't
work for finely-detailed still images that get close examination;
interlace on a computer monitor leads to distracting flicker.
>
> I always learned the scanning of the frame was in one direction, in Europe.
> but actually it's also possible an ultrafast scanning must happen when the
> beam goes back to the start of the next frame. It's supposedly then that
> complementary info, eventually other data not relevant for the frame
> fabrication is transmitted to the software of the receptor.
>
That's correct; there is a "retrace" time that allows for the magnetic
fields which direct the beam to be reversed. At the beginning of the
scan, in the "black" area at the top of the screen, additional
information can be transmitted. A lot of information can be stuffed
into the retrace; the basic bandwidth of a television channel is 6
Mhz, and even a few scan lines can contain quite a few bits.
Randolph