[geeks] Whee! Lightning strikes, AGAIN!

[gsm at mendelson.com]

On Wed, Jul 29, 2009 at 10:46:37AM -0400, Phil Stracchino wrote:

>Not for nothing is NTSC sardonically known as Never The Same Color...


That's for a different reason. The color signal was transmitted a phase 
encoded 3.57 mHz signal overlayed with the video signal. The phase is reset
to "zero" at the end of each field. 

The 60 fields a second came from matching the power line frequency and
the 262.5 lines per field came from the time need to draw a line and return the
beam to the other side of the screen (retrace).

The half lines come from the fact that the beam started it's scan in the
middle of the screen not the edges, so that if something was missed due
to differences in the resolution of the receiving equipment, it would be
at the edges where it would not be noticed.

Due to transmission errors, reflections, etc, the signal wanders a bit,
although most color errors were really caused by poorly set up equipment.

The BBC who did not start transmission of colour until 1965 learned from
the experience of the NTSC broadcasters and every line inverted the phase 
effectivley reseting it to "zero". 

That's where the name came from, PAL (phase alternating line).

The frame count 25 came from 50 fields a second to match the power line
frequency, and the resolution of 625 lines per frame (312.5 per field)
came from using the same scan rates as NTSC, but since they were doing 50
fields a second instead of 60, they had time for more scan lines.

The BBC had wider channels and wanted more bandwith between the color
carrier and the white signal (0 luminance (brightness) so they placed
theirs at 4.43 mHz.

VHS tapes recorded the color separately, so they did not need to have the
full bandwidth for the included subcarier. At one time you could buy NTSC
VCR's that had an extra phase shifter so that if your PAL TV could play 
60Hz composite video, you would get every other line with phase shifted
color and it would be watchable.

Geoff.

-- 
Geoffrey S. Mendelson, Jerusalem, Israel gsm at mendelson.com  N3OWJ/4X1GM