"Sylvia Else" wrote in message
...
On 6/04/2010 12:53 AM, William Sommerwerck wrote:
However... If the burst phase is wrong, then there is no cancellation
of
errors, because there are no "errors" /in the signal itself/. (Right?
(???))
Therefore, I don't see how line averaging can be used to eliminate the
need
for a manual hue control.

Think of the chroma signal as a vector with its y coordinate equal the
red difference component, and the x coordinate equal to the blue
difference component. A phase error rotates that vector about the z
axis. Effectively, the blue difference component receives a bit of the
red difference component, and vice versa.

On alternate lines the phase of the red difference component *only* is
inverted. In our view, this has the effect of reflecting the vector in
the x axis - what was a positive y value becomes negative.

The same phase error causes this vector to rotate in the same direction
about the z axis, but because of the reflection, the mixing of the
components has the opposite sign.

If you then negate the resulting red difference component of the second
line, and average with the red difference component of the first line,
the parts received from the blue difference component cancel out,
leaving a red different component that equals the original, multiplied
by the cosine of the phase error. The same applies to the blue
component. The result is that the hues are correct, but not as saturated
as they shoud have been.

No argument. That's always been my understanding. But...

If the burst phase gets screwed up somewhere along the line, no amount of
line averaging will fix the problem, because there's nothing "wrong" with
the subcarrier to fix.

If the burst has a random phase relationship to the colour subcarrier on
each line, then my analysis falls apart because the vectors would have
random orientations. In such a situation a PAL receiver would do no better
than NTSC, and they'd both perform awfully.

If the burst just has a fixed phase offset from the true colour
subcarrier, then the averaging will work.

Indeed it will work if the colour subcarrier drifts in a consistent way
relative to the burst - or if the receiver's oscillator similarly drifts.
The effect of such a drift on an NSTC picture would be a variation of tint
from left to right. However, a tint control wouldn't be able to address
that problem - it would simply move the horizontal position on the screen
where the colours are accurate - suggesting that it doesn't occur in
practice except in equipment that is recognisably broken.


Many years back, Bush in the UK produced a colour decoder which was
'revolutionary' compared to other manufacturers' efforts, in that the
subcarrier was regenerated in the decoder directly from the burst, rather
than being a free-running oscillator just locked to the burst with a PLL.
They did this by deriving a phase-adjustable pulse from the H-flyback, and
using this to 'notch out' the burst from the back porch period. The 10
cycles of burst thus recovered, were then applied directly to the 4.43MHz
crystal, which caused it to ring at exactly the same frequency and in
exactly the same phase as the original subcarrier. Always seemed to work
pretty well, and they continued to use this system over a period of probably
10 years or more, covering three chassis designs / revisions.

Arfa


Granted, this problem hardly ever happens. But the argument that a fully
implemented PAL set is inherently immune to color errors is hard for me
to
swallow.



I don't think there's a claim that it is inherently immune to all colour
errors, only those caused by consistent differences between the phase of
the subcarrier and the burst.

Sylvia.