On 5/04/2010 10:56 PM, William Sommerwerck wrote:
I understand that prior to the expiry of the Telefunken PAL patent,
Sony Trinitron sets for the PAL market actually threw away the
chrominance signal on alternate scan lines, thus landing themselves
back in NTSC territory. Those sets had a tint control, and I know from
personal experience that they produced a perfectly satisfactory result
(I only learnt the other day why they had a tint control).

Depends on what you mean by "satisfactory". Passable, maybe.

When you discuss something at length, you become aware of those things you
thought you understood, but didn't. (Well, I do, anyway.)

I'd always read that one could construct a PAL receiver in such a way that
eliminated the need for a manual hue control. I never questioned this, but
now it makes little sense.

There are two reasons for having a manual hue control:

The user can adjust the color rendition to their personal (and usually
incorrect) taste. *
The user can correct for incorrect burst phase.

That seems to be "it". As we've seen, these errors can be corrected by
adjusting the hue control, whereas the other error -- differential phase
shift -- cannot be so-corrected, because the timing errors are not linear.

Here's where I get confused. The line-to-line polarity reversal ** causes
the differential phase errors to be equal and opposite, and thus cancel out
when added (at the cost of desaturation -- but that's another issue).

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.

Sylvia.