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'video subcarrier recovery (was Re: Crystal Tutoria'
1998\10\28@191309 by Eric Smith

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"Peter L. Peres" <spam_OUTplpTakeThisOuTspamACTCOM.CO.IL> wrote:
> incorrect for Australia and places which have PAL because the color
> decoder crystal is phase modulated at +/-90 degrees every other line, and
> this phase modulation is a pain to remove for other uses.

On alternate lines the phase of the burst relative to the color subcarrier
+/- 135 degrees.  The color subcarrier is continuous, and can be recovered
from the burst using suitable circuitry.  This would be a pain if you had
to do it in a discrete implementation, but there are plenty of chips
available that will do it for you.

In NTSC, the subcarrier frequency is 455/2 times the line rate, so the
subcarrier phase differs by 180 degress on successive lines.  Since there
are 525 lines in a complete frame, this also means that alternate frames
have alternate subcarrier phase, for a two-frame (four-field) color frame
sequence.

In PAL, the subcarrier frequency is 1135/4 of the line rate, plus an
additional 25 Hz.  This results in the subcarrier phase advancing 90.576
degrees per line.  Since the 25 Hz offset adds up to a complete subcarrier
cycle per frame, the total subcarrier advance per frame is 90 degrees
((90.576 * 625) modulo 360), so there are four frames (8 fields) per
color-frame sequence.

Reference: _A Technical Introduction to Digital Video_, by Charles Poynton

1998\10\29@132120 by Peter L. Peres

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On Thu, 29 Oct 1998, Eric Smith wrote:

> "Peter L. Peres" <.....plpKILLspamspam@spam@ACTCOM.CO.IL> wrote:
> > incorrect for Australia and places which have PAL because the color
> > decoder crystal is phase modulated at +/-90 degrees every other line, and
> > this phase modulation is a pain to remove for other uses.
>
> On alternate lines the phase of the burst relative to the color subcarrier
> +/- 135 degrees.  The color subcarrier is continuous, and can be recovered
> from the burst using suitable circuitry.  This would be a pain if you had
> to do it in a discrete implementation, but there are plenty of chips
> available that will do it for you.

The color subcarrier is quadrature modulated with the color information,
so it is continuous (if there is color in the image) but its phase is not.
Excepting for the burst and some test signals sent in certain invisible TV
lines the color carrier proper is never sent over the air in PAL. As far
as I know, there are no chips made for mass market TV demodulators that
recover the color subcarrier proper. The only shortcut is the double
frequency TDA Philips chip series that may or may not correct for color
framing.

The burst is indeed at +135 and -135 degrees, alternated every line, but
the difference between +135 and -135 is exactly 90 degrees and the crystal
in ordinary PAL decodes is direct-locked to the burst (at 0 or 180 degrees
usually) on every line, which means that it jumps 90 degrees in phase
forward and backward 15625 times per second, if we neglect the color
framing jumps.

Peter

1998\10\29@154655 by Eric Smith

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"Peter L. Peres" <plpspamKILLspamACTCOM.CO.IL> wrote:
> The color subcarrier is quadrature modulated with the color information,
> so it is continuous (if there is color in the image) but its phase is not.

No, the PAL color subcarrier *IS* continuous phase.  It is the color *burst*
that isn't continuous phase.

Since the horizontal line time is not a multiple of the subcarrier phase, the
subcarrier phase *relative to the line timing* varies from one line to
another.  As I described previously, the subcarrier phase for each successive
line advances 90.576 degrees (not the 90 degrees that is usually given
in simplified explanations).

> The burst is indeed at +135 and -135 degrees, alternated every line, but
> the difference between +135 and -135 is exactly 90 degrees and the crystal

Yes, but as stated above, burst phase is relative to the subcarrier phase,
which is continuous in time but advances relative to each scan line.  If you
looked at the same point within two consecutive scan lines (e.g., the exact
midpoint of the color burst), there would NOT be a 90 degree phase difference
between those two points:

line     subcarrier phase       color burst phase            color burst phase
        relative to line    relative to subcarrier phase    relative to line

1            0.000                +135                          135.000
2           90.576                -135                          315.576
3          181.152                +135                          316.152
4          271.728                -135                          136.728
5            2.304                +135                          137.304
6           92.880                -135                          317.880
etc.

Eric

1998\10\30@062001 by Peter L. Peres

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On Thu, 29 Oct 1998, Eric Smith wrote:

> "Peter L. Peres" <.....plpKILLspamspam.....ACTCOM.CO.IL> wrote:
> > The color subcarrier is quadrature modulated with the color information,
> > so it is continuous (if there is color in the image) but its phase is not.
>
> No, the PAL color subcarrier *IS* continuous phase.  It is the color *burst*
> that isn't continuous phase.

Yes, you are right, I did not express myself accurately. However, for
extraction purposes it still is a pain in the butt.

> Since the horizontal line time is not a multiple of the subcarrier phase, the
> subcarrier phase *relative to the line timing* varies from one line to
> another.  As I described previously, the subcarrier phase for each successive
> line advances 90.576 degrees (not the 90 degrees that is usually given
> in simplified explanations).

Ok, if it advances in jumps, not continuously. Otherwise there would be
color aberration on the horizontal scan line, to the tune of 0.5 degrees
per line (roughly). This aberration does not exist in physically
functioning systems today. The 0.576 degrees per line add up to 180
degrees over the 312 lines of a frame and this is the beginning of the
color framing sequence. After 4 frames it starts over.

{Quote hidden}

All my experience with broadcast and cameras and SSGs and all that
suggests that the master color subcarrier oscillator in the transmitter
does indeed run at a stable frequency, but it has a slave that is synced
to it during the BG pulse and that is used to prepare the actual color
information for each line. (The slave is synced to the appropriate phase
of the 4 phase master signal). This is to avoid the 0.5 degree per line
color shift which is very un-German (the PAL standard was elaborated in
Germany).

Peter

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