Philips PM 5508 Operating Manual page 39

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Bars 1 and 2 contain the same amount of (B-Y) information, while bars 1 and 3 con-
tain the same amount of (R-Y) information. Because this signal is encoded as an
NTSC signal, the (R-Y) output of the delay line circuit in the receiver should elimi-
nate all (R-Y) information during bars 1 and 3, see Fig. VII-13.
That is why bars 1 and 2 have the same colour (of 180°) on the screen and bar 3 is
grey like bar 4 (if the receiver is correctly adjusted). In case of incorrectly adjusted
amplitude and phase controls of the delay-line, the (R-Y) demodulator will produce
an undesired output signal which, due to the action of the (R-Y) switch, will cause a
line-sequentially varying colour on the screen. This results in the appearance of
"Venetian blinds".
If only bars 1 and 3 contain "Venetian blinds" the only possible cause is an ampli-
tude deviation of the delay-line circuit. A phase error, however, will produce
"Venetian blinds" in bar 2 as well, as only a phase shift between two successive
lines can produce an (R-Y) output signal, in spite of the fact that the signal from the
generator does not contain any (R-Y) information during that bar.
In this way it is possible to distinguish between phase and amplitude errors.
The PAL-alternating burst is required to ensure the proper operation of the (R-Y)
switch. Without alternating burst, no "Venetian blinds" would appear.
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