(PAL) The
video signal format used in the UK
PAL uses
Amplitude Modulation for the video information, and
Frequency Modulation for the
audio information. The phase
of the colour subcarrier is reversed on alternate lines
which (together with the use of a delay line) allows the
receiver to cancel any phase errors introduced in the path
between the studio and the end-user's receiver. Such phase
errors are quite common and would cause the displayed colours
to shift in
hue.
The US equivalent,
NTSC, does not have this feature and thus
requires a user control to correct for transmission phase
errors, hence the mis-expansion "Never Twice the Same Colour".