Phase alternating line
<television> (PAL) The video signal format used in the UK [where else?].
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".
