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".
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