Hanover bars
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Hanover bars, in the simplest PAL television video format, is an undesirable artefact in the reception of a television image.
The two B-Y (U) and R-Y (V) signals in the PAL system carry the color information for a picture, with the phase of the V signal is reversed 180 degrees on each alternate line (hence the name phase alternate line). This is done to cancel minor errors in the reception process. However if gross errors occur, complementary errors from the V signal carry into the U signal, and thus stripes - Hanover bars occur.
Later PAL systems introduced such alterations to ensure that Hanover bars do not occur, introducing a "swinging burst" to the color synchronization. Other PAL systems may handle this problem differently.