Hanover bars, in the simplest PAL

PAL

PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...

 television video

Video

Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...

 format, are an undesirable artifact in the reception of a television image.

Two signals, B-Y (U) and R-Y (V) are used in the PAL system to carry the color information for a picture, with the phase

Phase (waves)

Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...

of the V signal (hue) reversed (i.e. shifted through 180 degrees) on alternate lines (hence the name PAL, or phase alternate line). This is done to cancel minor phase errors in the reception process. However, if gross errors occur, complementary errors from the V signal carry into the U signal, and thus stripes occur.

Later PAL systems introduced 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.

Suppression of Hanover bars

To suppress Hanover bars, PAL color decoders use a delay line which repeats the chroma information from each previous line, and blends it with the current line. This causes phase errors to cancel out at the cost of vertical color resolution, and in early designs also a loss of color saturation proportional to the phase error.