Editing Hanover bars

{{Short description|Undesirable visual artifact in television}}
[[Image:Hanoverbars without PAL delay.png|thumb|Simulated strong Hanover bars shown on a [[Philips circle pattern|Philips PM5544]] [[Test card|test pattern]]. Note: Hanover bars can only be seen by viewing full-size image]]
[[Image:Hanover_bars_with_PAL_delay.png|thumb|Simulated cancellation of Hanover bars through a chroma delay line]]

'''Hanover bars''', in one of the [[PAL]] television [[video]] formats, are an undesirable visual artifact in the reception of a television image.<ref>{{Cite web|url=https://www.radiomuseum.org/forum/45_years_anniversary_of_walter_bruchs_pal_color_television.html|title=45 Years Anniversary of Walter Bruch's PAL Color Television|website=www.radiomuseum.org}}</ref><ref name="auto">{{Cite book|url=https://books.google.com/books?id=U9aH92iLEZAC&dq=Hanover+bars+pal+-wiki&pg=PA112|title=TV and Video Engineering|first=A. M.|last=Dhake|date=May 1, 1999|publisher=Tata McGraw-Hill Education|isbn=9780074601051 |via=Google Books}}</ref> The name refers to the city of [[Hannover]], in which the PAL system developer [[Telefunken|Telefunken Fernseh und Rundfunk GmbH]] was located.

The PAL system encodes color as ''[[YUV]]''. The ''U'' (corresponding to ''[[B-Y]]'') and ''V'' (corresponding to [[B-Y|''R-Y'']]) signals carry the color information for a picture, with the [[phase (waves)|phase]] of the ''V'' signal 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 (waves)|phase errors]] in the reception process. However, if gross errors occur, complementary errors from the ''V'' signal carry into the ''U'' signal, and thus visible stripes occur.<ref>{{Cite book|url=https://books.google.com/books?id=pY2jBQAAQBAJ&dq=Hanover+bars+pal+-wiki&pg=PA71|title=Newnes Guide to TV and Video Technology|first=Eugene|last=Trundle|date=May 12, 2014|publisher=Elsevier|isbn=9781483183169 |via=Google Books}}</ref>

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 that repeats the [[Chrominance|chroma]] information from each previous line and blends it with the current line.<ref>
{{cite book
  | title     = National Association of Broadcasters Engineering Handbook
  | chapter   = Worldwide Standards for Analog Television
  | publisher = Academic Press
  | date      = 2007
}}
</ref> 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.<ref name="auto"/>

== References ==
{{Reflist}}

== See also ==
*[[Dot crawl]]
*[[PAL]]
*[[PAL-S]]

[[Category:Television technology]]

{{Analogue TV transmitter topics}}