Editing Overscan

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==Datacasting==
 
==Datacasting==
Analog TV overscan can also be used for datacasting. The simplest form of this is closed captioning and teletext, both sent in the vertical blanking interval (VBI).  Electronic program guides, such as TV Guide On Screen, are also sent in this manner. Microsoft's HOS uses the horizontal overscan instead of the vertical to transmit low-speed program-associated data at 6.4 kbit/s, which is slow enough to be recorded on a VCR without data corruption.<ref>{{cite web|url=http://www.microsoft.com/about/legal/intellectualproperty/search/details.mspx?ip_id=IDAEQMXD&techType=&ipCat=Any&feeStructure=Any&keywords=&ipVenture=true |title=Microsoft Intellectual Property and Licensing |publisher=Microsoft.com |date=2011-10-27 |accessdate=2012-01-04}}</ref>  In the United States, National Datacast used Public Broadcasting Service network stations for overscan and other datacasting, but they migrated to digital TV due to the digital television transition in 2009.
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Analog TV overscan can also be used for datacasting. The simplest form of this is closed captioning and teletext, both sent in the vertical blanking interval (VBI).  Electronic program guides, such as TV Guide On Screen, are also sent in this manner. Microsoft's HOS uses the horizontal overscan instead of the vertical to transmit low-speed program-associated data at 6.4 kbit/s, which is slow enough to be recorded on a VCR without data corruption.<ref>{{cite web|url=http://www.microsoft.com/about/legal/intellectualproperty/search/details.mspx?ip_id=IDAEQMXD&techType=&ipCat=Any&feeStructure=Any&keywords=&ipVenture=true |title=Microsoft Intellectual Property and Licensing |publisher=Microsoft.com |date=2011-10-27 |accessdate=2012-01-04}}</ref>  In the United States, National Datacast used Public Broadcasting Service network stations for overscan and other datacasting, but they Digital television transition|migrated to digital TV due to the digital television transition in 2009.
  
 
==Overscan amounts==
 
==Overscan amounts==
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[[File:overscan examples.svg|thumb|500px|Illustration of Action Safe and Title Safe areas for 4:3 and 16:9 aspect ratios according to the BBC.]]
 
There is no hard technical specification for overscan amounts for the low definition formats. Some say 5%, some say 10%, and the figure can be doubled for title safe, which needs more margin compared to action safe.  The overscan amounts are specified for the high definition formats as specified above.
 
There is no hard technical specification for overscan amounts for the low definition formats. Some say 5%, some say 10%, and the figure can be doubled for title safe, which needs more margin compared to action safe.  The overscan amounts are specified for the high definition formats as specified above.
  
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* 625-line analogue video contains 575 active video lines<ref>ITU-R BT.470-6</ref> (this includes two half lines). When the half lines are rounded up to whole lines for ease of digital representation, this gives 576 lines, which is also the nearest mod(16) value to 575. To maintain the same picture aspect ratio, the number of active pixels could be increased to 703.2, which can be rounded up to 704.
 
* 625-line analogue video contains 575 active video lines<ref>ITU-R BT.470-6</ref> (this includes two half lines). When the half lines are rounded up to whole lines for ease of digital representation, this gives 576 lines, which is also the nearest mod(16) value to 575. To maintain the same picture aspect ratio, the number of active pixels could be increased to 703.2, which can be rounded up to 704.
  
* 525-line analogue video contains 485 active video lines<ref name="SMPTE 170M"/> (this include two half lines, though typically only 483 picture lines are present due to Closed Captions data taking up the first "active picture" line on each field). The nearest mod(16) value is 480. To maintain the same picture aspect ratio, the number of active pixels could be decreased to 706.2, which can be rounded down to 704 for mod(16).
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* 525-line analogue video contains 485 active video lines<ref name="SMPTE 170M"/> (this include two half lines, though typically only 483 picture lines are present due to [[Closed Captions]] data taking up the first "active picture" line on each field). The nearest mod(16) value is 480. To maintain the same picture aspect ratio, the number of active pixels could be decreased to 706.2, which can be rounded down to 704 for mod(16).
  
 
The "standard" pixel aspect ratio data found in video editors, certain ITU standards, MPEG etc. is usually based on an approximation of the above, fudged to allow either 704 or 720 pixels to equate to the full 4x3 or 16x9 picture at the whim of the author.<ref>{{cite web|url=http://forum.doom9.org/showthread.php?p=1110419#post1110419 |author=facialz |title=Table of PAL PARs - DVD conversion |page=2 |publisher=Doom9's Forum |date=2008-03-09 |accessdate=2012-05-01}}</ref>
 
The "standard" pixel aspect ratio data found in video editors, certain ITU standards, MPEG etc. is usually based on an approximation of the above, fudged to allow either 704 or 720 pixels to equate to the full 4x3 or 16x9 picture at the whim of the author.<ref>{{cite web|url=http://forum.doom9.org/showthread.php?p=1110419#post1110419 |author=facialz |title=Table of PAL PARs - DVD conversion |page=2 |publisher=Doom9's Forum |date=2008-03-09 |accessdate=2012-05-01}}</ref>
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====625 / 525 or 576 / 480====
 
====625 / 525 or 576 / 480====
In broadcasting, analogue system descriptions include the lines ''not used'' for the visible picture, whereas the digital systems only 'number' and encode signals that contain something to see.
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In [[broadcasting]], analogue system descriptions include the lines ''not used'' for the visible picture, whereas the digital systems only 'number' and encode signals that contain something to see.
  
The 625 (''PAL'') and 525 (''NTSC'') frame areas therefore contain even more overscan, which can be seen when vertical hold is lost and the picture rolls.
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The 625 (''PAL'') and 525 (''NTSC'') frame areas therefore contain even more overscan, which can be seen when vertical hold is lost and the picture rolls.{{Citation needed|date=August 2009}}
  
A portion of this interval available in analogue, known as the vertical blanking interval, can be used for older forms of analogue datacasting such as Teletext services (like Ceefax and subtitling in the UK). The equivalent service on Digital television does not use this method and instead often uses MHEG.
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A portion of this interval available in analogue, known as the [vertical blanking interval, can be used for older forms of analogue datacasting such as Teletext services (like Ceefax and subtitling in the UK). The equivalent service on Digital television does not use this method and instead often uses MHEG.
  
Horizontally, the difference between 702/704 and 720 pixels/line is referred to as nominal analogue blanking.
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Horizontally, the difference between 702/704 and 720 pixels/line is referred to as [[nominal analogue blanking]].
  
 
====480 vs 486====
 
====480 vs 486====

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