Analogue television synchronization: Difference between revisions
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#REDIRECT [[Analog television]] |
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[[Image:Videosignal_porch.jpg|right|250px|thumb|Portion of a [[PAL]] videosignal. From left to right: end of a video line, [[front porch]], horizontal sync pulse, [[back porch]] with [[color burst]], and beginning of next line]] |
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[[Image:Videosignal_vsync.jpg|right|250px|thumb|Beginning of the frame, showing several scan lines; the terminal part of the vertical sync pulse is at the left]] |
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[[Image:Videosignal_frame.jpg|right|250px|thumb|PAL videosignal frames. Left to right: frame with scan lines (overlapping together, horizontal sync pulses show as the doubled straight horizontal lines), vertical blanking interval with vertical sync (shows as brightness increase of the bottom part of the signal in almost the leftmost part of the vertical blanking interval), entire frame, another VBI with VSYNC, beginning of third frame]] |
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A '''sync pulse''' is a component of a [[television]] [[scan line]]. |
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They have a voltage of approximately -300mV and occur at the end of each scan line, and exist in order to reset the [[electron]] beam at the end of each scan line. They comprise of a square shaped dip in the line just after the [[front porch]], and before the [[back porch]].<ref name="tveng" /><ref name="analogtv" /><ref name="tvreception" /> |
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It has a period of approximately 4.7 microseconds and is also known as the sync tip. |
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There are two kinds of sync pulses: '''horizontal sync''' ('''HSYNC'''), which is responsible for [[horizontal synchronization pulse]] and separating the [[scan line]]s, and '''vertical sync''' ('''VSYNC'''), responsible for [[vertical synchronization]] and separating the [[video frame]]s. In PAL and NTSC, the vertical sync pulse occurs within the [[vertical blanking interval]]. The vertical sync pulses are made by prolonging the length of HSYNC pulses through almost the entire length of the scan line. |
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In early days of television, vertical synchronization was derived from the phase of AC mains voltage. Therefore the [[frame rate]] matches the mains frequency in the respective countries: [[50 Hz]] for [[Europe]], [[60 Hz]] for USA and [[Japan]]. |
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==Horizontal synchronization pulse== |
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In [[television]], a '''horizontal synchronization pulse''' is sent whenever the television set should start scanning a new line. It disciplines a [[flyback transformer]] within the set. |
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The format of such a pulse consists of: |
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===525-line system (American)=== |
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# a 4.85 [[microsecond|µs]]-long synchronization pulse, where the signal level is set to 0 [[volt|V]] |
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# a 8 µs calibration delay, where the signal level is set to 0.3 V. The delay is used by the television set to adjust the level of black. (see [[System M]] ) |
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The rest of the [[scan line]] follows, with the signal ranging from 0.3 V (black) to 1 V (white), until the next horizontal or [[vertical synchronization pulse]]. |
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===405-line (obsolete British)=== |
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9 µs |
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===625-line (British and European)=== |
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4.7 µs synchronization pulse where the signal level is set to 0 [[volt|V]], the total blanking duration being approximatelly 12 µs. (see [[System B]]) |
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===819-line (obsolete French)=== |
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2.5 µs |
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==Vertical synchronization pulse== |
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In [[television]], a '''vertical synchronization pulse''' is sent whenever the television set should start scanning a new image from the top of the screen. |
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The format of such a signal consists in: |
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===525-line system (American)=== |
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* pre-equalizing pulses (6 to start scanning odd lines, 5 to start scanning even lines) |
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* long-sync pulses (5 pulses) |
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* post-equalizing pulses (5 to start scanning odd lines, 4 to start scanning even lines) |
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Each pre- or post- equalizing pulse consists in half a [[scan line]] of black signal: 2µs at 0V, followed by 30µs at 0.3V. |
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Each long sync pulse consists in an equalizing pulse with timings inverted: 30µs at 0V, followed by 2µs at 0.3V. |
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==See also== |
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*[[Back porch]] |
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*[[Colour burst]] |
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*[[Front porch]] |
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*[[Overscan]] |
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*[[TV transmitter]] |
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*[[Vertical blanking interval]] |
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*[[Vertical synchronization]] |
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*[[Video frame]] |
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==References== |
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{{Reflist |refs= |
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<ref name="tveng"> |
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{{cite book |
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| last = Gupta |
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| first = R. G. |
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| title = Television Engineering and Video Systems |
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| publisher = Tata McGraw-Hill |
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| date = 2006 |
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| page = 62 |
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| isbn = 0070585962 |
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| url = http://books.google.com/books?id=P6BlcWaizHUC |
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}} |
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</ref> |
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<ref name="analogtv"> |
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{{cite web |
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| url = http://www.pembers.freeserve.co.uk/World-TV-Standards/index.html |
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| title = World Analogue Television Standards and Waveforms |
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| first = Alan |
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| last = Pemberton |
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| date = 30 November 2008 |
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| work = Pembers' Ponderings |
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| publisher = |
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| location = [[Sheffield, England]] |
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| archiveurl = http://web.archive.org/web/20080220113153/http://www.pembers.freeserve.co.uk/World-TV-Standards/index.html |
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| archivedate = Feb 20, 2008 |
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| accessdate = 25 September 2010 |
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}} |
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</ref> |
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<ref name="tvreception"> |
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{{cite book |
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| last = Wharton |
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| first = W. |
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| coauthors = Douglas Howorth |
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| title = Principles of Television Reception |
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| url = http://books.google.com/books?id=dxOCAAAACAAJ |
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| archiveurl = |
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| archivedate = |
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| edition = illustrated |
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| year = 1971 |
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| publisher = Pitman Publishing |
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| isbn = 0273361031 |
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| oclc = 16244216 |
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}} |
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</ref> |
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}} |
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{{Analogue TV transmitter topics}} |
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{{DEFAULTSORT:Sync Pulse}} |
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[[Category:Video signal]] |
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[[Category:Television technology]] |
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[[Category:Television terminology]] |
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{{electronics-stub}} |
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{{tv-tech-stub}} |
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Revision as of 21:26, 6 October 2010
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