Archive:How television works - Revision history

wikademia>Wikademia: moved How television works to Archive:How television works

2009-06-12T16:18:26Z

moved How television works to Archive:How television works

← Older revision	Revision as of 16:18, 12 June 2009
(No difference)

wikademia>Wikademia at 21:59, 5 June 2009

2009-06-05T21:59:05Z

← Older revision		Revision as of 21:59, 5 June 2009
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	==Displaying a picture==		==Displaying a picture==
			{{?}}
	A [[cathode-ray tube\|CRT]] television displays an image by scanning a beam of [[electrons]] across the screen in a pattern of horizontal lines known as a [[raster scan\|raster]]. At the end of each line the beam returns to the start of the next line; at the end of the last line it returns to the top of the screen. As it passes each point the intensity of the beam is varied, varying the brightness (technically, [[luminance]]) of that point. A [[color television]] system is identical except that an additional signal known as [[chrominance]] controls the color of the spot.		A [[cathode-ray tube\|CRT]] television displays an image by scanning a beam of [[electrons]] across the screen in a pattern of horizontal lines known as a [[raster scan\|raster]]. At the end of each line the beam returns to the start of the next line; at the end of the last line it returns to the top of the screen. As it passes each point the intensity of the beam is varied, varying the brightness (technically, [[luminance]]) of that point. A [[color television]] system is identical except that an additional signal known as [[chrominance]] controls the color of the spot.

wikademia>Wikademia at 05:34, 27 May 2009

2009-05-27T05:34:40Z

← Older revision		Revision as of 05:34, 27 May 2009
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	~~<!-- Please do not remove or change this AfD message until the issue is settled -->~~
	~~{{AfDM\|page=How television works\|logdate=2009 May 24\|substed=yes }}~~
	~~<!-- For administrator use only: {{oldafdfull\|page=How television works\|date=24 May 2009\|result='''keep'''}} -->~~
	~~<!-- End of AfD message, feel free to edit beyond this point -->~~
	~~{{Unreferenced\|date=April 2008}}~~

	Until the advent of [[digital television]] and [[flat panel displays]] in the 1990s, all television was based on the transmission and reception of [[analog signals]], displayed on a [[cathode-ray tube]]. Although a number of different [[broadcast television systems]] were in use worldwide, the same principles of operation apply.		Until the advent of [[digital television]] and [[flat panel displays]] in the 1990s, all television was based on the transmission and reception of [[analog signals]], displayed on a [[cathode-ray tube]]. Although a number of different [[broadcast television systems]] were in use worldwide, the same principles of operation apply.

wikademia>Wikademia: 43 revisions

2009-05-25T04:44:50Z

43 revisions

← Older revision	Revision as of 04:44, 25 May 2009
(No difference)

wikademia>ViperSnake151: Nominated for deletion; see Wikipedia:Articles for deletion/How television works. w/TW

2009-05-24T02:34:02Z

Nominated for deletion; see Wikipedia:Articles for deletion/How television works. w/TW

← Older revision		Revision as of 02:34, 24 May 2009
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			<!-- Please do not remove or change this AfD message until the issue is settled -->
			{{AfDM\|page=How television works\|logdate=2009 May 24\|substed=yes }}
			<!-- For administrator use only: {{oldafdfull\|page=How television works\|date=24 May 2009\|result='''keep'''}} -->
			<!-- End of AfD message, feel free to edit beyond this point -->
	{{Unreferenced\|date=April 2008}}		{{Unreferenced\|date=April 2008}}

194.144.106.78: /* Synchronisation */ Link added to "Black level"

2009-05-07T22:09:12Z

Synchronisation: Link added to "Black level"

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	{{Mergefrom\|vertical synchronization pulse\|date=May 2008}}		{{Mergefrom\|vertical synchronization pulse\|date=May 2008}}

	Synchronisation is transmitted via negative-going pulses; in a composite video signal these are approximately 0.3V below the 'black' level. The ''horizontal sync'' signal is a single short pulse which indicates the start of every line. Two timing intervals are defined - the ''front porch'' between the end of displayed video and the start of the sync pulse, and the ''back porch'' after the sync pulse and before displayed video. These and the sync pulse itself are called the ''horizontal blanking'' (or ''retrace'') ''interval'' and represent the time that the electron beam in the CRT is returning to the start of the next display line.		Synchronisation is transmitted via negative-going pulses; in a composite video signal these are approximately 0.3V below the [[Black_level\|"black level"]]. The ''horizontal sync'' signal is a single short pulse which indicates the start of every line. Two timing intervals are defined - the ''front porch'' between the end of displayed video and the start of the sync pulse, and the ''back porch'' after the sync pulse and before displayed video. These and the sync pulse itself are called the ''horizontal blanking'' (or ''retrace'') ''interval'' and represent the time that the electron beam in the CRT is returning to the start of the next display line.

	The ''vertical sync'' signal is a series of much longer pulses, indicating the start of a new field. The sync pulses occupy the whole of line interval of a number of lines at the beginning and end of a scan; no picture information is transmitted during vertical retrace. The pulse sequence is designed to allow horizontal sync to continue during vertical retrace; it also indicates whether each field represents even or odd lines in interlaced systems (depending on whether it begins at the start of a horizontal line, or mid-way through).		The ''vertical sync'' signal is a series of much longer pulses, indicating the start of a new field. The sync pulses occupy the whole of line interval of a number of lines at the beginning and end of a scan; no picture information is transmitted during vertical retrace. The pulse sequence is designed to allow horizontal sync to continue during vertical retrace; it also indicates whether each field represents even or odd lines in interlaced systems (depending on whether it begins at the start of a horizontal line, or mid-way through).

90.207.176.55: /* References */

2009-04-20T11:53:16Z

References

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	==References==		==References==
	{{Reflist}}		{{Reflist}}
	~~pooing on cheese~~
	~~Eat my bum~~

	==External links==		==External links==

90.207.176.55: /* References */

2009-04-20T11:53:04Z

References

← Older revision		Revision as of 11:53, 20 April 2009
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	{{Reflist}}		{{Reflist}}
	pooing on cheese		pooing on cheese
			Eat my bum

	==External links==		==External links==

90.207.176.55: /* References */

2009-04-20T11:51:56Z

References

← Older revision		Revision as of 11:51, 20 April 2009
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	==References==		==References==
	{{Reflist}}		{{Reflist}}
			pooing on cheese

	==External links==		==External links==

wikademia>Lomn: restore unreferenced tag, adjust header levels

2009-03-30T13:09:34Z

restore unreferenced tag, adjust header levels

← Older revision		Revision as of 13:09, 30 March 2009
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			{{Unreferenced\|date=April 2008}}

			Until the advent of [[digital television]] and [[flat panel displays]] in the 1990s, all television was based on the transmission and reception of [[analog signals]], displayed on a [[cathode-ray tube]]. Although a number of different [[broadcast television systems]] were in use worldwide, the same principles of operation apply.

			The first analog television systems were [[monochrome]]; they were enhanced to include color beginning in the 1960s (see article [[History of television]]).

			==Displaying a picture==

	A [[cathode-ray tube\|CRT]] television displays an image by scanning a beam of [[electrons]] across the screen in a pattern of horizontal lines known as a [[raster scan\|raster]]. At the end of each line the beam returns to the start of the next line; at the end of the last line it returns to the top of the screen. As it passes each point the intensity of the beam is varied, varying the brightness (technically, [[luminance]]) of that point. A [[color television]] system is identical except that an additional signal known as [[chrominance]] controls the color of the spot.		A [[cathode-ray tube\|CRT]] television displays an image by scanning a beam of [[electrons]] across the screen in a pattern of horizontal lines known as a [[raster scan\|raster]]. At the end of each line the beam returns to the start of the next line; at the end of the last line it returns to the top of the screen. As it passes each point the intensity of the beam is varied, varying the brightness (technically, [[luminance]]) of that point. A [[color television]] system is identical except that an additional signal known as [[chrominance]] controls the color of the spot.
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	The physics of the CRT require that a finite time interval is allowed for the spot to move back to the start of the next line (''horizontal retrace'') or the start of the screen (''vertical retrace''). The timing of the luminance signal must allow for this.		The physics of the CRT require that a finite time interval is allowed for the spot to move back to the start of the next line (''horizontal retrace'') or the start of the screen (''vertical retrace''). The timing of the luminance signal must allow for this.

	Raster scanning has to be performed sufficiently quickly that [[persistence of vision]] allows the eye to view a stable image, and such that moving images can be displayed without appearing jerky. The maximum [[frame rate]] achievable depends on the [[Bandwidth (signal processing)\|bandwidth]] of the electronics and transmission system, and the number of lines in the image. In practice, a rate of 50 or 60 [[hertz]] is a satisfactory compromise, with [[interlacing]] used to double the apparent number of lines .		Raster scanning has to be performed sufficiently quickly that [[persistence of vision]] allows the eye to view a stable image, and such that moving images can be displayed without appearing jerky. The maximum [[frame rate]] achievable depends on the [[Bandwidth (signal processing)\|bandwidth]] of the electronics and transmission system, and the number of lines in the image. In practice, a rate of 50 or 60 [[hertz]] is a satisfactory compromise, with [[interlacing]] used to double the apparent number of lines.

	===Components of a television system===		==Components of a television system==

	A practical television system needs to take luminance, chrominance (in a color system), synchronization (horizontal and vertical), and [[Sound recording and reproduction\|audio]] signals, and [[broadcast]] them over a [[radio]] transmission. The transmission system must include a means of [[television channel\|channel]] selection.		A practical television system needs to take luminance, chrominance (in a color system), synchronization (horizontal and vertical), and [[Sound recording and reproduction\|audio]] signals, and [[broadcast]] them over a [[radio]] transmission. The transmission system must include a means of [[television channel\|channel]] selection.
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	[[Image:TV-block-diagram.svg]]		[[Image:TV-block-diagram.svg]]

	====Receiving the signal====		===Receiving the signal===

	The television system for each country will specify a number of ''channels'' within the [[UHF]] or [[VHF]] frequency ranges. A channel actually consists of two signals: the picture information is transmitted using [[amplitude modulation]] on one frequency, and the sound is transmitted with [[frequency modulation]] at a frequency at a fixed offset (typically 4.5 to 6MHz) from the picture signal.		The television system for each country will specify a number of ''channels'' within the [[UHF]] or [[VHF]] frequency ranges. A channel actually consists of two signals: the picture information is transmitted using [[amplitude modulation]] on one frequency, and the sound is transmitted with [[frequency modulation]] at a frequency at a fixed offset (typically 4.5 to 6MHz) from the picture signal.
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	The FM sound carrier is then demodulated, amplified, and used to drive a loudspeaker. Until the advent of [[NICAM]] sound transmission was invariably monophonic.		The FM sound carrier is then demodulated, amplified, and used to drive a loudspeaker. Until the advent of [[NICAM]] sound transmission was invariably monophonic.

	====Picture and Synchronisation====		===Picture and Synchronisation===

	The video carrier is demodulated to give a [[composite video]] signal; this contains luminance (brightness), chrominance (color) and synchronisation signals; this is identical to the video signal format used by analog video devices such as [[VCRs]] or [[closed-circuit television\|CCTV cameras]]. Note that the RF signal modulation is inverted compared to the conventional AM: the minimum video signal level corresponds to maximum carrier amplitude, and vice versa. The carrier is never shut off altogether; this is to ensure that intercarrier sound demodulation can still occur.		The video carrier is demodulated to give a [[composite video]] signal; this contains luminance (brightness), chrominance (color) and synchronisation signals; this is identical to the video signal format used by analog video devices such as [[VCRs]] or [[closed-circuit television\|CCTV cameras]]. Note that the RF signal modulation is inverted compared to the conventional AM: the minimum video signal level corresponds to maximum carrier amplitude, and vice versa. The carrier is never shut off altogether; this is to ensure that intercarrier sound demodulation can still occur.
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	[[Image:Video-line.svg]]		[[Image:Video-line.svg]]

	====Synchronisation====		===Synchronisation===
	{{Mergefrom\|vertical synchronization pulse\|date=May 2008}}		{{Mergefrom\|vertical synchronization pulse\|date=May 2008}}

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	The lack of precision timing components available in early television receivers meant that the timebase circuits occasionally needed manual adjustment. The adjustment took the form of ''horizontal hold'' and ''vertical hold'' controls, usually on the rear of the set. Loss of horizontal synchronisation usually resulted in an unwatchable picture; loss of vertical synchronisation would produce an image rolling up or down the screen.		The lack of precision timing components available in early television receivers meant that the timebase circuits occasionally needed manual adjustment. The adjustment took the form of ''horizontal hold'' and ''vertical hold'' controls, usually on the rear of the set. Loss of horizontal synchronisation usually resulted in an unwatchable picture; loss of vertical synchronisation would produce an image rolling up or down the screen.

	====Monochrome video====		===Monochrome video===

	The luminance component of a composite video signal varies between 0V and approximately 0.7V above the 'black' level. (In the NTSC system, there is in fact a ''blanking'' signal level used during the front porch and back porch, and a ''black'' signal level 75mV above it; in PAL and SECAM these are identical).		The luminance component of a composite video signal varies between 0V and approximately 0.7V above the 'black' level. (In the NTSC system, there is in fact a ''blanking'' signal level used during the front porch and back porch, and a ''black'' signal level 75mV above it; in PAL and SECAM these are identical).
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	In a monochrome receiver the luminance signal is simply amplified (with brightness and contrast controls determining DC shift and amplification, respectively) and used to drive the [[control grid]] in the [[electron gun]] of the CRT. This changes the intensity of the electron beam and therefore the brightness of the spot being scanned.		In a monochrome receiver the luminance signal is simply amplified (with brightness and contrast controls determining DC shift and amplification, respectively) and used to drive the [[control grid]] in the [[electron gun]] of the CRT. This changes the intensity of the electron beam and therefore the brightness of the spot being scanned.

	====Power Supply====		===Power Supply===

	Most of the receiver's circuitry (at least in [[Transistor]] or [[Integrated circuit\|IC]] based designs) operates from a comparatively low-voltage [[direct-current\|DC]] power supply. However, the [[anode]] connection requires a very high voltage (typically 10-30kV) for correct operation.		Most of the receiver's circuitry (at least in [[Transistor]] or [[Integrated circuit\|IC]] based designs) operates from a comparatively low-voltage [[direct-current\|DC]] power supply. However, the [[anode]] connection requires a very high voltage (typically 10-30kV) for correct operation.
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	Typically, the flyback transformer and rectifier circuitry are incorporated into a single unit with a captive output lead, so that all high-voltage parts are enclosed. The high frequency (15Khz or so) of the horizontal scanning allows reasonably small components to be used.		Typically, the flyback transformer and rectifier circuitry are incorporated into a single unit with a captive output lead, so that all high-voltage parts are enclosed. The high frequency (15Khz or so) of the horizontal scanning allows reasonably small components to be used.

	===Color video===		==Color video==

	A color signal conveys picture information for each of the red, green, and blue components of an image (see the article on [[Color space]] for more information). However, these are not simply transmitted as three separate signals, because:		A color signal conveys picture information for each of the red, green, and blue components of an image (see the article on [[Color space]] for more information). However, these are not simply transmitted as three separate signals, because: