IdeaWazaWiki - User contributions [en]

Archive:How television works

2009-04-20T11:53:16Z

90.207.176.55: /* References */

{{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.

Raster scanning is shown in a slightly simplified form below.

[[Image:Raster-scan.svg]]

When analog television was developed, no affordable technology for storing any video signals existed; the luminance signal has to be generated and transmitted at exactly the point in time at which is displayed on the CRT. It is therefore essential to keep the raster scanning in the camera (or other device for producing the signal) in exact [[synchronization]] with the scanning in the television.

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.

==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 typical analog television receiver is based around the block diagram shown below:

[[Image:TV-block-diagram.svg]]

===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 channel frequencies chosen represent a compromise between allowing enough [[Bandwidth (signal processing)|bandwidth]] for video (and hence satisfactory picture resolution), and allowing enough channels to be packed into the available frequency band. In practice a technique called [[vestigial sideband]] is used to reduce the channel spacing, which would be at least twice the video bandwidth if purely AM was used.

Signal reception is invariably done via a [[superhet]] receiver: the first stage is a ''tuner'' which selects a channel and frequency-shifts it to a fixed [[intermediate frequency]] (IF). Signal [[amplifier|amplification]] (from the microvolt range to fractions of a volt) is then performed largely by the IF stages.

At this point the IF signal consists of a video [[carrier wave|carrier]] at one frequency and the sound carrier at a fixed offset. Early systems would feed this to a simple [[demodulator]], which produced a video signal at [[baseband]] and the sound as an FM signal at the offset frequency (this is known as ''intercarrier sound''). Later systems [[electronic filter|filter]] the IF first to prevent interference between sound and vision.

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===

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.

Each line of the displayed image is transmitted using a signal as shown below. The same basic format (with minor differences mainly related to timing and the encoding of color) is used for [[PAL]], [[NTSC]] and [[SECAM]] television systems. A monochrome signal is identical to a color one, with the exception that the elements shown in color in the diagram (the color burst, and the chrominance signal) are not present.

[[Image:Video-line.svg]]

===Synchronisation===
{{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.

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).

In the TV receiver, a ''sync separator'' circuit detects the sync voltage levels and sorts the pulses into horizontal and vertical sync. These are fed to horizontal and vertical ''timebase'' circuits which generate [[sawtooth]] current waveforms, which are each reset by the appropriate sync pulse. These waveforms are fed to the horizontal and vertical ''scan coils'' wrapped around the CRT tube. These produce a [[magnetic field]] proportional to the changing current, and this deflects the electron beam, scanning it across the tube surface.

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===

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).

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===

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.

This voltage is not generally produced by the main [[power supply]] circuitry; instead the receiver makes use of the circuitry used for horizontal scanning. At the end of each horizontal scan line, the [[magnetic field]] which has built up in the scan coils contains electromagnetic [[energy]]. This must be dissipated when the field is reversed during horizontal retrace. Instead of being dissipated as waste heat, the horizontal scan coil is discharged into the [[transformer|primary winding]] of a [[flyback transformer]]. The secondary of this is fed to a high-voltage [[rectifier]] which produces the required [[extra high tension|EHT]] supply (see [[flyback converter]] for a detailed description of this form of power supply).

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==

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:
* such a signal would not be compatible with monochrome receivers (an important consideration when color broadcasting was first introduced)
* it would occupy three times the bandwidth of existing television, requiring a decrease in the number of channels available
* typical problems with signal transmission (such as differing received signal levels between different colors) would produce unpleasant side-effects.

Instead, the RGB signals are converted into [[YUV]] form, where the Y signal represents the overall brightness, and can be transmitted as the luminance signal. This ensures a monochrome receiver will display a correct picture. The U signal then represents how 'blue' the color is, and the V signal how 'red' it is. As the eye is more sensitive to errors in luminance than in color, the U and V signals can be transmitted in a relatively [[lossy compression|lossy]] (specifically: bandwidth-limited) way with acceptable results.

In the [[NTSC]] and [[PAL]] color systems, U and V are transmitted by adding a ''color subcarrier'' to the composite video signal, and using [[quadrature amplitude modulation]] on it. In NTSC, the subcarrier is at approximately 3.58 MHz, in PAL it is roughly 4.43 MHz - these are chosen to be above the baseband luminance signal, but below the FM sound carrier.

The two signals (U and V) modulate both the [[amplitude]] and [[phase]] of the color carrier, so to demodulate them it is necessary to have a reference signal against which to compare it. For this reason a short burst of reference signal known as the ''color burst'' is transmitted during the back porch of each line. A reference oscillator in the receiver locks onto this signal (see [[phase-locked loop]]) to achieve a phase reference, and uses its amplitude to set an [[Automatic gain control|AGC]] system to achieve an amplitude reference.

The U and V signals are then demodulated by band-pass filtering to retrieve the color subcarrier, mixing it with the in-phase and [[quadrature phase|quadrature]] signals from the reference oscillator, and low-pass filtering the results.

NTSC uses this process unmodified; unfortunately this often results in poor color reproduction due to phase errors in the received signal. The PAL system corrects this by reversing the phase of the signal on each successive line and averaging the result over pairs of lines. Phase errors therefore tend to be cancelled out.

In the [[SECAM]] television system, U and V are transmitted on ''alternate'' lines, using simple [[frequency modulation]] of the color subcarrier.

==See also==
* [[History of television]]
* [[Broadcast television systems]]
* [[NTSC]], [[PAL]] and [[SECAM]]

==References==
{{Reflist}}

==External links==
*[http://zone.ni.com/devzone/cda/tut/p/id/4750 Video signal measurement and generation]
*[http://freespace.virgin.net/ljmayes.mal/var/tvsync.htm Television synchronisation]
*[http://www.penders.cwc.net/otfrequenc.html UK TV frequency listings for (obsolete) 405-line, and 625-line systems]
*[http://www.hometechanswers.com/video/standards.html Video broadcast standard frequencies and country listings]
*[http://www.edn.com/article/CA6335297.html EDN magazine describing design of a 1958 transistorised TV receiver]

[[Category:Television technology]]

[[fa:طرز کار تلویزیون]]

Archive:How television works

2009-04-20T11:53:04Z

90.207.176.55: /* References */

{{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.

Raster scanning is shown in a slightly simplified form below.

[[Image:Raster-scan.svg]]

When analog television was developed, no affordable technology for storing any video signals existed; the luminance signal has to be generated and transmitted at exactly the point in time at which is displayed on the CRT. It is therefore essential to keep the raster scanning in the camera (or other device for producing the signal) in exact [[synchronization]] with the scanning in the television.

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.

==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 typical analog television receiver is based around the block diagram shown below:

[[Image:TV-block-diagram.svg]]

===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 channel frequencies chosen represent a compromise between allowing enough [[Bandwidth (signal processing)|bandwidth]] for video (and hence satisfactory picture resolution), and allowing enough channels to be packed into the available frequency band. In practice a technique called [[vestigial sideband]] is used to reduce the channel spacing, which would be at least twice the video bandwidth if purely AM was used.

Signal reception is invariably done via a [[superhet]] receiver: the first stage is a ''tuner'' which selects a channel and frequency-shifts it to a fixed [[intermediate frequency]] (IF). Signal [[amplifier|amplification]] (from the microvolt range to fractions of a volt) is then performed largely by the IF stages.

At this point the IF signal consists of a video [[carrier wave|carrier]] at one frequency and the sound carrier at a fixed offset. Early systems would feed this to a simple [[demodulator]], which produced a video signal at [[baseband]] and the sound as an FM signal at the offset frequency (this is known as ''intercarrier sound''). Later systems [[electronic filter|filter]] the IF first to prevent interference between sound and vision.

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===

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.

Each line of the displayed image is transmitted using a signal as shown below. The same basic format (with minor differences mainly related to timing and the encoding of color) is used for [[PAL]], [[NTSC]] and [[SECAM]] television systems. A monochrome signal is identical to a color one, with the exception that the elements shown in color in the diagram (the color burst, and the chrominance signal) are not present.

[[Image:Video-line.svg]]

===Synchronisation===
{{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.

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).

In the TV receiver, a ''sync separator'' circuit detects the sync voltage levels and sorts the pulses into horizontal and vertical sync. These are fed to horizontal and vertical ''timebase'' circuits which generate [[sawtooth]] current waveforms, which are each reset by the appropriate sync pulse. These waveforms are fed to the horizontal and vertical ''scan coils'' wrapped around the CRT tube. These produce a [[magnetic field]] proportional to the changing current, and this deflects the electron beam, scanning it across the tube surface.

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===

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).

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===

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.

This voltage is not generally produced by the main [[power supply]] circuitry; instead the receiver makes use of the circuitry used for horizontal scanning. At the end of each horizontal scan line, the [[magnetic field]] which has built up in the scan coils contains electromagnetic [[energy]]. This must be dissipated when the field is reversed during horizontal retrace. Instead of being dissipated as waste heat, the horizontal scan coil is discharged into the [[transformer|primary winding]] of a [[flyback transformer]]. The secondary of this is fed to a high-voltage [[rectifier]] which produces the required [[extra high tension|EHT]] supply (see [[flyback converter]] for a detailed description of this form of power supply).

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==

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:
* such a signal would not be compatible with monochrome receivers (an important consideration when color broadcasting was first introduced)
* it would occupy three times the bandwidth of existing television, requiring a decrease in the number of channels available
* typical problems with signal transmission (such as differing received signal levels between different colors) would produce unpleasant side-effects.

Instead, the RGB signals are converted into [[YUV]] form, where the Y signal represents the overall brightness, and can be transmitted as the luminance signal. This ensures a monochrome receiver will display a correct picture. The U signal then represents how 'blue' the color is, and the V signal how 'red' it is. As the eye is more sensitive to errors in luminance than in color, the U and V signals can be transmitted in a relatively [[lossy compression|lossy]] (specifically: bandwidth-limited) way with acceptable results.

In the [[NTSC]] and [[PAL]] color systems, U and V are transmitted by adding a ''color subcarrier'' to the composite video signal, and using [[quadrature amplitude modulation]] on it. In NTSC, the subcarrier is at approximately 3.58 MHz, in PAL it is roughly 4.43 MHz - these are chosen to be above the baseband luminance signal, but below the FM sound carrier.

The two signals (U and V) modulate both the [[amplitude]] and [[phase]] of the color carrier, so to demodulate them it is necessary to have a reference signal against which to compare it. For this reason a short burst of reference signal known as the ''color burst'' is transmitted during the back porch of each line. A reference oscillator in the receiver locks onto this signal (see [[phase-locked loop]]) to achieve a phase reference, and uses its amplitude to set an [[Automatic gain control|AGC]] system to achieve an amplitude reference.

The U and V signals are then demodulated by band-pass filtering to retrieve the color subcarrier, mixing it with the in-phase and [[quadrature phase|quadrature]] signals from the reference oscillator, and low-pass filtering the results.

NTSC uses this process unmodified; unfortunately this often results in poor color reproduction due to phase errors in the received signal. The PAL system corrects this by reversing the phase of the signal on each successive line and averaging the result over pairs of lines. Phase errors therefore tend to be cancelled out.

In the [[SECAM]] television system, U and V are transmitted on ''alternate'' lines, using simple [[frequency modulation]] of the color subcarrier.

==See also==
* [[History of television]]
* [[Broadcast television systems]]
* [[NTSC]], [[PAL]] and [[SECAM]]

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

==External links==
*[http://zone.ni.com/devzone/cda/tut/p/id/4750 Video signal measurement and generation]
*[http://freespace.virgin.net/ljmayes.mal/var/tvsync.htm Television synchronisation]
*[http://www.penders.cwc.net/otfrequenc.html UK TV frequency listings for (obsolete) 405-line, and 625-line systems]
*[http://www.hometechanswers.com/video/standards.html Video broadcast standard frequencies and country listings]
*[http://www.edn.com/article/CA6335297.html EDN magazine describing design of a 1958 transistorised TV receiver]

[[Category:Television technology]]

[[fa:طرز کار تلویزیون]]

Archive:How television works

2009-04-20T11:51:56Z

90.207.176.55: /* References */

{{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.

Raster scanning is shown in a slightly simplified form below.

[[Image:Raster-scan.svg]]

When analog television was developed, no affordable technology for storing any video signals existed; the luminance signal has to be generated and transmitted at exactly the point in time at which is displayed on the CRT. It is therefore essential to keep the raster scanning in the camera (or other device for producing the signal) in exact [[synchronization]] with the scanning in the television.

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.

==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 typical analog television receiver is based around the block diagram shown below:

[[Image:TV-block-diagram.svg]]

===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 channel frequencies chosen represent a compromise between allowing enough [[Bandwidth (signal processing)|bandwidth]] for video (and hence satisfactory picture resolution), and allowing enough channels to be packed into the available frequency band. In practice a technique called [[vestigial sideband]] is used to reduce the channel spacing, which would be at least twice the video bandwidth if purely AM was used.

Signal reception is invariably done via a [[superhet]] receiver: the first stage is a ''tuner'' which selects a channel and frequency-shifts it to a fixed [[intermediate frequency]] (IF). Signal [[amplifier|amplification]] (from the microvolt range to fractions of a volt) is then performed largely by the IF stages.

At this point the IF signal consists of a video [[carrier wave|carrier]] at one frequency and the sound carrier at a fixed offset. Early systems would feed this to a simple [[demodulator]], which produced a video signal at [[baseband]] and the sound as an FM signal at the offset frequency (this is known as ''intercarrier sound''). Later systems [[electronic filter|filter]] the IF first to prevent interference between sound and vision.

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===

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.

Each line of the displayed image is transmitted using a signal as shown below. The same basic format (with minor differences mainly related to timing and the encoding of color) is used for [[PAL]], [[NTSC]] and [[SECAM]] television systems. A monochrome signal is identical to a color one, with the exception that the elements shown in color in the diagram (the color burst, and the chrominance signal) are not present.

[[Image:Video-line.svg]]

===Synchronisation===
{{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.

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).

In the TV receiver, a ''sync separator'' circuit detects the sync voltage levels and sorts the pulses into horizontal and vertical sync. These are fed to horizontal and vertical ''timebase'' circuits which generate [[sawtooth]] current waveforms, which are each reset by the appropriate sync pulse. These waveforms are fed to the horizontal and vertical ''scan coils'' wrapped around the CRT tube. These produce a [[magnetic field]] proportional to the changing current, and this deflects the electron beam, scanning it across the tube surface.

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===

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).

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===

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.

This voltage is not generally produced by the main [[power supply]] circuitry; instead the receiver makes use of the circuitry used for horizontal scanning. At the end of each horizontal scan line, the [[magnetic field]] which has built up in the scan coils contains electromagnetic [[energy]]. This must be dissipated when the field is reversed during horizontal retrace. Instead of being dissipated as waste heat, the horizontal scan coil is discharged into the [[transformer|primary winding]] of a [[flyback transformer]]. The secondary of this is fed to a high-voltage [[rectifier]] which produces the required [[extra high tension|EHT]] supply (see [[flyback converter]] for a detailed description of this form of power supply).

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==

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:
* such a signal would not be compatible with monochrome receivers (an important consideration when color broadcasting was first introduced)
* it would occupy three times the bandwidth of existing television, requiring a decrease in the number of channels available
* typical problems with signal transmission (such as differing received signal levels between different colors) would produce unpleasant side-effects.

Instead, the RGB signals are converted into [[YUV]] form, where the Y signal represents the overall brightness, and can be transmitted as the luminance signal. This ensures a monochrome receiver will display a correct picture. The U signal then represents how 'blue' the color is, and the V signal how 'red' it is. As the eye is more sensitive to errors in luminance than in color, the U and V signals can be transmitted in a relatively [[lossy compression|lossy]] (specifically: bandwidth-limited) way with acceptable results.

In the [[NTSC]] and [[PAL]] color systems, U and V are transmitted by adding a ''color subcarrier'' to the composite video signal, and using [[quadrature amplitude modulation]] on it. In NTSC, the subcarrier is at approximately 3.58 MHz, in PAL it is roughly 4.43 MHz - these are chosen to be above the baseband luminance signal, but below the FM sound carrier.

The two signals (U and V) modulate both the [[amplitude]] and [[phase]] of the color carrier, so to demodulate them it is necessary to have a reference signal against which to compare it. For this reason a short burst of reference signal known as the ''color burst'' is transmitted during the back porch of each line. A reference oscillator in the receiver locks onto this signal (see [[phase-locked loop]]) to achieve a phase reference, and uses its amplitude to set an [[Automatic gain control|AGC]] system to achieve an amplitude reference.

The U and V signals are then demodulated by band-pass filtering to retrieve the color subcarrier, mixing it with the in-phase and [[quadrature phase|quadrature]] signals from the reference oscillator, and low-pass filtering the results.

NTSC uses this process unmodified; unfortunately this often results in poor color reproduction due to phase errors in the received signal. The PAL system corrects this by reversing the phase of the signal on each successive line and averaging the result over pairs of lines. Phase errors therefore tend to be cancelled out.

In the [[SECAM]] television system, U and V are transmitted on ''alternate'' lines, using simple [[frequency modulation]] of the color subcarrier.

==See also==
* [[History of television]]
* [[Broadcast television systems]]
* [[NTSC]], [[PAL]] and [[SECAM]]

==References==
{{Reflist}}
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==External links==
*[http://zone.ni.com/devzone/cda/tut/p/id/4750 Video signal measurement and generation]
*[http://freespace.virgin.net/ljmayes.mal/var/tvsync.htm Television synchronisation]
*[http://www.penders.cwc.net/otfrequenc.html UK TV frequency listings for (obsolete) 405-line, and 625-line systems]
*[http://www.hometechanswers.com/video/standards.html Video broadcast standard frequencies and country listings]
*[http://www.edn.com/article/CA6335297.html EDN magazine describing design of a 1958 transistorised TV receiver]

[[Category:Television technology]]

[[fa:طرز کار تلویزیون]]

Archive:Nagina Group

2008-04-22T19:48:46Z

90.207.169.88:

{{Infobox_Company |
| company_name =Nagina Group
| company_logo =[[Image:Nagina group.JPG]]
| company_type =[[Public corporation]]
| foundation = [[1972]]
| location_city = Gilburg-III, Lahore
| location_country = Pakistan
| key_people = Shaikh Inam Ellahi, co-founder<br /> Shaukat Ellahi Shaikh, Shafqat Ellahi Shaikh, Shahzada Ellahi Shaikh, co-founder<br />
| industry = [[textiles, energy, trading,]]
| revenue = {{profit}} [[Pakistani rupee|Rs.]] 832 million,<br />$100 million [[United States dollar|USD]] ([[fiscal year|FY]] 2006)
| homepage = http://www.nagina.com/
}}

'''Nagina Group''' is a [[public corporation]] based in [[Pakistan]]i that manufactures a wide variety of [[textile]] products. Founded in [[1972]] by Inam Ellahi Shaikh and Ashan Ellahi Shaikh, it was listed by [[Forbes Magazine]] as the best-performing company with less than US$1 billion in revenue in [[2003]].

==History==
What is now known as The Nagina Group began as a single company, Nagina Mills Limited, established by Shaikh Inam Ellahi and Ashan Ellahi Shaikh in [[1967]]. Focusing on the production of [[cotton]] [[yarn]], it grew with the acquisition of [[Tanning|tanneries]], [[factory|factories]] that produced shoes and ice, and Pakistan's largest [[jute|jute mill]]. These assets were lost, however, when [[East Pakistan]] separated from the Pakistani Federation to become [[Bangladesh]] in [[1971]].

The company reformed in [[1971]] as the Nagina Group, consolidating its assets under a single [[umbrella organization]]. The group currently manages Nagina Cotton Mills Limited, Ellcot Spinning Mills Limited, and Prosperity Weaving Mills Limited. Together, the facilities hold approximately 100,000 modern [[spindle]]s, and produce forty-eight million pounds of carded and combed yarn annually.

Nagina Group also operates three private [[power plants]] to provide electricity for its mills. The installed capacity of its generators is 30.57 [[Megawatt]]s. Excess energy is sold to other industrial facilities in the area.

Nagina Group has an ambitious 5 year strategic objective. This objective includes expanding into logistics, consolidating position in energy production, and opening a 6 star beach resort in Karachi.

Nagina Cotton Mills Limited (Winner of Forbes Global Award "Best under a Billion-200") is the mother company of the Group, established in 1967. The company operates state of art spinning machinery comprising 47,040 Spindles with related process machines. Latest equipment from Japan, Europe and China allow production of high quality Spun Yarns in Carded and Combed versions for use in high speed looms and knitting machines. MUNDRI and ANGOTHI are popular brands of the company.

directors are pleased to report that prestigious International Business Magazine FORBES Global included the company in their list of "Best under a billion 200".

According to the magazine "Our list of 200 successful companies outside U.S. is drawn up from among 19,000 publicly traded concerns with annual below $ 1 billion. We screened out most corporation that didn't have operating margin of at least 5% over the past five aiming for the businesses that were profitable and that rewarded shareholders. A handful of companies that failed these tests were considered if they had outstanding results otherwise. The remaining 4000-odd concerns were graded against one another using various dollar based performance measures, and the top 500 reviewed for inclusion on this list".

Vision

Nagina Group’s vision is to provide an avenue for our partners and stakeholders to excel. We aspire to serve our country and to help build a bright future for millions of Pakistanis. Nagina Group strives genuinely to achieve the premier benchmarks of excellence and quality in the services we provide to our clients and to constantly improve our operations. Nagina Group provides our clients with a commitment to be the best.
==External links==

Nagina Group was founded on May 16th 1967 with the incorporation of Nagina Cotton Mills Limited. Since then Nagina Group has expanded its operations in textile manufacturing and has diversified into other sectors. Currently, Nagina Group’s textile manufacturing capacity is 100,956 spindles and 324 air jet looms.

A steady and well thought out growth strategy coupled with a solid commitment to quality has earned Nagina Group a family of satisfied customers, hardworking employees, appreciative shareholders and the confidence of the banking community. The core businesses of Nagina Group generate an annual turnover in excess of 100 Million US dollars. Due to decades of dealing with Pakistani, North American, European and Far Eastern customers Nagina Group has created a skilled and knowledgeable management fully capable of understanding and responding to customer needs in record time.

Nagina Group consists of three publicly listed companies on the Karachi Stock Exchange (Nagina Cotton Mills Limited, Ellcot Spinning Mills Limited, Prosperity Weaving Mills Limited), five private companies (Monell (Pvt.) Limited, Icaro (Pvt.) Limited, Haroon Omer (Pvt.) Limited, Ellahi International (Pvt.) Limited), ARH (Pvt.) Limited), and employs over 3,500 employees. Nagina Group was selected in 2003 by Forbes Magazine as one of the best 200 international companies with an annual turnover of under a billion US dollars.
{{Primarysources|date=February 2007}}
* http://www.nagina.com/

[[Category:Companies of Pakistan]]