How television works
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[edit] Analog television
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).
[edit] Displaying a picture
A CRT television displays an image by scanning a beam of electrons across the screen in a pattern of horizontal lines known as a 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.
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 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.
[edit] Components of a television system
A practical television system needs to take luminance, chrominance (in a color system), synchronization (horizontal and vertical), and audio signals, and broadcast them over a radio transmission. The transmission system must include a means of channel selection.
A typical analog television receiver is based around the block diagram shown below:
[edit] 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 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 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 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 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.
[edit] 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 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.
[edit] Synchronisation
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It has been suggested that vertical synchronization pulse be merged into this article or section. (Discuss) |
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.
[edit] 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.
[edit] Power Supply
Most of the receiver's circuitry (at least in Transistor or IC based designs) operates from a comparatively low-voltage 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 primary winding of a flyback transformer. The secondary of this is fed to a high-voltage rectifier which produces the required 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.
[edit] 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 (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 3.58MHz, in PAL it is roughly 4.43MHz - 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 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 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.
