E-Notes Audio Video Systems

By-Suresh Kumar

Lecturer ECE

G.P Hisar

Chapter 1

Microphones & Loud Speakers

Introduction: A microphone, colloquially nicknamed mic or mike is a transducer that converts sound into an electrical signal.

Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public events, motion picture production, live and recorded audio engineering, sound recording, two-way radios, megaphones, radio and television broadcasting, and in computers for recording voice, speech recognition, VoIP, and for non-acoustic purposes such as ultrasonic sensors or knock sensors.

Several different types of microphone are in use, which employ different methods to convert the air pressure variations of a sound wave to an electrical signal. The most common are the dynamic microphone, which uses a coil of wire suspended in a magnetic field; the condenser microphone, which uses the vibrating diaphragm as a capacitor plate, and the piezoelectric microphone, which uses a crystal of piezoelectric material. Microphones typically need to be connected to a preamplifier before the signal can be recorded or reproduced.

Types of microphones

Carbon : The carbon microphone was the earliest type of microphone. The carbon button microphone (or sometimes just a button microphone), uses a capsule or button containing carbon granules pressed between two metal plates like the Berliner and Edison microphones. A voltage is applied across the metal plates, causing a small current to flow through the carbon. One of the plates, the diaphragm, vibrates in sympathy with incident sound waves, applying a varying pressure to the carbon.

Wireless Microphone : A wireless microphone is a microphone without a physical cable connecting it directly to the sound recording or amplifying equipment with which it is associated. Also known as a radio microphone, it has a small, battery-powered radio transmitter in the microphone body, which transmits the audio signal from the microphone by radio waves to a nearby receiver unit, which recovers the audio. The other audio equipment is connected to the receiver unit by cable. Wireless microphones are widely used in the entertainment industry, television broadcasting, and public speaking to allow public speakers, interviewers, performers, and entertainers to move about freely while using a microphone to amplify their voices.

Loud speaker : A loudspeaker (or loud-speaker or speaker) is an electroacoustic transducer;[1] which converts an electrical audio signal into a corresponding sound.[2] The most widely used type of speaker in the 2010s is the dynamic speaker, invented in 1925 by Edward W. Kellogg and Chester W. Rice. The dynamic speaker operates on the same basic principle as a dynamic microphone, but in reverse, to produce sound from an electrical

signal. When an alternating current electrical audio signal is applied to its voice coil, a coil of wire suspended in a circular gap between the poles of a permanent magnet, the coil is forced to move rapidly back and forth due to Faraday's law of induction, which causes a diaphragm (usually conically shaped) attached to the coil to move back and forth, pushing on the air to create sound waves. Besides this most common method, there are several alternative technologies that can be used to convert an electrical signal into sound. The sound source (e.g., a sound recording or a microphone) must be amplified or strengthened with an audio power amplifier before the signal is sent to the speaker.

Horn Loudspeaker : A horn loudspeaker is a loudspeaker or loudspeaker element which uses an acoustic horn to increase the overall efficiency of the driving element(s). A common form (right) consists of a compression driver which produces sound waves with a small metal diaphragm vibrated by an electromagnet, attached to a horn, a flaring duct to conduct the sound waves to the open air. Another type is a woofer driver mounted in a loudspeaker enclosure which is divided by internal partitions to form a zigzag flaring duct which functions as a horn; this type is called a folded horn speaker. The horn serves to improve the coupling efficiency between the speaker driver and the air. The horn can be thought of as an "acoustic transformer" that provides impedance matching between the relatively dense diaphragm material and the less-dense air. The result is greater acoustic output power from a given driver.

Multi Room Speaker System : Smart speakers might be the toast of the smart home now, but previously it was the top multi-room speakers that filled their role in many ways – uniting the home by sharing music between rooms and devices, all controlled from your phone.

For ages, demand was small and it was largely taken care of by a smart startup called Sonos. Fast-forward a few years, and the market has opened up to a much larger range of kit, making it harder than ever to choose the best speaker setup.

Despite the boom in smart speakers, if music rather than smart functionality is your main requirement from a speaker, a top multi-room system with audio at its heart could still be the best place to start.

Things to look out for include the streaming services it supports, any inputs you might need or any extra connectivity like Bluetooth. Some even support high-res music if you consider yourself something of an audiophile. You might also like to check the size of the range on offer should you want to upgrade or bolster your collection further down the line.

Sound recordings

Sound recording and reproduction is an electrical, mechanical, electronic, or digital inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording.

Acoustic analog recording is achieved by a microphone diaphragm that senses changes in atmospheric pressure caused by acoustic sound waves and records them as a mechanical representation of the sound waves on a medium such as a phonograph record (in which a stylus cuts grooves on a record). In magnetic tape recording, the sound

waves vibrate the microphone diaphragm and are converted into a varying electric current, which is then converted to a varying magnetic field by an electromagnet, which makes a representation of the sound as magnetized areas on a plastic tape with a magnetic coating on it. Analog sound reproduction is the reverse process, with a bigger loudspeaker diaphragm causing changes to atmospheric pressure to form acoustic sound waves.

Magnetic recording : Magnetic recording, method of preserving sounds, pictures, and data in the form of electrical signals through the selective magnetization of portions of a magnetic material. The principle of magnetic recording was first demonstrated by the Danish engineer Valdemar Poulsen in 1900, when he introduced a machine called the telegraphone that recorded speech magnetically on steel wire.

In the years following Poulsen’s invention, devices using a wide variety of magnetic recording mediums have been developed by researchers in Germany, Great Britain, and the United States. Principal among them are magnetic tape and disk recorders, which are used not only to reproduce audio and video signals but also to store computer data and measurements from instruments employed in scientific and medical research. Other significant magnetic recording devices include magnetic drum, core, and bubble units designed specifically to provide auxiliary data storage for computer systems.

Magnetic tape devices. Magnetic tape provides a compact, economical means of preserving and reproducing varied forms of information. Recordings on tape can be played back immediately and are easily erased, permitting the tape to be reused many times without a loss in quality of recording. For these reasons, tape is the most widely used of the various magnetic recording mediums. It consists of a narrow plastic ribbon coated with fine particles of iron oxide or other readily magnetizable material. In recording on tape, an electrical signal passes through a recording head as the tape is drawn past, leaving a magnetic imprint on the tape’s surface. When the recorded tape is drawn past the playback or reproducing head, a signal is induced that is the equivalent of the recorded signal. This signal is amplified to the intensity appropriate to the output equipment.

Digital recording : In digital recording, audio signals picked up by a microphone or other transducer or video signals picked up by a camera or similar device are converted into a stream of discrete numbers, representing the changes over time in air pressure for audio, and chroma and luminance values for video, then recorded to a storage device. To play back a digital sound recording, the numbers are retrieved and converted back into their original analog waveforms so that they can be heard through a loudspeaker. To play back a digital video recording, the numbers are retrieved and converted back into their original analog waveforms so that they can be viewed on a video monitor, television or other display.

Optical recording : The process of recording signals on a medium through the use of light, so that the signals may be reproduced at a subsequent time. Photographic film has been widely used as the medium, but in the late 1970s development of another medium, the so-called optical disk, was undertaken. The introduction of the laser as a light source greatly improves the quality of reproduced signals. The pulse-code modulation (PCM) techniques make it possible to obtain extremely high-fidelity reproduction of sound signals in optical disk recording systems.

Optical film recording is also termed motion picture recording or photographic recording. A sound motion picture recording system consists basically of a modulator for producing a modulated light beam and a mechanism for moving a light-sensitive photographic film relative to the light beam and thereby recording signals on the film corresponding to the electrical signals. A sound motion picture reproducing system is basically a combination of a light source, an optical system, a photoelectric cell, and a mechanism for moving a film carrying an optical record by means of which the recorded photographic variations are converted into electrical signals of approximately similar form.

CD System : A CD player is an electronic device that plays audio compact discs, which are a digital optical disc data storage format. CD players were first sold to consumers in 1982. CDs typically contain recordings of audio material such as music. CD players are often a part of home stereo systems, car audio systems, and personal computers. With the exception of CD boomboxes, most CD players do not produce sound by themselves. Most CD

players only produce an output signal via a headphone jack or RCA jacks. To listen to music using a CD player with a headphone output jack, the user plugs headphones or earphones into the headphone jack. To use a CD player in a home stereo system, the user connects an RCA cable to the RCA jacks or other outputs and connects it to a hi-fi (or other amplifier) and loudspeakers for listening to music. They are also manufactured as portable devices, which are battery powered and typically used with headphones.

DVD System : A DVD player is a device that plays DVD discs produced under both the DVD-Video and DVD-Audio technical standards, two different and incompatible standards. Some DVD players will also play audio CDs. DVD players are connected to a television to watch the DVD content, which could be a movie, a recorded TV show, or other content. The first DVD-Audio players were released in Japan by Pioneer in late 1999, but they did not play copy-protected discs. Matsushita (under the Panasonic and Technics labels) first released full-fledged players in July 2000 for $700 to $1,200. DVD-Audio players are now also made by Aiwa, Denon, JVC, Kenwood, Madrigal, Marantz, Nakamichi, Onkyo, Toshiba, Yamaha, and others. Sony released the first SACD players in May 1999 for $5,000. Pioneer's first DVD-Audio players released in late 1999 also played SACD. SACD players are now also made by Accuphase, Aiwa, Denon, Kenwood, Marantz, Philips, Sharp, and others.

Television Television (TV) is a telecommunication medium used for transmitting moving images in monochrome (black and white), or in colour, and in two or three dimensions and sound. The term can refer to a television set, a television program ("TV show"), or the medium of television transmission. Television is a mass medium for advertising, entertainment and news.

Types of television : 1. Monochrome television 2. Colour television

Monochrome television : Television in which the final reproduced picture is monochrome, having only shades of gray between black and white. Also known as black-and-white television.

Elements of a television system

An image source. This is the electrical signal representing the visual image, and may be from a camera in the case of live images, a video tape recorder for playback of recorded images, or a film chain-telecine-flying spot scanner for transmission of motion pictures (films).

A sound source. This is an electrical signal from a microphone or from the audio output of a video tape recorder or motion picture film scanner.

A transmitter, which generates radio signals (radio waves) and encodes them with picture and sound information.

An antenna coupled to the output of the transmitter for broadcasting the encoded signals. An antenna to receive the broadcast signals. A receiver (also called a tuner), which decodes the picture and sound information from the broadcast signals,

and whose input is coupled to the antenna. A display device, which turns the electrical signals into visual images. An audio amplifier and loudspeaker, which turns electrical signals into sound waves (speech, music, and

other sounds) to accompany the images.

NECESSITY OF SCANNING Scanning is necessary because if we want to transmit the whole picture frame in a oncetime it require large bandwidth . so, we divide the whole frame into pixel . so, as the scanning is start the picture is pixel by pixel transmit and it require less bandwidth and it is done at a faster to remove the flicker and to show continuity in picture.

Blanking, Retrace, Synchronizing and Equalizing Pulses:

In TV, ‘blanking’ means ‘going to black’ as part of the video signal, the blanking voltage is at the black level. Video voltage at the black level cuts off the beam currents in the picture tube to black out the light from screen. The purpose of providing the blanking pulses is to make invisible the retraces of the scanning process. The horizontal blanking pulse at the frequency of 15625 Hz, blanks out the retrace from right to left for each line. The vertical blanking pulses at 50 Hz blank out the retrace from bottom to top for each field.

The time period of blanking pulses is 16% of the each horizontal line, i.e., 16 % of 64 µs = 10.2 µs. In other words, retrace from left to right must be completed in 10.2 µs.

The time period of vertical blank pulses is 8% of each vertical field. It comes equal to 8% of 1/50 s = 0.0016 s. In other words, the vertical retrace must be completed within 1.6 ms.

A blanking pulse comes first to put the video signal at black level, then a sync pulse comes to start the retrace. This sequence applies to blanking, horizontal and vertical retraces.

To produce a true and undistorted picture, it is necessary that the scanning process at the transmitter camera tube should be quite in step with that at the receiver picture tube. Thus the timing pulses generated by the synchronizing generator to trigger the saw tooth generator for vertical and horizontal plates are not only applied to the transmitter camera tube system but also transmitted to the receiver along with the image signals. At the receiver, these triggering pulses are separated from the signal components, which are then differentiated (horizontal synchronizing pulses) to trigger saw-tooth wave generators for the application of saw-tooth voltage to horizontal and vertical deflection plates of picture tube respectively.

Vestigial sideband (VSB)

Vestigial sideband (VSB) is a type of amplitude modulation ( AM ) technique (sometimes called VSB-AM ) that encodes data by varying the amplitude of a single carrier frequency . Portions of one of the redundant sidebands are removed to form a vestigial sideband signal - so-called because a vestige of the sideband remains.

In AM, the carrier itself does not fluctuate in amplitude. Instead, the modulating data appears in the form of signal components at frequencies slightly higher and lower than that of the carrier. These components are called sidebands . The lower sideband (LSB) appears at frequencies below the carrier frequency; the upper sideband (USB) appears at frequencies above the carrier frequency. The actual information is transmitted in the sidebands, rather than the carrier; both sidebands carry the same information. Because LSB and USB are essentially mirror images of each other, one can be discarded or used for a second channel or for diagnostic purposes.

VSB transmission is similar to single-sideband (SSB) transmission, in which one of the sidebands is completely removed. In VSB transmission, however, the second sideband is not completely removed, but is filtered to remove all but the desired range of frequencies .

Composite video signal

Composite video (one channel) is an analog video transmission (without audio) that carries standard definition video typically at 480i or 576i resolution. Video information is encoded on one channel, unlike the higher-quality S-video (two channels) and the even higher-quality component video (three or more channels).

Composite video mostly comes in three standard formats: NTSC, PAL, and SECAM. It has come to be designated by the initials CVBS for composite video baseband signal or color, video, blanking and sync,[1][2] or is simply referred to as SD video for the standard-definition television signal it conveys.

Picture tube

The picture tube is the largest component of a television set, consisting of four basic parts. The glass face panel is the screen on which images appear. Suspended immediately behind the panel is a steel shadow mask, perforated with thousands of square holes. (Connected to the mask is a metal shield to neutralize disruptive effects of the Earth's magnetic field.) The panel is fused to a glass funnel, which comprises the rear of the picture tube. The very rear of the funnel converges into a neck, to which an electron gun assembly is connected.

The inside of the panel is painted with a series of very narrow vertical stripes, consisting of red, green and blue phosphors. These stripes are separated by a narrow black graphite stripe guardband. When struck by an electron beam, the phosphors will illuminate, but the graphite will not. This prevents color impurity by ensuring that the electron beam only strikes the phosphor stripes it is intended to light.

The electron beam is generated by the electron gun assembly, which houses three electron guns situated side-by-side. Each of the three guns emits an electron beam (also called a cathode ray) into the tube, through the mask and onto the panel.

Because the three beams travel side-by-side, the holes in the mask ensure that each beam, because of its different angle of attack, will hit only a specific phosphor stripe - red, green or blue. The three phosphors, lighted in different combinations of intensity, can create any visible color when viewed from even a slight distance.

The three electron beams are directed across the screen by a series of electromagnets, called a yoke, which draw the beams horizontally across the screen a line at a time. Depending on the screen size, the beam draws about 500 lines across the entire screen. Each time, the phosphors light up to produce an image.

The electron guns and the yoke are electronically synchronized to ensure the lines of phosphors are lighted properly to produce an accurate image. The image lasts only for about a 30th of a second. For that reason, the picture must be redrawn 30 times a second. The succession of so many pictures produces the illusion of movement, just like the frames on movie film.

Color combinations produced by a picture tube Red Green Blue

Red X Yellow X X Blue X Green X White X X X Black Magenta X X Cyan X X

Camera tube

Video camera tubes were devices based on the cathode ray tube that were used to capture television images prior to the introduction of charge-coupled devices (CCDs) in the 1980s.[1][2][3] Several different types of tubes were in use from the early 1930s to the 1980s.

In these tubes, the cathode ray was scanned across an image of the scene to be broadcast. The resultant current was dependent on the brightness of the image on the target. The size of the striking ray was tiny compared to the size of the target, allowing 483 horizontal scan lines per image in the NTSC format, or 576 lines in PAL.[4]

1.Vidicon

This camera tube based on the photo conductive properties of semiconductors i.e., decrease in resistance with the amount of incident light. The tube is shown in figure. It consists of

(a). Signal Plate: Which is a conducting metallic film very thin so as to be transparent. The side of this film facing cathode is coated with a very thin layer of photoconductive material (amorphous selenium). This side is scanned by electron beam. The optical image is focused on the other side of this film.

(b).Scanning System: The electron beam for scanning is formed by the combination of cathode, control grid-1, accelerating grid-2 and anode grid-3. The focusing coil produces an axial field which focuses the beam on the film. Vertical and horizontal deflection of the beam, so as to the scan the whole film, is accomplished by passing saw-tooth current waves through deflecting coils which thus produce transverse horizontal and vertical magnetic fields respectively. The alignment coils are for initial adjustment of the direction of electron beam.

Operation: When the scanning beam passes over the photo conductive material of the signal plate, it deposits electrons so that the potential of this side of plate is reduced to that of the cathode. But the otherside of the film (plate) is still at its original potential. Consequently a potential difference across a given pointon the

photoconductive material is created. It is approximately 30 V. Before the next scanning (which may be done after an interval of 1/50 or 1/25 sec.) the charge leaks through photoconductive material at a rate determined by the conductivity of the material which, in turn depends upon the amount of incident light.

White portions of the object will project more light on the film and make it more conductive. This charge leaked to photoconductive side of the film will vary according to illumination of the object. As a result, potential at every point on the photoconductive side will vary. Now the electron beam again starts scanning the photoconductive side of the film but this time the charge deposited by the beam in order to reduce its potential towards zero (cathode potential) will vary with time. Therefore current through RL (and hence the output voltage) will follow the changes in potential difference between two surfaces of the film and hence follows the variations of light intensity of successive points in the optical image.

Advantages:

1. Low cost.2. Simple Adjustment.3. Sensitivity is large.4. Resolution of the order of 350 lines can be achieved under practical conditions.

Disadvantages:1. Owing to the fact that the resistance of the photoconductive film does not change instantaneously with change of light intensity, different levels of light intensity are adjusted with slight time slag.2. The response characteristic is non-linear.

2.Plumbicon

The construction of a plumbicon camera tube is similar to that of a standard vidicon except for the target material. The plumbicon has a new type of photo-conductive target, i.e., lead oxide of the form PbO. The figure below shows the constructional features of a plumbicon camera.

Operation: The operation of a plumbicon camera tube can be best explained from the diagram. Initially, when there is no light input, the PIN diode is reverse biased due to a positive potential appearing on SnO2 coating (n-type) and p-type stabilized at a potential slightly below the cathode due to negatively charged scanning beam. This results in a very small output current which is almost negligible. This is the greatest advantage of a plumbicon camera tube especially when used with color systems. The photo electronic conversion is almost similar to that of a standard vidicon except for the method of discharging each storage element. In standard vidicon each element acted as a

leaky capacitor with leakage resistance decreasing with more light. Here when light falls on the target, the diode becomes forward biased upon the extent depending upon light intensity. The forward bias on each diode results from the photo excitation of the pure PbO and doped PbO junction. Thus the target behaves as a capacitor in series with PIN diode.

Merits and Demerits:

1. In plumbicons, the uniluminated or the dark current is negligible and also it is temperature independent.2. It has got high sensitivity and a high signal to noise ratio.3. Resolution is good but not as good as that of a vidicon.4. Operational gamma is unity.5. It is compact and exhibits simplicity of operation.6. It is free of spurious signals.7. Susceptibility to damage by over loads is not as severe as it is in vidicons.8. There are some forms of PbO which have spectral limitations.

TV Receiver

1. TV Receiving Aerial: the TV signal radiated by the transmitter has to be intercepted. For this an antenna with high gain, Broad Band, highly directional is used. Yagi-uda multi-element array is preferred. Impedance of the aerial should match the impedance of the transmission line. The aerial also selects the required signal and rejects unwanted signals.

2. Tuner: Also called RF Tuner/ Front end. Signals from Aerial are amplified, down converted to intermediate frequency. Band and channel selection is also carried out here. This is a separate, sealed and riveted unit mounted away from other circuits. Unit’s body is grounded to avoid interference by other strong signals. Delayed AGC is used at RF amplifier.

3. VIF Amplifier: The output of RF Tuner has two intermediate frequencies i.e. video IF (38.9 MHz) and sound IF (33.5 MHz), occupying a band width of 7 MHz. Most of the amplification and selection is done here. This is also a separate, enclosed unit. Keyed AGC is used at the base of 1st IF Amplifier.

4. Video Detector: The output of VIF section is around 2 to 5VPP. This is fed to video detector to separate various signals i.e., video, AGC, inter carrier SIF. A special diode driven into saturation has many harmonics at its output. Using LPF, signals above 5 MHz are filtered out. OA 79, Ge, heavily doped, point contact diode is used for detection.

5. Video Output Amplifier: Output of video detector which is around 2 to 5 VPP is amplified by a single stage broad band, voltage amplifier to give an output of 80 VPP. Wave Trap is used to prevent 5.5 MHz inter – carrier SIF (38.9 – 33.4 MHz = 5.5 MHz) and disturb the picture. Output of this amplifier is fed to picture tube.

6. Picture Tube: This is a special type of CRT, having widescreen with wide deflection angle. Electromagnetic deflection and electro-static Focusing is used. H and V deflection coils moulded into a single unit ‘Yoke’ is mounted on the neck. Final anode is applied with Extra high Tension (EHT) which is around 12 to 18 kV. Screen is formed by long persistence P4 phosphor, which is a mixture of cadmium tungstate and zinc sulphide. The light radiated is Yellowish white. With proper output from syn section, picture is displayed on the screen.

7. Sync Section: Output of video detector which is composite video signal has H and V sync pulses either at top or bottom edges. By feeding video signal to sync separator, sync pulses can be separated. We can use clipper circuit when the video signal is with negative sync. Further, by using LPF, V- sync can be separated. Using HPF, H sync pulses can be separated. These pulses are used to synchronize the H and V saw-tooth oscillators. Output of these oscillators are amplified and used to drive the deflection coils on the picture tube.

8. Inter Carrier SIF Section: Output of detector has 5.5 MHz, which is the result of heterodyning between VIF and SIF (38.9 MHz – 33.4 MHz). This has frequency modulated Audio Signal. By using highly selective tuned circuit (sound-take-off coil), 5.5 MHz is separated. This is amplified and frequency demodulated to recover Audio signal.

9. Audio Section: Output from SIF section which is weak AF signal is amplified both in terms of voltage and power to drive the loud speaker. Finally, we get sound output.

10. Power Supply: Various levels of DC voltages are required for the operation of TV receiver. So, 230 V, 1 φ AC is rectified, filtered and regulated to provide ripple free steady voltage to various stages. However due to many advantages switch mode power supplied (SMPS) are widely used now-a-days. EHT and BHT required at picture tube is obtained from Auxiliary power supply using Line output transformer (LOT/EHT).

Colour television

Color television is a television transmission technology that includes information on the color of the picture, so the video image can be displayed in color on the television set. It is an improvement on the earliest television technology, monochrome or black and white television, in which the image is displayed in shades of gray

(grayscale). Television broadcasting stations and networks in most parts of the world upgraded from black and white to color transmission in the 1970s and 1980s. The invention of color television standards is an important part of the history of television, and it is described in the technology of television article.

Transmission of color images using mechanical scanners had been conceived as early as the 1880s. A practical demonstration of mechanically-scanned color television was given by John Logie Baird in 1928, but the limitations of a mechanical system were apparent even then. Development of electronic scanning and display made an all-electronic system possible. Early monochrome transmission standards were developed prior to the Second World War, but civilian electronics developments were frozen during much of the war. In August 1944, Baird gave the world's first demonstration of a practical fully electronic color television display. In the United States, commercially competing color standards were developed, finally resulting in the NTSC standard for color that retained compatibility with the prior monochrome system. Although the NTSC color standard was proclaimed in 1953 and limited programming became available, it was not until the early 1970s that color television in North America outsold black and white or monochrome units. Color broadcasting in Europe was not standardized on the PAL format until the 1960s.

Around 2006 countries began to switch from analog color television technology to digital television. This changeover is now complete in many developed countries, but analog television is still the standard in many developing countries.

Primary & secondary colours

Colours which cannot be produced by mixing other colours are called primary colours. It is not found possible to produce either red, blue or green colours by mixing two other colours. For this reason red, green and blue are called primary colours.

Mixing of the three primary colours

A secondary colour can be produced by mixing other colours. Thus yellow colour can be produced by mixing red and green colours. Magenta colour can be produced by mixing red and blue colours. By the same token, cyan colour can be produced by mixing blue and green colours.

Concept of mixing & colour triangle

A colour triangle is an arrangement of colours within a triangle, based on the additive combination of three primary colors at its corners.

An additive colour space defined by three primary colors has a chromaticity gamut that is a color triangle, when the amounts of the primaries are constrained to be nonnegative.[1][2]

Before the theory of additive color was proposed by Thomas Young and further developed by James Clerk Maxwell and Hermann von Helmholtz, triangles were also used to organize colors, for example around a system of red, yellow, and blue primary colors.[3]

After the development of the CIE system, color triangles were used as chromaticity diagrams, including briefly with the trilinear coordinates representing the chromaticity values.[4] Since the sum of the three chromaticity values has a fixed value, it suffices to depict only two of the three values, using Cartesian co-ordinates. In the modern x,y diagram, the large triangle bounded by the imaginary primaries X, Y, and Z has corners (1,0), (0,1), and (0,0), respectively; colour triangles with real primaries are often shown within this space.

Camera Tube

Note that this page describes the UK colour system (PAL).

Light enters via the lens on the left and is split into three paths by mirrors and semi-transparent mirrors.

The light in each path passes through a colour filter.

These filters are like the transparent coloured papers in which chocolates are wrapped.

If you look through a red one, everything looks red.

This is because it lets only red light through.

Blue or green objects look black.

Colours which contain some red, such as purple, look dark red.

Red, blue and green filters are used.

The coloured images are focused on the faces of the three colour tubes which scan the images.

Each tube gives a signal out, proportional to the amount of colour.

Some of the red, green and blue signals from the camera tubes are added in the luminance matrix.

This means that the separated colours are recombined electronically.

This gives a luminance (brightness) signal.

The luminance signal is labeled Ey, and is used by black and white receivers.

The colour signals are known as Er, Eg, and Eb.

The red and blue signals are converted into two new signals called the red and blue colour difference signals.

They are (Er - Ey) and (Eb - Ey).

These two signals are modulated onto a "sub carrier" at 4.43 MHz which becomes the chrominance (colour) signal.

The luminance, chrominance and sync signals are combined and are then used to amplitude modulate a carrier in the UHF band.

An associated sound signal frequency modulates a second carrier, which is 6 MHz apart from the vision carrier.