Broadcast Engineering: AM Broadcasting Standards (FCC) AM and FM Broadcasting Frequency allocation: 535 to 1605 kHz (525 to Broadcasting 1705 kHz) divided into 106 (130) channels Channel spacing: 10 kHz (9 kHz) • Broadcasting is the distribution of audio Permitted channel bandwidth: 30 kHz and/or video signals (programs) to a number (maximum modulating signal frequency: 15 of recipients ("listeners" or "viewers") that kHz) belong to a large group. *note: geographically co-located stations must • Broadcasting to a very narrow range of be spaced at least 3 channels apart for sideband audience is called narrowcasting. interference protection) • The term "broadcast" was coined by early Carrier tolerance: ± 20 Hz radio engineers from the midwestern United Unmodulated carrier power: 100 W to 50 kW States. "Broadcasting", in farming, is one Type of emission: A3E (double sideband, full method of spreading seed using a wide toss carrier) of the hand, in a broad cast. Intermediate Frequency: 455 kHz • Television and radio programs are distributed through radio broadcasting or AM Station Performance cable, often both simultaneously. Requirements (FCC) • Broadcasting forms a very large segment of mass media. Modulation percentage: 85% to 95% Audio frequency distortion harmonics Amplitude Modulation (AM) (AFDH): <5% rms amplitude up to 84% modulation Audio frequency response: transmission • Amplitude Modulation characteristic must be flat from 100 Hz to 5 kHz The process of varying the amplitude of a (± 2 dB referred to 1 kHz) high-frequency carrier wave in accordance Carrier stability: ± 20 Hz with the amplitude of the modulating signal Service Areas in AM • Uses of AM • Primary 1. AM broadcast (535 – 1605 kHz) Area in which the groundwave field is measured at 1 mV/m, and is not subject to 2. Citizen’s Band Radio (27 MHz) objectionable interference and fading 3. Aircraft communications (108 – 136 MHz) • Secondary 4. International shortwave (3 – 30 MHz) The area serviced by the skywave, having broadcast – via sky wave a skywave field strength equal to or greater than 500 μV/m for 50% or more of the time. 5. TV picture (using vestigial sideband) May be subject to fading but no objectionable cochannel interference Limiting condition for AM: • Intermittent The amplitude of the modulating signal must not The area receiving service from the exceed that of the carrier, else overmodulation groundwave but beyond the primary service area (clipping) occurs. and subject to objectionable interference and fading Times of Day in AM Broadcasting Classification of Powers
Daytime 1. Licensed power or authorized operating • From local sunrise to local sunset power 6 AM to 6 PM (2200 UTC to 1000 UTC) That which is actually supplied or fed to the antenna by the transmitter. Should Nighttime have a tolerance of -5% or +10% • Between local sunset to local sunrise 6 PM to 6 AM (1000 UTC to 2200 UTC) 2. Maximum rated carrier power The maximum power that the transmitter Experimental period is capable of supplying to the antenna • Midnight to local sunrise and still operate satisfactorily 12 MN to 6 AM (1600 UTC to 2200 UTC) • Used for experimental purposes in testing 3. Plate input power and maintaining apparatuses by the licensee, Product of the voltage and current at the provided that no interference is caused to output of the last radio stage, measured other stations maintaining a regular operating without modulation schedule within such a period 4. Antenna input power 3 Important Requirements in Broadcasting Product of the square of the antenna current and the antenna resistance at the 1. Timing point where the current is measured. • Programming schedule must be followed (programs should start and end within the Two methods of measuring antenna input allotted time) power 2. Fidelity 1. Direct method • Program material shall not have any
reasonable distortion P =I 2R 3. Modulation depth o a a • The audio signal must modulate the transmitter properly Where: Ia = antenna current w/ no modulation Power Allocations Ra = impedance or resistance of the antenna where the current is measured
2. Indirect method • used to determine the output power of FM broadcast stations and TV aural The transmitter of existing Metro Manila transmitters • stations may be located outside of Metro Manila, used for AM broadcast stations in provided that Metro Manila remains within the emergencies 80 dBu contour of the transmitter P =V I F o p p • The station shall not operate more than 5% and lower than 10% of its authorized Where: operating power Vp = plate voltage of the final amplifier Ip = plate current of the final amplifier F = power factor correction AM Antennas and Site Considerations • Standard AM broadcast stations use • Log either a single omnidirectional vertical A listing of the date and time of events, antennas, or multi-element, phased vertical programs, equipment parameters, tests, directional arrays malfunctions, corrections, and other such information • Generally, antennas are erected on flat lands, preferably those having good ground Types of Logs in Broadcast Operations conductivities (e.g. marsh lands) • Program Log • Earth mat – a network of buried wires Contains entries with regard to the nature of directly under the antenna, extending the program, its name and title, start and end outward from the base, buried about 6 to 12 times, source, sponsors of announcements, inches (15 to 30 cm) below the ground duration of advertisements, etc.
• Counterpoise – a smaller version of the • Operating Log earth mat above ground Contains the technical details of the transmitter during operation, such as operating Antenna Towers: parameters (Vp, Ip, Ia, etc.), the time the Obstruction Painting and Lighting transmitter is put on and off the air, the time antenna lights are turned on or off, etc. • Must be painted with equal-width stripes of aviation (emergency) orange and white, each • Maintenance Log stripe approximately one-seventh of the height of Contains the results of transmitter and other the tower, but not over 100 ft (30 m) in width on equipment tests, repairs, calibration, checks, etc. tall towers. The top and bottom stripes must be orange Frequency Modulation (FM) • To mark the tower at night (sunset to sunrise), towers up to 150 ft (46 m) must have • Frequency Modulation two (2) steady-burning 116-W or 125-W lamps The process of varying the frequency of a in an aviation red light globe at the top of the high-frequency carrier wave in accordance tower (beacon) with the amplitude variations of the modulating signal • For towers over 150 ft, the top beacon light consists of 620-W or 700-W PS-40 • Uses of FM Flashing Code Beacon lamps with aviation red filters 1. FM broadcast (88 – 108 MHz)
• At half-, third-, quarter-, etc. tower height 2. Television sound points (depending on the height of the tower), flashing 620-W to 700-W beacons are installed 3. Satellite Television (both audio and video)
• Lights should be automatically controlled 4. Mobile radio services by a device sensitive to the night sky.
• Lights should be inspected at least once a day, or by automatic indicators Logs FM Broadcasting Standards (FCC) Frequency allocation: 88 to 108 MHz divided • A tower, elevated structures (buildings) into 100 channels can be used to elevate the antenna Channel spacing: 200 kHz Permitted channel bandwidth: 200 kHz (±75 • Hills and mountains can also act as kHz deviation for maximum modulating natural towers. frequency of 15 kHz, and a 25 kHz guard band on both sides) Pre-emphasis and De-emphasis Type of emission: F3E (monoaural) F8E (stereophonic) • Since noise sideband power in FM Intermediate Frequency: 10.7 MHz (10.61 to decreases inversely with the modulating 10.79 MHz) frequency, higher modulating frequencies are Pilot Subcarrier Frequency: 19 kHz more susceptible to noise than the lower ones. Classes of FM Stations • A method of artificially boosting the • Class-A affected frequencies with respect to a pre- Shall have an authorized transmitter power arranged curve before transmission to not exceeding 25 kW and an ERP not improve noise immunity is termed as pre- exceeding 125 kW. Minimum transmitter emphasis. power is 10 kW • The compensation at receiver side is • Class-B called de-emphasis Shall have an authorized transmitter power not exceeding 10 kW and an ERP not • The amount of pre/de-emphasis for FM exceeding 30 kW. Minimum transmitter broadcasting has been standardized as 75μs power is 1 kW • In the UK, the amount of emphasis is • Class-C standardized to 50μs A non-commercial, community station having an ERP not exceeding 1 kW Stereophonic FM • Class-D Stereo (also stereoscope) Shall have an authorized transmitter power not exceeding 10 W. Used for educational • Originally referred to a special purposes. photographic technique used to give the viewer the impression of observing a scene in FM Antennas and Site Considerations three dimensions • The antenna used in FM broadcasting is a • In stereophonic audio, a sound source is half-wave dipole. recorded from two different angles (in this case, the left and right sides). During • The transmitting antenna location should playback, these signals simulate the sound to be chosen so that line-of-sight can be the left and right ears, giving the illusion of a obtained from the antenna over the general three-dimensional sound source. service area. • To provide LOS within the principal Problems encountered with initial design of area, the antenna must be conveniently above stereophonic FM broadcasts: the average terrain. 1. Original FM broadcasts were monophonic. • Too much of the total composite The system should be compatible with existing modulating signal would be taken up by the monophonic receivers subcarrier voltage
2. The stereo information had to be transmitted 5. What’s with the 19 kHz pilot subcarrier? within the 200 kHz bandwidth allotment • It is used as the reference to obtain the difference signal. Block Diagram of a Stereophonic Transmission System 6. Why 19 kHz? • Most people cannot hear beyond 15 kHz, much more 19 kHz.
Subsidiary Communications Authorization (SCA)
• The transmission of programs which are of a broadcast nature, but which are of interest primarily to limited segments of the public wishing to subscribe thereto.
• Typical applications of SCA 1. Background music, weather, time Spectrum of a Stereo FM Multiplex System Signals 2. Educational information 3. Talk-back for remote stations 4. Telemetry 5. Facsimile 6. Slow-scan TV • Uses a subcarrier of 67 kHz and is Some notes on FM Stereo modulated to a depth of 7.5 kHz
1. Why are the L & R signals not sent The Broadcast Studio independently and simultaneously? • So that the system will be compatible • The studio usually contains equipment with existing monoaural receivers for program origination.
2. What’s the use of the 38 kHz subcarrier? • It is a usual practice to co-locate the • To serve as the carrier for the balance studio and transmitter in a single facility, modulation process of the difference signal mostly for economic purposes.
3. Why 38 kHz? Remote Studio Facilities • The difference signal must not interfere with the original sum signals • In cases where the studio and the transmitter cannot be located in the same facility, or it is better for the transmitter to be located elsewhere (economically or 4. Why suppress the 38 kHz subcarrier? technically), then a studio-to-transmitter link (STL) may be employed. Broadcast Engineering: Introduction to Television Picture Elements Television • A still picture is fundamentally an • The word television means “to see at a arrangement of many small dark and light distance” areas • Each small area of light or shade is a • In a TV broadcasting system, the visual picture detail, or picture element – a pixel for information in a scene is converted to an short, or pel electrical signal which is transmitted to a • The arrangement of these elements define receiver the information in a scene • If these elements are transmitted and • At the receiver, this video signal is used reproduced in the same degree of light or to reassemble the image on the screen of a shade as in the original in the proper picture tube. position, then the picture is reproduced.
Transmitter Block Diagram
Scanning
• A television picture is scanned in a sequential series of horizontal lines, one under another. • A TV picture is scanned in the same way as you would read a page of text to cover all Receiver Block Diagram the words in a line and all the lines on the page. • For greater detail, a large number lines is required to include the greatest number of picture elements.
The Television Picture Persistence of Vision
• The impression made by any light seen by the eye persists for a small fraction of a second after the light source is removed
• If many views are presented to the eye during this interval, the eye will integrate, giving the impression of seeing all the images at the same time Motion Pictures • • Using this effect, we can send the entire With all the pixels in the frame televised picture information one pixel at a time given by means of the scanning process, it is also that a scene is scanned rapidly enough necessary to present the picture to the eye in such a way that any motion in the scene • In addition, to create the illusion of appears as a smooth, continuous change. • motion, enough complete pictures must be The technique of moving pictures is shown during each second – about 24 taken from motion pictures (movies) pictures per second, as used in motion • One of the first people to experiment pictures with moving pictures is Eadweard Muybridge, by taking a series of Blanking and Flicker in Motion Pictures photographs of a moving horse • In order for successive images to blend Muybridge’s Photographs smoothly to one another, the switching of the pictures must not be seen by the eye
• A shutter blanks out the screen during this interval so that the eye sees only the images and not the switching
• However, the frame rate of 24 fps is not rapid enough to allow the brightness of one picture to blend smoothly when the screen is black between frames, resulting in flicker
• To eliminate flicker, the pictures are run at 24 fps, but showing each frame twice, so • Notice that the series consists of still that 48 pictures are shown each second pictures with each picture slightly different • from the preceding one The screen is blanked out every time a • Each picture is projected individually as a picture is shown. 48 pictures per second still picture means the screen is blanked 48 times per second • However, if projected in rapid succession, it produces the illusion of • continuous motion There are 48 views of a scene each second, and the screen is blanked 48 times per second, although there are still the same 24 picture frames per second • This increased blanking rate eliminates the flicker
A Frame and a Field
• A similar process is used to reproduce motion in television
• Instead of the 24 frames per second used for motion pictures, a picture repetition rate of 30 pictures per second is used for A picture composed of even lines (Even Field) television Frame and Field Frequencies • To eliminate flicker, a blanking rate of 60 times per second is used • The frame repetition rate of 30 is chosen in television to be in sync with the power line • But instead of showing each picture frequency of 60 Hz. For a frame rate of 30 twice, each picture is divided into two parts: fps, the field rate is 60. an odd field and an even field • This field rate of 60 Hz is the vertical • Therefore, 60 views of a scene is scanning frequency - the rate at which the presented to the viewer, as two fields are electron beam completes its cycle of vertical scanned during one frame period of 1/30 s. motion from top to bottom and back to top again
• A frame consists of 525 lines, which gives 262.5 lines for a single vertical field
• Therefore, the number of lines per second is 262.5 x 60 = 15,750
Scanning and Interlacing
• A frame is scanned every other line, and One picture (Frame) when the scanning beam reaches the bottom of the screen, it goes back to the top to scan the lines that were skipped previously
• These odd set and even set of lines are shown alternately to form the frame – a technique known as interlacing
A picture composed of odd lines (Odd Field) More on Trace, Retrace, and Blanking
• The motion of the scanning beam traces the scene from the left to the right of the frame, one line after another
• After a line is scanned (or traced), the beam quickly goes back to the left of the frame to scan the next line below, termed as retrace or flyback
• When the beam reaches the bottom of the screen, it then returns to the top-left corner to scan a new frame Detail of Interlaced Scanning • A scanned horizontal line is called a horizontal trace, while the vertical motion of the beam is a vertical trace
• The screen is blanked out every retrace period (horizontal and vertical) to hide the electron beam
Detail of Interlaced Scanning
Odd lines in 1st vertical trace 1st field = total of 262.5 lines The Camera Tube • When the beam hits the phosphor screen, it illuminates a spot on the screen • The optical image is focused on a representing a picture element photosensitive plate using a lens system • The beam is controlled (deflected) from • This photosensitive plate converts the left to right and top to bottom of the screen image to an electric charge pattern by external coils at the neck of the tube
• This charge pattern is scanned A Cathode-ray Tube (CRT) sequentially by an electron beam, controlled by a series of coils at the back of the tube
• The electron beam discharges each picture element, which produces a signal from the output electrode
A Vidicon Camera Tube
Beam Deflection and the Sawtooth Waveform
• To deflect the electron beam from left to right (and top to bottom) of the screen, a steadily increasing current (magnetic field) is used The Picture Tube • This linear increase provides a smooth movement of the electron beam • The picture tube is a cathode-ray tube (CRT) with an electron gun and a phosphor • screen inside an evacuated glass envelope The flyback is the sharp decrease in the current from the peak value to zero • The beam is accelerated to the screen by a positive anode voltage • After the horizontal trace, the camera signal is blacked out (blanking) to cover the retrace
• After the retrace, the blanking signal is removed, and another line is ready to be scanned
Detail of a Composite Video Signal
Synchronizing Pulses
• In order for the picture elements to be reassembled in the correct order in the picture tube as it was scanned in a camera tube, synchronism is in order
• Synchronizing pulses are sent for each horizontal line scanned, in order to keep the horizontal scanning synchronized Sync Polarity and the Composite Video Signal
• Also, a vertical synchronizing pulse is • The video signals can have two transmitted for each field, to synchronize the polarities: vertical scanning motion 1. A positive sync with the sync pulses in the up position • Sync pulses are part of the video signal, 2. A negative sync with the sync pulses but they occur during the blanking period in the down position (since they are not part of the viewable information) • Video signals with negative sync is fed into the control grid of the picture tube, while positive-sync signals are fed to the cathode
• Furthermore, negative sync is standard for signals in and out of video equipment (cameras, etc.)
• For either polarity, the white parts of the video signal are opposite to those of the sync The Composite Video Signal pulse, with the blanking level at black.
• As the scanning beam of the camera tube • Sync amplitudes are sometimes called moves from left to right (and top to bottom) blacker than black of the screen, the amplitude of the camera signal vary for shades of white, gray, or black in the picture Video signal with positive sync
The IRE Scale
• Video signal amplitude is usually checked with negative sync polarity to fit the IRE scale
Video signal with negative sync • IRE stands for Institute of Radio Engineers
• The composite video signal can have a peak-to-peak amplitude of 140 IRE units
Blanking • The sync pulse has an amplitude of 40 IRE units • The composite video signal contains blanking pulses to make the retrace lines • The black peaks of the camera signal are invisible by blacking the signal out when the offset from the blanking level by 7.5 IRE scanning circuits produce retraces units. This is to make sure that the color bursts do not interfere with the sync • Horizontal blanking pulses are included to blank out retraces from right to left of the • Peak white signal goes up to 100 IRE screen units
• Vertical blanking pulses blank out • Subtracting the black offset level from retraces from the bottom to the top of the the peak white, 100 – 7.5 = 92.5 IRE units screen for camera signal variations
• Each blanking pulse changes the video signal to black during blanking time Horizontal Blanking Time
Vertical Blanking Time
• The time to complete one field is 1/60 s, or 0.0167 s. Since the vertical trace is much longer than a horizontal trace, the vertical retrace is also longer
• The width of the vertical blanking pulse is 8% of the vertical trace time, which gives us 1.33 ms Color Television
• This long blanking period not only blanks Initial Problems with Color Television the vertical retrace but also blanks out several horizontal scanning lines (21 lines per field) • As with stereophonic FM broadcasts, color television was wrought with Sync Pulses in V Blanking Time compatibility problems with the older monochrome receivers • The sync pulses in the V blanking time include equalizing pulses, vertical sync • Also, color information must fit in the 6 pulses, and some horizontal sync pulses MHz space allocated for television broadcast services • The vertical blanking period begins with a group of six equalizing pulses, spaced at • A similar technique is used, one that uses half-line intervals a matrix to produce a signal compatible to monochrome receivers, and a subcarrier to • The next is a serrated vertical sync pulse carry the color information that produces the vertical flyback. This is 3 lines wide A Color Television Camera
• Following the vertical sync is another set • A color camera is actually three cameras of equalizing pulses, then a train of in one housing – one for each primary color horizontal pulses of red, green, and blue • An optical separator behind the main lens (taking lens) breaks the incoming light into its component colors • Separate preamplifiers and processors green phosphor, and the third is for the blue handle these R, G, and B signals phosphor • Trios of dots are formed by the color phosphors
A Tricolor Picture Tube
Dichroic Mirrors
• Dichroic mirrors are special tinted mirrors that allow certain wavelengths to pass and reflects others • A certain dichroic mirror would reflect blue but passes the remainder • The remaining light is passed onto another set of dichroic mirrors, one that reflects red, and another that reflects green • The R, G, and B components of the light are then fed to separate camera tubes
Color Separation and the Shadow Mask
• Separation of colors is maintained by the shadow mask principle. • The mask is a perforated steel sheet mounted on the back of the screen • Only the electrons that converge at a proper angle can strike the phosphor screen Color Picture Tubes to produce the correct color. Other electrons are blocked by the mask • Instead of having the screen coated with a single color phosphor and one electron gun, The Shadow Mask Principle color tubes have red, green, and blue • phosphors, with three electron guns for each If we were to suspend a light source at an primary color angle above a perforated plate, then the light • They are essentially three guns in a single source would pass through the holes and hit glass envelope specific areas on the layer below. Assume that the illuminated points are painted red • One gun controls electrons that strike • only the red phosphor, the second is for the If another light source is suspended at a different angle from the first, it would illuminate a different set of areas on the layer below. This time assume that the points are • As a result, a monochrome picture blue. produced by the Y signal looks correct in • The same idea can be applied to a third shades of gray and white source, this time for green spots Color Information
• Color information is encoded using a 3.579545 MHz carrier • This signal is called the chrominance signal or C signal • The 3.58 MHz carrier is referred to as the chroma subcarrier • This signal is multiplexed with the Y signal and modulate the picture carrier together • The amplitude-modulated C signal is transmitted with a suppressed carrier, to reduce interference with the Y signal.
Encoding the Color Information
• As with the luminance signal, the R, G, and B signals are fed to a matrix to combine them in specific proportions • Two color signals come out of the matrix, which are color mixtures
I = 0.60R − 0.28G − 0.32B
• Note that each source itself is not a Q = 0.21R − 0.52G + 0.31B primary color; the mask makes one beam serve for red, blue, and green • These two signals correspond to hue • It is the relative position of the light values sources with respect to the mask that • The Q signal stands for quadrature. It is determines the separate colors transmitted with a phase shift of 90° from the I signal Compatibility with Monochrome Receivers
• Color television is compatible with black and white because essentially the same scanning standards are used. • The R, G, and B information signals are encoded to be a luminance signal Y which is the signal for black and white television
Y = 0.3R + 0.59G + 0.11B
• These percentages approximate the brightness sensation for different colors • The B – Y video signal is a color mixture close to blue. The phase angle of this signal Color Sync Burst is exactly 180° opposite of the color sync burst, making it easy to lock on to • To demodulate the colorplexed • The R – Y video signal is close to red, information, a 3.58 MHz oscillator at the and is 90° from the B – Y signal receiver generates the subcarrier signal • The B – Y and the R – Y are combined to • Furthermore, a sample of the 3.58 MHz get the G – Y signal subcarrier signal is transmitted with the C • These three signals are called color signal as a phase reference for the color difference signals oscillator in the receiver • Color synchronization of the color hues in the picture is accomplished by a burst of 8 to 11 cycles of the 3.58 MHz subcarrier signal on the back porch of each horizontal blanking pulse
Decoding the Picture Information
• As with stereophonic FM, part of the Addition of Colors picture signal is used for monochrome reception. The remainder is used to decode the color information for colored receivers • The colorplexed video information is separated into the Y and C signal using bandpass filters • The Y information is sent to monochrome circuits • The C signal is decoded further to extract the R, G and B signals that is fed to the guns at the back of the picture tube Color Circle (Vectorscope)
Television Systems Around the World
The Americas, Western Europe, C.I.S. Japan, Philippines Germany, Italy, England France (formerly Spain U.S.S.R.) Lines per frame 525 625 625 625 625 Frames per second 30 25 25 25 25 Field frequency, Hz 60 50 50 50 50 Line frequency, Hz 15,750 15,625 15,625 15,625 15,625 Video bandwidth, MHz 4.2 5 or 6 5.5 6 6 Channel width, MHz 6 7 or 8 8 8 8 Video modulation (sync) Negative Negative Negative Positive Negative Sound Signal FM FM FM AM FM Color System NTSC PAL PAL SECAM SECAM Color subcarrier, MHz 3.58 4.43 4.43 4.43 4.43
Color Systems
• NTSC Stands for National Television Standards Committee
• PAL Stands for Phase Alternation by Line
• SECAM Stands for Systeme Electronique pour Coloeur Avec Memoire In English, Electronic System for Color with Memory