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.