How Electronic Mass Media Work

Electromagnetism

•	There are three fundamental characteristics of all forms of electromagnetic energy
o	They radiate outward in all directions from a source
o	They travel at the speed of light
o	They have the properties of waves

Radio waves 

•	Travel at the speed of light: 186,282.3959 miles per second 
•	Circles the Earth seven times in a second
•	Waves have two characteristics: 
o	Frequency -- the number of separate waves produced each second
o	Wavelength -- distance from one point on a cycle to the same point on the next cycle  
•	All waves have measurable velocities, wavelengths, and frequencies

Frequency Spectrum 

•	As frequency goes up, the difficulty of using the wave increases
•	Satellites use up to 30 to 40 gigahertz, still comparatively low in the spectrum
•	Spectrum use is routinely reallocated for various reasons.  One recent trend:  
•	FCC auctions some of the spectrum now to raise money for the government 
•	That's what’s happened to the former analog TV spectrum
•	About $55-$60 billion has been raised, primarily from wireless operators

Radio Waves

•	One complete wave is a cycle – one cycle per second is a Hertz (Hz)
•	Frequency determines wavelength: the more waves, the shorter the wavelength.
•	Broadcast services can be described in terms of 
o	Wavelength – short-wave or microwave
o	Frequency bands (LF, MF, HF, VHF, UHF, SHF, EHF)
•	Attenuation: loss of strength as the wave travels
•	The higher the frequency the greater the attenuation – inverse square proportion.

Radio Frequencies and Channels

•	FCC allocates a channel – a certain frequency range – to each broadcast outlet. 
•	At the center is the carrier wave (because it carries information) – the frequency assigned to the station.  
•	The carrier and its sidebands are the channel.  
•	TV stations have two carrier waves -- one for video and the other for audio. 
•	The carrier wave is broadcast whether it is modulated or not.

Audio Waves

•	Sound waves propagate mechanically – need a medium such as air or water.
•	Radio waves propagate electromagnetically
o	Would rather have a vacuum
o	They also pass through physical objects.
•	Sound waves travel at about 1,050 feet per second.
•	Sound waves electrified travel close to the speed of light (resistance) through wires, which radiate electromagnetic waves.

Modulation

•	Modulation refers to the methods by which information is imposed on a carrier wave.
•	Modulation produces a pattern of energy that can be interpreted as meaningful
•	The telegraph is just the same as a blinking flashlight or a smoke signal
•	Amplitude patterns convey loudness
•	Frequency patterns convey pitch
•	The A/F variations modulate the transmitter's RF carrier.  The result is a radio wave

AM vs. FM

•	Amplitude modulation is called standard because it came first
•	NTSC TV uses AM for the picture and FM for the sound
•	AM is easily distorted by electrical interference causing static
•	There is no discernable affect on FM

Bandwidth and Sidebands

•	Bandwidth is the capacity of a communications system
•	Full motion video requires the most, FM radio less than video, AM less than FM, phone or other voice communications even less
•	For radio, the bandwidth is the two sidebands
•	Modulation creates equal sidebands above and below the carrier wave
•	The info in each sideband is the same, so you only need one
•	TV – the lower sideband is suppressed to conserve spectrum, leaving a "vestigial" sideband to prevent interference.

Multiplexing

•	Multiplexing is a process of maximizing the capacity of the channel
•	A channel may be divided into subchannels
•	In broadcasting, that means propagating a subcarrier
o	The main video carrier in TV sends the luminence information
o	A subcarrier carries the chroma information
o	Another subcarrier carries the audio
o	FM radio uses multiplexing to carry stereo information.

Wave Propagation

•	Propagation – signals traveling outward from an antenna
•	Coverage contours – under ideal conditions are nearly circular, but in fact they are not
•	They are affected by terrain, frequency of signal, weather, and sunspots.
•	Objects longer than the wave cause "shadows" in coverage
•	Frequency causes the most important propagation differences.

Direct Waves

•	Applies to FM radio and TV – all in the VHF and UHF ranges
•	Direct waves extend only to the horizon – usually about 30 miles
•	Raising the transmitter or the receiving antenna or both extends the horizon.

Ground Waves

•	Applies only to AM radio in the MF range
•	Propagated through and along the Earth's surface
•	Covers much larger areas than direct waves
•	A lot depends on soil conductivity.

Sky Waves

•	A transmitter propagates in all directions, including up
•	MF (AM radio) and HF (shortwave radio) waves refract back down from the ionosphere – sky waves
•	Makes clear channel stations possible
•	But also creates interference and skip zones
•	AM radio sky waves occur only at night
•	Shortwave antennas make use of the ionosphere 24 hours a day and broadcast on many frequencies.

Antennas

•	Are needed to both transmit and receive
•	Size and location are everything
•	Transmitter antenna size must be mathematically related to the wavelength and be no more than one fourth of the wavelength
•	The higher the frequency, the more you need an efficient antenna, but almost anything can act as an antenna

Directional Propagation

•	Beaming reinforced signals in the desired direction
•	Increases signal strength in the desired direction and reduces interference in the other directions
•	The increase is called antenna gain
•	Directing AM radio signal requires more antennas and is very expensive, but it is the service that requires it the most
•	Receiving antennas can also be directed for efficient reception -- some must be turned to receive different stations.

Co-channel Stations 

•	Two stations share the same frequency are co-channel
•	They have to be far enough apart to not interfere with each other
•	The interference zone extends well beyond the service zone
•	Sky wave phenomena – many stations reduce power at night or go off the air
•	This is the primary limitation on the number of stations we can license

Adjacent-Channel Stations

•	It’s too expensive to build very selective receivers, so:
•	Some outlets suffer interference from stations on adjacent frequencies
•	AM stations suffer most from this interference
•	They have to be hundreds of miles apart in some cases

AM Radio: Band, Channels, and Classes

•	The bandwidth of an AM radio channel is 10 kHz
•	There are 117 AM channels – ten were added in 1988
•	88 stations moved to the expanded bands to reduce interference
•	There are three types of channels: clear, regional, and local
•	There are four classes from A to D – designate allowable transmitter power and operation hours

AM Radio: Transmission

•	AM stations range in power from 250 watts to 50,000 watts
•	Antennas must be hundreds of feet tall
•	Complex engineering studies have to be made to assess soil conductivity, airplane flight paths, electrical interference, etc
•	Low-power AM signals may be fed into a building's electrical grid or through pipes
•	These are called carrier-current stations – often used for travelers info services

FM Radio: Bands and Channels

•	The bandwidth of an FM radio channel is 200 kHz
•	There are 100 200 kHz channels for a total of 20 MHz of spectrum
•	The 20 lowest channels are reserved for noncommercial use by radio stations such as WRFL (at 88.1, it's the lowest channel in the spectrum), WUKY, and WEKU
•	The lower the frequency, the more bang for your transmission buck

FM Radio: Transmission

•	FM radio uses direct waves, which reach only to the horizon
•	The coverage contour depends on transmitter power and the height of the antenna
•	No sky waves involved, so the coverage is stable both day and night
•	There are three classes of FM station that define power, antenna height, and zone  
o	"A" stations are limited to a 15 mile range
o	"B" stations are limited to a 30 mile range
o	"C" stations are limited to a 60 mile range

FM Radio: Transmission II

•	The maximum power for a C station is 100,000 watts; maximum antenna height is 2000 feet.  
•	The frequencies used by FM are much higher than AM
•	So the antennas are much smaller
•	They are on very high towers or on top of buildings or mountains.

FM Stereo Reception

•	FM radio delivers high-fidelity sound with much better dynamic and frequency response than AM
•	It's also immune to static and other interference
•	FM transmitters have a stereo generator that produces pilot and subcarrier signals
•	No information is compressed – they separate the two channels in your stereo receiver
•	FM channels can also be multiplexed to carry non-broadcast music and data information that is picked up using special equipment.

Camera Technology

•	Like other electronic devices, TV cameras once used tubes
•	Switched from tubes to Charge-Coupled Devices (CCD)
•	CCD uses a photodiode that converts light into picture elements (pixels)
•	CCD reads out the picture in line-by-line scanning called interleaving
•	Two fields make a frame (one second)
•	Professional cameras have a separate CCD for red, green, and blue (RGB)
•	Consumer cameras generally have only one

Scanning and Frames 

•	U.S. (NTSC) television calls for 525 lines of resolution – the clarity of the image
•	Accomplished by interleaved scanning – devised to eliminate flickering
•	Two fields are scanned, the odd and the even lines of the display
•	They constitute a frame – 30 each second
•	A phenomenon called persistence of vision enables humans to perceive 30 still images per second as smoothly moving video.

TV Channels

•	All AM and FM radio channels together occupy less space than four six MHz TV channels
•	NTSC TV is poor quality imaging compared to still photography and film
•	The reason is the constraint of spectrum space
•	The bandwidth is said to be 6 MHz, but only about 4.5 MHz carry the video and audio
•	The rest is buffer to eliminate adjacent channel interference.

TV Transmission Propagation

•	TV and FM antennas are relatively small, but they must be placed as high as possible
•	TV and FM radio use a directionality technique called beam tilt
o	Concentrates most of the energy downward
o	It’s not lost in space where it does no good
•	FCC allots tremendous power (up to five million watts) to UHF stations because of the high attenuation of the higher frequencies

TV Reception

•	If you don't have cable, you have to use an antenna
•	If you live close to the TV station, the antenna in your TV will do
•	If you live farther away you need an outdoor antenna
•	They must point toward the station so if you want to receive more than one channel the antenna has to rotate.

TV Reception II

•	Your TV processes the audio and video separately
•	The audio comes through what is essentially an FM receiver, just like your stereo
•	In a CRT the video goes to an electron gun that excites pixels as it moves down the inside of the tube laying down lines of resolution
•	The color comes from exciting pixels that glow in combinations of red, green, and blue
•	Now we use active-matrix LCDs that are much higher quality – colorful, bright, flicker-free, and extremely high resolution

HDTV

•	Motivation was conservation of spectrum space
•	Field testing of HDTV was completed at 199 sites in 1994
•	The first commercial HDTV broadcast took place in Raleigh, N.C. in 1996
•	Mandatory conversion from analog to digital was supposed to take place in 2006, but the FCC extended the deadline to Feb. 17, 2009 
•	Conversion was actually completed by June 12, 2009

HDTV

•	At first, all HDTV TV signals were in the UHF range – caused concerns for broadcasters that their coverage area would decrease – and it did for some of them
•	Some problems occurred for people with VHF antennas
•	Luminance, chroma and audio carriers are all combined into one digital stream
•	Aspect ratio is 16:9, the geometric mean of analog 4:3 aspect ratio
•	Resolution in the U.S is 1080i, 1080p and 780p