question: How come radio waves can be used to transmit very high quality data for TV/internet but the radios used by police departments and air traffic control tend to be garbled and staticky?
comment: "Radio connections" can be of two types: analog or digital. In an analog connection, radio waves are converted to electricity by an amplifier and this turns directly into mechanical forces on a speaker. Any distortion in the radio waves during travel is represented faithfully to the speaker and you hear scratching. In digital connection, sound waves are translated into a sequence of packets which are transmitted in their encoded form to the other "radio" which then receives each packet and translates it back into sound. The use of "packets" in networking protocols allows for error checking and confirmation. Packets often contain a checksum which is the result of a mathematical operation on the packet's contents. If the receiver computes the checksum of a received packet and it is wrong, it can request the packet again from the sender. The result is a perfect recreation within the receiver, of the sound data stream sent generated by the sender. It's the same reason there isn't "static" in a Netflix video stream. Back in the previous millennium, TV was sent over analog radio signal and distortion of the image was quite common.
passages: U.S state patrols, county sheriffs' offices, and municipal police departments often run their own systems in parallel, presenting interoperability problems. The FCC assigns licenses to these entities in the public safety (PP and PX) allotments of the spectrum. These include allocations in the lower portion of the VHF spectrum (around 39 - 45 MHz), highly susceptible to "skip" interference but still used by state highway patrols; the VHF "hi-band", from 150 to 160 MHz; and various UHF bands. Many systems still use conventional FM transmissions for most traffic; others are trunked analog or digital systems. Recently, there has been a move towards digital trunked systems, especially those based around the public-safety standard Project 25 format set by the Association of Public-Safety Communications Officials-International. A minority of other police radio systems, the largest examples being the Milwaukee Police Department and Pennsylvania State Police use the incompatible OpenSky format. TETRA, the standard in many European countries as well as other places in the world, is virtually unused in the United States.
   Section::::Broadcast technologies.:Infrastructure-independent.
  
  Broadcast technologies that do not depend on man-made infrastructure to convey communication may be least susceptible to disruption during disasters and emergencies. Some examples of infrastructure-independent technologies are:
  
  BULLET::::- Short-wave Radio
  
  BULLET::::- Two-way Radio
  
  BULLET::::- Weather Radio
  
  BULLET::::- Internet Based Communication
  
  Each device has its own pros and cons given the emergency situation.
  
  Section::::Broadcast technologies.:Infrastructure-dependent.
  
  Broadcast technologies that depend on man-made infrastructure to convey communication are susceptible to disruption if any part of that infrastructure is overloaded, damaged or otherwise destroyed. Some examples of infrastructure-dependent technologies are:
  
  BULLET::::- Audio Public Address Systems
  
  BULLET::::- WMT Public Address Systems
   BULLET::::- Usable range compared to FM
  
  D-STAR, like any digital voice mode has comparable usable range to FM, but it degrades differently. While the quality of FM progressively degrades the further a user moves away from the source, digital voice maintains a constant voice quality up to a point, then essentially "falls off a cliff". This behavior is inherent in any digital data system, and it demonstrates the threshold at which the signal is no longer correctable, and when data loss is too great, audio artifacts can appear in the recovered audio.
  
  BULLET::::- Emergency Communications Concerns
   BULLET::::- DAB radios are out of step with live events, so the experience of listening to live commentaries on events being watched is impaired;
  
  BULLET::::- Listeners using a combination of analogue (AM or FM) and DAB radios (e.g. in different rooms of a house) will hear a mixture when both receivers are within earshot.
   Even if the technical problems are solved, the issue is more complicated than just adding radio channels or talk groups. It is also a cultural problem. In one local incident, a large number of officers from three police agencies were fielded to search for a violent criminal who had evaded officers from one of the agencies. The officers did not coordinate by switching to a shared radio channel. After the incident, one participant said the users thought their radios were incompatible and did not understand how the shared channel worked. This possibly reflects a training problem or a technology literacy problem. The problem seems to have been remedied since then.
   With large high power transmitters, the protective circuits can comprise a significant fraction of the total design complexity and cost.
  
  Section::::Power supplies.
   In the event a "Code 6 Charles" is broadcast, the unit in question must verify their location, advise if they are "Code 4" and the nature of the "Code 4" (e.g. "Suspect in custody", "Common Name", "Information Only" or "Wrong suspect".)
  
  A noticeable characteristic of police broadcasts is the expedited nature of crime broadcasts; due to the number of broadcasts that need to be made at any given moment of the day, each transmission is necessarily as brief as possible. As a standard of police professionalism, RTOs are trained to use a tone that is strictly business-like.
  
  Section::::Radio.:Digital frequencies.
   Compromises in audio quality are inherent in the use of any codebook-based speech coder, particularly when used in conditions of high background noise. Incremental improvements are being made in the algorithms, which may lead to differences in performance even while the basic method remains unchanged. In the US, the Department of Commerce Public Safety Communications Research laboratory regularly reports on progress in this field. While their work specifically pertains to Project 25 radios, it is directly applicable to any system using similar multi-band excitation coders.
  
  Section::::NXDN Forum.
   Digital Radio Mondiale, commonly used in shortwave broadcasting, can use less, the same, or more bandwidth as AM, to provide high quality audio. Digital Radio Mondiale requires digital detection circuitry not present in conventional AM radios to decode programming. 
  
  Special CAM-D receivers provide the benefit of better frequency response and a slow auxiliary data channel for display of station ID, programming titles, etc.
  
  Section::::Issues.
  
  Section::::Issues.:Availability of Receivers.
   Police, fire, and military communications on the VHF bands are also DX'ed to some extent on multi-band radio scanners, though they are mainly listened to strictly on a local basis. One difficulty is in identifying the exact origins of communications of this nature, as opposed to commercial broadcasters which must identify themselves at the top of each hour, and can often be identified through mentions of sponsors, slogans, etc. throughout their programming.
  
  Section::::Types of DXing.:Amateur radio DX.
   In the United States, the 4.9 GHz band is reserved for use by public safety wireless applications. The transmission mask is narrower for the public safety band than for consumer part 15 applications. Thus one cannot simply operate 802.11j equipment in the public safety band and be FCC compliant. Public safety agencies are working with manufacturers and the FCC in order to leverage Commercial Off The Shelf (COTS) equipment. There are public safety groups working closely with the manufacturing community, federal interests, and standards bodies to create an 802.11 series standard for public safety.
  
  Section::::See also.
  
  BULLET::::- IEEE 802.11a
   Section::::Interference and CTCSS.
  
  In non-critical uses, CTCSS can also be used to hide the presence of interfering signals such as receiver-produced intermodulation. Receivers with poor specifications—such as scanners or low-cost mobile radios—cannot reject the strong signals present in urban environments. The interference will still be present and may block the receiver, but the decoder will prevent it from being heard. It will still degrade system performance but the user will not have to hear the noises produced by receiving the interference.
   FM 24-6 further states, "Readability and signal strength reports indicate the desired method of transmission and should be used in that sense. Readability and signal strength reports are not exchanged unless "3" or less, lack of any report being assumed to indicate satisfactory communications."
  
  The report example given below uses military call signs and the obsolete Q Code of QJS for readability instead of the current QRK:
  
  Section::::CCB signal strength and readability.
   In 2011, the "Wall Street Journal" published an article describing research into security flaws of the system, including a user interface that makes it difficult for users to recognize when transceivers are operating in secure mode. According to the article, "(R)esearchers from the University of Pennsylvania overheard conversations that included descriptions of undercover agents and confidential informants, plans for forthcoming arrests and information on the technology used in surveillance operations." The researchers found that the messages sent over the radios are sent in segments, and blocking just a portion of these segments can result in the entire message being jammed. "Their research also shows that the radios can be effectively jammed (single radio, short range) using a highly modified pink electronic child’s toy and that the standard used by the radios 'provides a convenient means for an attacker' to continuously track the location of a radio’s user. With other systems, jammers have to expend a lot of power to block communications, but the P25 radios allow jamming at relatively low power, enabling the researchers to prevent reception using a $30 toy pager designed for pre-teens."
   P25-compliant systems are being increasingly adopted and deployed. Radios can communicate in analog mode with legacy radios, and in either digital or analog mode with other P25 radios. Additionally, the deployment of P25-compliant systems will allow for a high degree of equipment interoperability and compatibility.
   From the 1980s, the ever-increasing use of mobile communications and broadcast media channels has put huge pressure on the available airspace. Regulatory authorities are squeezing band allocations closer and closer together, relying on increasingly sophisticated EMC control methods, especially in the digital communications arena, to keep cross-channel interference to acceptable levels. Digital systems are inherently less susceptible than analog systems, and also offer far easier ways (such as software) to implement highly sophisticated protection measures.
   Section::::Frequencies and transmitters.:Shortwave (USB).
  
  BULLET::::- Diego Garcia:
  
  BULLET::::- 12,579 kHz daytime
  
  BULLET::::- 4,319 kHz nighttime
  
  BULLET::::- Guam:
  
  BULLET::::- 13,362 kHz daytime
  
  BULLET::::- 5,765 kHz nighttime
  
  BULLET::::- Key West, Florida: ("decommissioned")
  
  BULLET::::- "12,133.5 kHz day & night"
  
  BULLET::::- "7,811.0 kHz day & night"
  
  BULLET::::- "5,446.5 kHz day & night"
  
  BULLET::::- Pearl Harbor, Hawaii ("site currently out of service"):
  
  BULLET::::- "10,320 kHz daytime"
  
  BULLET::::- "6,350 kHz nighttime"
  
  See: AFN Shortwave Frequencies
  
  Section::::See also.
  
  BULLET::::- AFN Berlin
  
  BULLET::::- AFN Bremerhaven
  
  BULLET::::- British Forces Broadcasting Service
  
  BULLET::::- Canadian Forces Radio and Television
  
  BULLET::::- Far East Network
  
  BULLET::::- Israel Army Radio
  
  BULLET::::- DoD News Channel
  
  BULLET::::- Radio Forces Françaises de Berlin
  
  Section::::Further reading.
   Over 570,000 radios have been purchased. There have been several system improvement programs, including the Integrated Communications Security (ICOM) models, which have provided integrated voice and data encryption, the Special Improvement Program (SIP) models, which add additional data modes, and the advanced SIP (ASIP) models, which are less than half the size and weight of ICOM and SIP models and provided enhanced FEC (forward error correction) data modes, RS-232 asynchronous data, Packet Data formats, and direct interfacing to Precision Lightweight GPS Receiver (PLGR) devices providing radio level situational awareness capability.
   A development is the use of the RDS, a system of car radios, whereby the vehicle can be fitted with a short range FM transmitter, set to RDS code 31, which interrupts the radio of all cars within range, in the manner of a traffic broadcast, but in such a way that the user of the receiving radio is unable to opt out of the message (as with traffic broadcasts). This feature is built into all RDS radios for use in national emergency broadcast systems, but short range units on emergency vehicles can prove an effective means of alerting traffic to their presence, although is not able to alert pedestrians, non-RDS radio users, or drivers with their radios turned off.
   The FCC restricts channel 9 to emergency communications and roadside assistance. Most highway travelers monitor channel 9. Truck drivers still use CB, which is an effective means of obtaining information about road construction, accidents and police speed traps.
  
  Section::::History.:Australia.
  
  Section::::History.:Australia.:History.
   Section::::Electric current.
  
  Electric currents that oscillate at radio frequencies ("RF currents") have special properties not shared by direct current or alternating current of lower frequencies.
  
  BULLET::::- Energy from RF currents in conductors can radiate into space as electromagnetic waves (radio waves). This is the basis of radio technology.
  
  BULLET::::- RF current does not penetrate deeply into electrical conductors but tends to flow along their surfaces; this is known as the skin effect.
   BULLET::::- Resistance to eavesdropping. The spreading code (in DS systems) or the frequency-hopping pattern (in FH systems) is often unknown by anyone for whom the signal is unintended, in which case it obscures the signal and reduces the chance of an adversary making sense of it. Moreover, for a given noise power spectral density (PSD), spread-spectrum systems require the same amount of energy per bit before spreading as narrowband systems and therefore the same amount of power if the bitrate before spreading is the same, but since the signal power is spread over a large bandwidth, the signal PSD is much lower — often significantly lower than the noise PSD — so that the adversary may be unable to determine whether the signal exists at all. However, for mission-critical applications, particularly those employing commercially available radios, spread-spectrum radios do not intrinsically provide adequate security; "...just using spread-spectrum radio itself is not sufficient for communications security".
   Section::::Security Issues.
  
  In VANET scenarios, every vehicle including emergency vehicles will be equipped with communication capabilities. In addition to collision avoidance, BSMs can also be used by the authorities for purposes such as locating a vehicle that is broken down, chasing vehicle of a criminal, etc. Therefore, it makes tampering with location information in BSMs very attractive for attackers. They can produce fake locations to cause accidents, mask the true location of a criminal's vehicle in a police chase and disrupt many other VANET safety applications.
  
  Section::::Public Key Infrastructure.
   By the last quarter of 2006 police forces had migrated radio networks from the UHF frequencies to TeTRa on the Airwave network, followed by ambulance services in 2007 and fire services in 2010. Airwave now has a nationwide network of more than 3,000 sites and provides secure voice and data communications to over 300 public safety organisations.
   Section::::Different Signals.
  