Emergency Alert System
The Emergency Alert System (EAS) is a national system in the U.S. put into place in 1994, superseding the Emergency Broadcast System (EBS) and is jointly coordinated by the Federal Communications Commission (FCC), Federal Emergency Managemant Agency (FEMA), and the National Weather Service (NWS).  The official EAS is designed to enable the President of the United States to speak to the United States within 10 minutes (this official federal EAS has never been activated).  The EAS regulations and standards are governed by the Public Safety and Homeland Security Bureau of the FCC. Each State and several territories have their own EAS plan.
The EAS covers both AM/FM/ACSSB(R)(LM(R)) radio and VHF Low/VHF Medium/VHF High/UHF/television (including low-power stations), HRC/IRC/ICC/STD/EIA, cable television and wireless cable television companies.  Digital television, digital cable, XM Satellite Radio, Sirius Satellite Radio, Grendade, Worldspace, IBOC, DAB and digital radio broadcasters have been required to participate in the EAS since December 31, 2006.  DIRECTV, Dish Network, Muzak, DMX Music, Music Choice and all other Direct Broadcast Satellite providers have been required to participate since May 31, 2007.  Video Dial Tone (OVS) has been required to participate since July 1, 2007.
Listen to:Template:Audio

# Technical concept
Messages in the EAS are comprised of four parts: a digitally encoded SAME header, an attention signal, an audio announcement, and a digitally encoded end-of-message marker.
The Template:Audio is the most critical part of the EAS design. It contains information about who originated the alert (the President, state or local authorities, the National Weather Service, or the broadcaster), a short, general description of the event (tornado, flood, severe thunderstorm), the areas affected (up to 32 counties or states), the expected duration of the event (in minutes), the date and time it was issued (in UTC), and an identification of the originating station.  (See SAME for a complete breakdown of the header.)
30+ radio stations are designated as National Primary Stations in the Primary Entry Point (PEP) System to distribute Presidential messages to other broadcast stations and cable systems. The Emergency Action Notification is the notice to broadcasters that the President of the United States or his designee will deliver a message over the EAS via the PEP system. "You {AM and FM broadcasters} will hear the following Emergency Action Notification
Message from the EAS decoder. This is an Emergency Action Notification requested by the White House. All broadcast stations will follow activation procedures in the EAS Operating Handbook for a national
level emergency. The President of the United States or his representative will shortly deliver a message over the Emergency Alert System." 
Because the header lacks error detection codes it is repeated three times for redundancy.  EAS decoders compare the received headers against one another, looking for an exact match between any two, eliminating most errors which can cause an activation to fail.  The decoder then decides whether to ignore the message or whether to relay it on the air based on whether the message applies to the local area served by the station (following parameters set by the broadcaster).
The SAME header bursts are followed by an Template:Audio which lasts between eight and 25 seconds, depending on the originating station. The tone is Template:Audio on a NOAA Weather Radio station, while on commercial broadcast stations, it consists of a "two tone" combination of 853 Hz and 960 Hz sine waves and is the same attention signal used by the older Emergency Broadcast System.  The "two tone" system is no longer required as of 1998 and is to be used only for audio alerts before EAS messages. Like the EBS, the attention signal is followed by a voice message describing the details of the alert.
The message ends with three bursts of the AFSK "EOM", or End of Message, which is the text NNNN, preceded each time by the binary 10101011 calibration.
The White House has endorsed the migration to the Common Alerting Protocol and FEMA is in the process of testing implementation.

# Station requirements
The FCC requires all broadcast stations to install and maintain EAS decoders and encoders at their control points.  These decoders continuously monitor the signals from other nearby broadcast stations for EAS messages.  For reliability, at least two other source stations must be monitored, one of which must be a designated local primary.  Stations are to retain the latest version of the EAS handbook.
Stations are required by law to keep full logs of all received and transmitted EAS messages.  Logs may be kept by hand but are usually kept automatically by a small receipt printer in the encoder/decoder unit.  Logs may also be kept electronically inside the unit as long as there is access to an external printer or method to transfer them to a personal computer.
In addition to the audio messages transmitted by radio stations, television stations must also transmit a visual message. A text "crawl" is displayed at the top of the screen.  A color coded "crawl" system is often used where the color signifies the  priority of the message.  Some television stations transmit only the visual message which is outside of the requirements.  A television station may be used for monitoring by another station and thus the audio is necessary.
Upon reception of an alert, a station must relay EAN (Emergency Action Notification) and EAT (Emergency Action Termination) messages immediately (US FCC 7).  Stations traditionally have been allowed to opt out of relaying other alerts such as severe weather and child abduction emergencies (AMBER Alerts) if they so choose.  Under new rules published on July 12, 2007, the FCC intends to require all stations to relay state and local alerts that are approved by their states' governors (pending approval of the CAP standard).
Some stations may be non-participating, and do not relay messages.  Instead they transmit a message instructing listeners/viewers to tune to another station for the information, and they must then suspend their operation.

# Testing the System
All EAS equipment must be tested weekly.  The required weekly test (RWT) consists of the header and the end-of-message SAME bursts.  The RWT need not contain an audio message nor are voice or graphic messages announcing the test are required, although many stations will provide them as a courtesy to the listener or viewer.  Television stations are not required to transmit a video message for weekly tests.  RWTs are scheduled by the station, alternating between night and day, and are not relayed.
On cable systems before the start of the EAS test, all of a system's channels are redirected to one analog channel which is received on all tiers of service, but doesn't usually give out news or weather information (such as the TV Guide Network, QVC, HSN, or a public access station), where the test occurs from the local headend office or from the system's master office elsewhere in the region.  Newer technology allows cable DVR and video on demand systems to interrupt playback of a program for an EAS test.  After the test ends, the one channel usually remains on screen for 5-10 addition seconds before the original station/network is returned to.
Required Monthly Tests (RMTs) are generally originated by the primary relay station or a State's EAS agency, relayed by broadcast and cable stations. RMTs are conducted with the following procedure:
- Normal programming is suspended, and an announcement may be made such as: "This is a test of the Emergency Alert System. This is only a test."
- The SAME Header burst is sent, perhaps followed by an attention signal.
- Another voice message is sent, which runs something like this:
"This is a coordinated monthly test of the broadcast stations in your area. Equipment that can quickly warn you during emergencies is being tested.  If this had been an actual emergency such as (insert types of messages that may occur in the geographic area) , official messages would have followed the alert tone.  This concludes this test of the Emergency Alert System."
- The SAME EOM burst is sent.
RMTs must be performed between 8:30AM and local sunset during odd numbered months, and local sunset to 8:30AM for even months.  Received tests must be retransmitted inside 60 minutes from receipt.
An RWT is not required during a calendar week in which an RMT is scheduled.  No testing has to be done at all during a calendar week in which the EAS has been legitimately activated. Occasionally a periodic national test may be conducted.

# Additions and proposals
The number of event types in the national system has grown to eighty.  At first, almost all but three of the events were weather-related, the remaining types dedicated for civil emergencies.  Since then, several classes of non-weather emergencies have been added, including, in most states, the AMBER Alert System for child abduction emergencies.
In 2004, the FCC issued a Notice of Proposed Rulemaking seeking comment on whether EAS in its present form is the most effective mechanism for warning the American public of an emergency and, if not, on how EAS can be improved.  As noted above, rules implemented by the FCC on July 12, 2007 provisionally endorse replacing the SAME protocol with CAP and allow governors to compel universal activation of the system within their own states.

# EAS for consumers
EAS is designed to be useful for the entire public, not just those with SAME-capable equipment.  However, several consumer-level radios do exist, especially weather radio receivers, which are available to the public through both mail-order and retailers like Radio Shack, Circuit City, and several others.  Other specialty receivers for AM/FM/ACSSB(R)(LM(R)) are available only through mail-order, or in some places from federal, state, or local governments, especially where there is a potential hazard nearby such as a nuclear plant or chemical factory.  These radios come pre-tuned to a station in each area that has agreed to provide this service to local emergency management officials and agencies, often with a direct link back to the plant's safety system or control room for instant activation should an evacuation or other emergency arise.
The ability to narrow messages down so that only the actual area in danger is alerted is extremely helpful in preventing false warnings, which was previously a major tune-out factor.  Instead of sounding for all warnings within a station's area, SAME-decoder radios now sound only for the counties they are programmed for.  When the alarm sounds, anyone with the radio knows that the danger is nearby and protective action should be taken.  For this reason, the goal of the National Weather Service is that each home should have both a smoke detector and a SAME weather radio.
A private website called the Emergency Email Network offers to send an email or SMS text message to registered users in the event of an EAS activation.  Some desktop weather monitoring programs, such as WeatherBug, offer a computer alert during emergencies.
Currently under development is new infrastructure called the Digital Emergency Alert System.  This system would allow the transmission of emergency alerts directly to citizens and responders without the need for a special receiver.  These alerts would be sent to users of computers, mobile phones, pagers, and other devices.

# Incidents
On September 11, 2001, ". . . the EAS was not activated nationally or regionally in New York or Washington during the terrorist attacks on the nation." Richard Rudman, then chairman of the EAS National Advisory Committee explained that near immediate coverage in the national media meant that the media itself provided the warning or alert of what had happened at what might happen as quickly as the information could be distributed. "Some events really do serve as their own alerts and warnings. With the immediate live media coverage, the need for an EAS warning was lessened." 34 PEP stations were kept  on high alert for use if the President had decided to order an Emergency Action Notification. "PEP is really is a last-ditch effort to get a message out if the president cannot get to the media." 
On February 1, 2005 someone inadvertently activated an EAS message over radio and television stations in Connecticut telling residents to evacuate the state immediately. Officials at the Office of Emergency Management announced that the activation and broadcast of the Emergency Alert System was in error due to possibly the wrong button being pressed. "State police said they received no calls related to the erroneous alert."
On June 26, 2007, the EAS in Illinois was activated at 7:35AM CDT and issued an Emergency Action Notification Message for the United States. This was followed by dead air and then WGN-AM (720) radio (the station designated to simulcast the alert message) being played on almost every television and radio station in the Chicago area and throughout much of Illinois . The accidental EAN activation was caused when a government contractor installing a new satellite receiver as part of a new national delivery path incorrectly left the receiver connected and wired to the state EOC's EAS transmitter before final closed circuit testing of the new delivery path had been completed. 

# References in pop culture
The EAS is recognized so widely that it has been implemented into popular culture, such as episodes of television shows, sometimes.  For example, an episode of Dexter's Laboratory (entitled "911") focuses on Dexter's attempts to rectify all possible emergencies happening at the time that an EAS test interrupted his favorite television show.  (Although it says in the episode that it was the Emergency Broadcast System, the episode was created after the change to the EAS.)  The sounds and tones in the EAS are also in some songs.