Substantially, EAS is an automated system. If the President of the United States (
POTUS) wishes to activate the system, it follows almost exactly like this, with a few details ommited:
The President's White House Communications Agency Trip Officer (Presidential Communications Officer - PCO) always accompanies the President along with the White House Military Office Aide or Emergency Action Officer, who himself carries the Nuclear Football (Presidential Emergency Action Documents) with information necessary to activate the system.
The PCO, with assistance of the Aide and at the request of POTUS, contacts FEMA Mount Weather, which has since about 1960 been the control point for Conelrad, and EAS that followed.
Following a series of steps, the POTUS voice only, using phone patch, radio, or any other connectivity available, will be "broadcast" via a dial-out conference bridge or private-line ring down to approximately 30 Primary Entry Point radio stations in the US. The receipt of the Emergency Action Notification by the PEP station EAS box at the transmitter begins that point at which the Presidential would be heard by the public.
These stations have an EAS box at the transmitter, and the "conference" once received will interrupt the transmitter immediately in what is termed a "seize-key" manner. Thereafter, radio and TV stations and cable systems pick-up the audio only of the POTUS message. The method of getting the message from PEP stations to the final destination varies from state to state, however in most cases once any radio station receives the message, it is broadcast live and immediately, and that station's EAS box or ENDEC is "locked-up" or seized until the POTUS message is ended and the system is "un-seized." The President may choose this seizure for as long as he deems necessary. All stations will broadcast that message, or go off air. Initially, most TV viewers will not see video of the President. The message will end and the system is unseized once an End of Message code is received at the PEP ENDEC.
Audio would likely sound like a phone call heard broadcast on AM radio, barely listenable.
There a a few common misconceptions about the system that still exist among engineers whose stations in major cities would carry the broadcast, if it ever occurred. The system is a last resort means for POTUS to communicate quickly to as many people as possible, often in major cities, in case of grave national crisis. For example WABC New York is a PEP station, as is WLS Chicago. This might occur while POTUS is in a motorcade or aboard Air Force One. No video would be received until POTUS were able to arrive at a studio. As it happens, on 9/11 the system was not used. The president delivered a televised message upon arrival at a secure location. The system is not primarily designed to warn of nuclear attack, although if NORAD issued a Declaration of Air Defense Emergency Condition Red (expected air attack imminent on the North America), POTUS or his designee could use the EAS to deliver a warning message.
Residents in New Orleans in the immediate hours after Katrina heard part of EAS in action, via WWL radio. WWL has been a PEP for some years, and equipped with a generator, 30 days supply of fuel and shelter at the transmitter, placed in what would have been outside a nuclear blast zone. Last resort radio links are in place from WHCA to the PEP stations. It is unclear why these were not used to broadcast messages directly from FEMA to WWL.
Neither the EBS nor EAS has ever had a nationwide open-circuit test. As such no one is 100 percent certain if it would work or how it would sound. However, President Kennedy did make a nationwide broadcast over Conelrad, in conjunction with Operation Alert 1961. Some states conduct monthly EAS Required Monthly tests which may originate from a PEP staion in that state. All broadcasters and cable systems relay the message in a daisy chain fashion within one hour of receipt. Outside of an actual warning, this is the only time the public hears the familiar EBS alert tone which was in use from the 1970's until the 1990's. During tests one sees a screen crawl of the text message. During an actual national emergency, this is the only video that TV viewers would likely see, until the POTUS arrives at a TV studio. Mt. Weather did have a Conelrad TV/Radio Studio as early as 1960, and a studio is presumed to still exist. On a parallel note, The Film "Seven Days in May" does rather accurately depict the Conelrad system at that time.
Only one full aircheck or recording of Conelrad, as tested during Operation Alert 1961, is known extant. This full aircheck is not known to be available on the internet, but is traded privately among collectors. A brief sample is available online.
Many broadcasters, with the exception of engineers at the PEP stations, misunderstand the system and how it works.
This announcement "This is an Emergency Action Notification. All broadcast stations and cable systems shall transmit this Emergency Action Notification Message. This station has interrupted its regular programming at the request of the White House to participate in the Emergency Alert System." or similar would be heard over all stations, once their boxes were seized, in order to allow time for stations downstream to join the conference. Shortly after, POTUS would deliver a live statement.
Persons listening to a PEP station will hear perhaps several minutes of this repeated announcement. It is intended to be essentially program fill material while waiting for "talk-up" or an audio message from POTUS. If you are listening to a station whose equipment is not automated, or it does not have a reliable link to PEP, you may have a long wait before hearing the announcement. You may see or hear it on TV or cable before radio. If you are watching satellite TV, or listening to satellite radio, you may or may not receive any warning. Much will depend upon if the satellite channel has connectivity to a PEP or WHCA/FEMA feed of the presidential message, and the capability and desire to air it. In recent years, there has been no requirement they do so. However regulations continue to evolve as HDTV is implemented. Under EBS, networks such as HBO and CNN had more direct connectivity to WHCA. That was done away with in the 1990's. Had you attended the CNN tour in Atlanta, you may have passed Master Control. On your right the teleprinter link to WHCA, with the EAS/EBS Authenticator Word List - Red Envelope. The Word Lists also were done away with in the 1990's
The important designation is that the radio stations, especially PEP, will not likely announce anything. If the ENDECs are set on automatic, the feed from WHCA / POTUS / Mt. Weather will be heard on the ENDEC speaker and all programming will broadcast automatically, unless someone were to actually unplug the ENDEC. This was well illustrated in 1997, when an accidental conference was invoked. What should have been a closed circuit test instead caused several PEP stations around the country to be "locked-up" for a period of about 1 hour. The only way to unseize them would have required actually driving to the transmitter (typically a rural area) and disconnecting it.
In many cases, especially late night, were a POTUS message to be received via EAS at a radio station, there might not even be a human in the building, especially if the station were fully automated. The alert would go out automatically, even if a false or accident.
Sources:
A brief sample of the Conelrad Alert from 1961. http://conelrad.com/media/audiodirectory.php
A simplified description of "How would an EAN / EAT work?" http://www.eas-wa.info/q&a.htm