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Specific Area Message Encoding

Specific Area Message Encoding or SAME is the protocol used to encode the Emergency Alert System in the U.S. for broadcast stations. It was originally created for NOAA Weather Radio by the National Weather Service, and was later adopted by the FCC for regular broadcasters on radio, television, and cable, as well as by Environment Canada for its weather radio service.

Functionality

The system transmits digital tones over normal audio using AFSK, with a 2083.3 Hz mark tone and 1562.5 Hz space tone, lasting 1920 μs (1.92 ms) each. The data is encoded in 7-bit ASCII but uses all 8 bits, with no parity bit and no stop bit ("8-N-0"), at a bitrate of 520.83 bits per second. The least significant bit of each byte is transmitted first, including the preamble. The tones are multiples of the bitrate.

In the SAME system, messages are constructed in four parts, the first and last of which are digital. The first part is the AFSK-encoded header message in 7-bit ASCII text. It is repeated three times, so that decoders can pick "best two out of three" for each byte, thereby eliminating most errors which can cause an activation to fail. The text of the header code contains:

A preamble of binary 10101011 (0xAB in hex) repeated sixteen times, used for "receiver calibration" (i.e., clock synchronization), then the letters ZCZC as an attention to the decoder
Information about the originator of the alert:

EAN - national authorities, including the President (Emergency Action Notification Network)
PEP - national authorities, including the President (Primary Entry Point System)
CIV - civil authorities, such as president, governor or local city/county emergency management agency
WXR - National Weather Service weatheradio
EAS - broadcasters, such as radio & TV stations, or cable TV companies

Event type (see below)
Areas affected (up to 32), designated by FIPS state code and indicating the county (parish in Louisiana, borough or census area in Alaska), but which may be designated for the whole state by using county number 000.
Duration of alert, normally in increments of 15 minutes from time of issue
Exact time of issue, in UTC, without time zone adjustments
Eight-character station callsign identification, with / used instead of - .

Full Message Breakdown

An EAS message contains these elements, in this transmitted sequence:

Header: (Preamble) ZCZC-org-eee-pssccc-pssccc+tttt-jjjhhmm-llllllll
Attention signal (audio tone for Weatheradio, combined tones for commercial radio/TV)
Message - audio, video image or video text
Tail: (Preamble) NNNN (EOM)

There is one second of blank audio between each section, and before and after each message.

(Preamble): Signal that clears and resets the receiving system; factory-programmed
ZCZC: Start of ASCII code; factory-programmed
org: Originator code; programmed per unit when put into operation
eee: Event code; programmed at time of event
pssccc: County-location codes (up to 32 locations); programmed at time of event

ss is the state or equivalent water area code
ccc is the county/borough/parish, independent city, or equivalent (000 for entire state)
p is the part of the county, or 0 for the entire county

tttt: Duration the alert is valid (hhmm format); programmed at time of event
jjjhhmm: Date and Time-of-Day; encoded automatically, based on settings

jjj is the Julian date day of the year, in UTC, with leading zeros
hhmm is 24-hour hours and minutes, in UTC, with leading zeros

llllllll: Eight-Character ID; programmed per unit when put into operation

broadcast station callsign, such as WXXX/FM
NWS location, such as KDEN/NWS for Denver, Colorado

Attention signal: Sent if any message is included (normally sent with all messages except RWT); must be at least eight seconds long
NNNN: End-of-message code (EOM)

Event codes

There are about 80 different event codes currently used in EAS. Originally, all but the first six of these were optional and could be programmed into encoder/decoder units at the request of the broadcaster. However, a July 12, 2007 memo by the FCC now requires mandatory participation in state and local level EAS.

Mandatory codes

EAN: Emergency Action Notification (Activates the EAS nationwide)
EAT: Emergency Action Termination (Ends national activation)
NIC: National Information Center statement (Used to distribute National level information after EAT has been sent)
RMT: Required Monthly Test
RWT: Required Weekly Test
NPT: National Periodic Test