Full Message Breakdown
An EAS message contains these elements, in this transmitted sequence:
- Header.
- Attention signal: Sent if any message is included (normally sent with all messages except RWT on broadcast radio/TV); must be at least eight seconds long. On Weatheradio in Canada the 1050Hz tone is only used with three event codes: RMT, SVR & TOR.
- Single 1050 Hz audio tone for Weatheradio.
- Combined 853 and 960 Hz tones for broadcast 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. For those used to packet communications systems where each packet has a checksum, note that there is no checksum used in the message format. Each message is supposed to be transmitted 3 times, and the receiver is obliged to implement columnar parity correction.
The combined tones date back to 1963 when they were part of the EAS' predecessor, the Emergency Broadcast System.
Read more about this topic: Specific Area Message Encoding
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