NRSC-4-B SAME Standard Reference

Canonical source: NRSC-4-B United States RBDS Standard (2005), Section 4 — Specific Area Message Encoding (SAME). This document summarises the standard for implementers working with the EAS Station™ codebase.

1. Overview

SAME (Specific Area Message Encoding) is the audio-layer protocol used by the US Emergency Alert System (EAS) to convey machine-readable alert headers on broadcast and cable channels. NRSC-4-B (Section 4 / Annex D) formally standardises SAME for FM broadcasters, aligning it with the FCC EAS rules at 47 CFR Part 11.

Every EAS activation begins with a SAME header, followed by the audio attention signal and voice message, and ends with the End-of-Message (EOM) code. Decoders must detect and validate the header, respect the purge time, and forward the alert downstream.

2. Physical Layer (§4.3.1)

Parameter	Value
Modulation	Audio-frequency-shift keying (AFSK)
Mark frequency	2083.3 Hz (binary 1)
Space frequency	1562.5 Hz (binary 0)
Centre frequency	1822.9 Hz
Symbol (baud) rate	520.83 baud (= 25000/48 symbols/second)
Bit order	LSB first
Preamble	16 bytes of `0xAB` (10101011 repeated)
Encoding	NRZ (non-return-to-zero)

The 16-byte preamble provides bit-clock synchronisation. No start or stop bits are used; the bit stream is continuous within each burst.

3. Transmission Protocol — Triple-Burst (§4.3.2)

NRSC-4-B mandates that every SAME header, including the End-of-Message code, be transmitted three times separated by one-second pauses:

[Burst 1] — 1 s silence — [Burst 2] — 1 s silence — [Burst 3]

Each burst consists of:

16-byte preamble (0xAB × 16)
The SAME ASCII message string
A trailing carriage return (0x0D)

Purpose: Redundancy allows receivers to reconstruct a correct header even when one burst is partially corrupted by noise. EAS Station™ applies 2-of-3 majority voting across the three bursts (see Section 7).

4. Header Format (§4.3.3)

A SAME header is a printable-ASCII string with the following dash-delimited structure:

ZCZC-ORG-EEE-PSSCCC+TTTT-JJJHHMM-LLLLLLLL-

4.1 Field Definitions

Field	Width	Description
`ZCZC`	4	Start code — marks the beginning of the header
`ORG`	3	Originator code
`EEE`	3	Event code
`PSSCCC`	6 per location	Location code(s), separated by `-`, maximum 31 codes
`+TTTT`	4	Purge time in HHMM format (e.g. `0100` = 1 hour)
`JJJHHMM`	7	Issue time: day-of-year (`JJJ`) + UTC hour (`HH`) + minute (`MM`)
`LLLLLLLL`	1–8	Station identifier (call letters / source ID)

4.2 Start Code

ZCZC — ASCII characters 5A 43 5A 43. Receivers scan the bit stream after the preamble for this four-byte sequence to begin header parsing.

4.3 Originator Codes (§4.3.3.2)

Code	Source
`EAS`	EAS Participant (encoder/decoder device)
`CIV`	Civil Authorities (e.g. state/local emergency management)
`WXR`	National Weather Service
`PEP`	Primary Entry Point System (FEMA/IPAWS)

Only these four codes are valid per NRSC-4-B. Any other value indicates a non-compliant or malformed header.

4.4 Event Codes (§4.3.3.3)

Three-letter codes defined by the FCC (47 CFR §11.31(e)). Examples:

Code	Event
`EAN`	Emergency Action Notification (national)
`EAT`	Emergency Action Termination
`NPT`	National Periodic Test
`RWT`	Required Weekly Test
`TOR`	Tornado Warning
`SVR`	Severe Thunderstorm Warning
`FFW`	Flash Flood Warning
`TOE`	911 Telephone Outage Emergency

EAS Station™ maintains a full registry in app_utils/eas.py → EVENT_CODE_REGISTRY.

4.5 Location Codes (§4.3.3.3)

Each six-digit FIPS-based code identifies a geographic area:

P  SS  CCC

Digits	Field
`P`	Subdivision/priority digit
`SS`	State FIPS (two digits)
`CCC`	County FIPS (001–999; 000 = entire state)

Constraints:

Minimum 1, maximum 31 location codes per header.
Codes are separated by -.
P=0 = all or most of the county; higher values indicate sub-county areas (not widely used).

4.6 Purge Time (§4.3.3.4)

Four-character field TTTT in HHMM format:

Code	Duration
`0015`	15 minutes
`0030`	30 minutes
`0045`	45 minutes
`0100`	1 hour
`0130`	1 hour 30 minutes
`0200`	2 hours
`0230`	2 hours 30 minutes
`0300`	3 hours
`0330`	3 hours 30 minutes
`0400`	4 hours
`0430`	4 hours 30 minutes
`0500`	5 hours
`0530`	5 hours 30 minutes
`0600`	6 hours

Only these fourteen values are valid. EAS Station™ validates the received code against NRSC4B_VALID_PURGE_TIMES in app_utils/eas.py.

Common mistake: TTTT is not decimal minutes. 0100 means 1 hour, not 100 minutes.

4.7 Issue Time (§4.3.3.5)

Seven-digit UTC field JJJHHMM:

Sub-field	Digits	Range	Meaning
`JJJ`	3	001–366	Day of year
`HH`	2	00–23	Hour (UTC)
`MM`	2	00–59	Minute (UTC)

EAS Station™ computes a datetime object by anchoring JJJ to the current calendar year and adds the UTC hours/minutes.

4.8 Station Identifier (§4.3.3.6)

1–8 printable ASCII characters identifying the originating station or device. Typically the FCC call sign of the broadcast station or the identifier of the originating ENDEC. The field is terminated by a trailing - before the carriage return.

5. End-of-Message (EOM) Code (§4.3.4)

NNNN

Transmitted three times (triple-burst, same one-second gaps as the header) to signal the end of the alert audio. Each EOM burst consists of:

16-byte 0xAB preamble
The four ASCII characters NNNN
No trailing carriage return (or an optional one — receivers must accept both)

6. NRSC-4-B Compliance Flags

describe_same_header() in app_utils/eas.py evaluates and exposes the following per-header compliance flags:

Field	Meaning
`nrsc4b_compliant`	`True` if all five field-level checks pass
`nrsc4b_valid_originator`	Originator is one of `EAS/CIV/WXR/PEP`
`nrsc4b_valid_event`	Event code is in the registered code table
`nrsc4b_valid_purge`	Purge code is one of the 14 defined HHMM values
`nrsc4b_valid_issue_time`	Day-of-year 1–366, hour 0–23, minute 0–59
`nrsc4b_valid_location_count`	1 ≤ location count ≤ 31
`nrsc4b_valid_station_id`	Station identifier is 1–8 printable ASCII chars

7. Error Correction — 2-of-3 Majority Voting (§4.5)

Because the header is transmitted three times, receivers can reconstruct a clean header even when individual bursts are corrupted:

Burst 1:  ZCZC-WXR-TOR-0...
Burst 2:  ZCZC-WXR-TOR-0...  ← bit error at byte 12
Burst 3:  ZCZC-WXR-TOR-0...

Algorithm (implemented in app_utils/eas_decode.py → _vote_on_bytes()):

Align all three bursts byte-for-byte.
For each byte position, collect the values from each burst.
If 2 or 3 bursts agree → use the agreed value.
If all three differ (no majority) or only 2 bursts were received → use the most recently received burst's value (the last burst is preferred, per §4.5 guidance, because later transmissions reflect any in-flight correction by the originating ENDEC).

The decode result exposes:

Field	Type	Description
`burst_count`	`int`	Number of bursts detected (0–3)
`voting_applied`	`bool`	`True` when ≥ 2 bursts were compared

A burst_count < 3 indicates a degraded or partial transmission; decoders should log a warning but still forward the decoded alert.

8. Decoded Header Fields

describe_same_header() returns a dict with every field parsed from the header. Key fields:

Key	Type	Description
`start_code`	`str`	`"ZCZC"`
`originator`	`str`	Three-letter originator code
`originator_description`	`str`	Human-readable originator name
`nrsc4b_valid_originator`	`bool`	Originator is a valid NRSC-4-B code
`event_code`	`str`	Three-letter event code
`event_name`	`str`	Human-readable event name
`nrsc4b_valid_event`	`bool`	Event is registered in the code table
`location_count`	`int`	Number of location codes in the header
`location_count_valid`	`bool`	1 ≤ count ≤ 31
`locations`	`list[dict]`	Detailed breakdown of each location code
`raw_locations`	`list[str]`	Raw six-digit FIPS strings as received
`purge_code`	`str`	Four-character HHMM string
`purge_hh`	`int`	Hours component of the purge time
`purge_mm`	`int`	Minutes component of the purge time
`purge_minutes`	`int`	Total purge duration in minutes
`purge_label`	`str`	Human-readable duration (e.g. `"1 hour"`)
`nrsc4b_valid_purge`	`bool`	Purge code is one of the 14 defined values
`issue_code`	`str`	Seven-character `JJJHHMM` string
`issue_day_of_year`	`int`	Day of year (1–366)
`issue_hour`	`int`	UTC hour (0–23)
`issue_minute`	`int`	UTC minute (0–59)
`issue_time_iso`	`str`	ISO-8601 UTC datetime string
`issue_time_label`	`str`	Human-readable label (e.g. `"Day 042 at 14:30 UTC"`)
`issue_components`	`dict`	`{day_of_year, hour, minute}`
`nrsc4b_valid_issue_time`	`bool`	Issue time passes all range checks
`station_identifier`	`str`	Station ID (trimmed)
`station_identifier_raw`	`str`	Station ID as received (may have trailing dashes)
`nrsc4b_valid_station_id`	`bool`	ID is 1–8 chars
`header_length`	`int`	Total character length of the raw header string
`nrsc4b_compliant`	`bool`	Overall NRSC-4-B compliance

Each entry in locations is a dict with:

Key	Description
`code`	Six-digit string
`p_digit`	Subdivision digit (0–9)
`p_meaning`	Human-readable meaning of the P digit
`state_fips`	Two-digit state FIPS code
`state_name`	State name
`state_abbr`	State abbreviation
`county_fips`	Three-digit county FIPS code
`is_statewide`	`True` if `county_fips == "000"`
`description`	Human-readable area name (from FIPS lookup)

9. Constants (app_utils/eas.py)

Constant	Value	Description
`NRSC4B_PREAMBLE_BYTE`	`0xAB`	Preamble byte value
`NRSC4B_PREAMBLE_LENGTH`	`16`	Number of preamble bytes per burst
`NRSC4B_BURST_COUNT`	`3`	Mandatory number of bursts
`NRSC4B_BAUD_RATE_FRAC`	`'25000/48'`	Exact baud rate as fraction (≈ 520.83 baud)
`NRSC4B_MARK_FREQ_HZ`	`2083.3`	Mark (1) frequency in Hz
`NRSC4B_SPACE_FREQ_HZ`	`1562.5`	Space (0) frequency in Hz
`NRSC4B_CENTER_FREQ_HZ`	`1822.9`	Centre frequency in Hz
`NRSC4B_MAX_LOCATIONS`	`31`	Maximum number of location codes
`NRSC4B_STATION_ID_MAX_LEN`	`8`	Maximum station identifier length
`NRSC4B_VALID_PURGE_TIMES`	(14-tuple)	All valid HHMM purge codes
`NRSC4B_VALID_ORIGINATORS`	(4-tuple)	`EAS`, `CIV`, `WXR`, `PEP`

10. Relationship to Other Standards

Standard	Relationship
47 CFR Part 11	FCC EAS rules; NRSC-4-B implements these at the FM broadcast layer
ECIG EAS-ENDEC	Equipment manufacturer guideline; mostly consistent with NRSC-4-B
CAP / IPAWS	XML-based national alert aggregation; IPAWS OPEN converts CAP to SAME
NRSC-4-B §3	RBDS physical layer (57 kHz subcarrier); SAME lives on the audio baseband
EAS Station™ RBDS decoder	Decodes EWS Group 9A which carries RBDS-layer emergency warning data correlated with SAME

11. Implementation Notes

Where the code lives

Function / Class	File	Purpose
`describe_same_header()`	`app_utils/eas.py`	Parse and validate a SAME header string
`_vote_on_bytes()`	`app_utils/eas_decode.py`	2-of-3 majority voting
`SAMEAudioDecodeResult`	`app_utils/eas_decode.py`	Decode result with `burst_count` / `voting_applied`
`_bits_to_text()`	`app_utils/eas_decode.py`	Convert demodulated bits → header text
`EVENT_CODE_REGISTRY`	`app_utils/eas.py`	Complete map of EAS event codes
`NRSC4B_VALID_PURGE_TIMES`	`app_utils/eas.py`	The 14 valid purge codes
`NRSC4B_VALID_ORIGINATORS`	`app_utils/eas.py`	The 4 valid originator codes

Field Descriptions metadata

SAME_HEADER_FIELD_DESCRIPTIONS in app_utils/eas.py is a list of dicts that describe every SAME header field for UI rendering and API documentation. Each entry has:

key — the field name in the describe_same_header() output
label — human-readable label
description — extended description
nrsc4b_section — the NRSC-4-B section that governs this field (where applicable)
valid_values / max_count / max_length — validation metadata for UI hints

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This document is served from docs/reference/NRSC4B_SAME_STANDARD.md in the EAS Station™ installation.