RBDS Standard Reference (NRSC-4-B)

1. Overview

RBDS (Radio Broadcast Data System) is the North American adaptation of the European RDS (Radio Data System, IEC 62106 / EN 50067) standard. The relevant US specification is NRSC-4-B, published by the National Radio Systems Committee.

RBDS embeds digital metadata in FM broadcasts using a 57 kHz subcarrier that is phase-locked to the third harmonic of the 19 kHz stereo pilot. The data stream carries the station name, program type, radio text, traffic announcements, clock time, alternative frequencies, and — critically for EAS stations — Emergency Warning System (EWS) alerts.

2. Physical Layer

Error Protection

Each 26-bit block contains:

• 16 bits of data
• 10 bits of CRC (generator polynomial: x¹⁰ + x⁸ + x⁷ + x⁵ + x⁴ + x³ + 1)
• A unique offset word (A, B, C, C', D) XORed into the syndrome to identify block position

Correct syndrome values for each block:

3. Data Model

PI Code (Program Identification)

The 16-bit PI code uniquely identifies the station. Block A of every group carries the PI code.

US call sign decoding (NRSC-4 Annex D):

• Range 0x1000–0x54A7: K-prefix stations (algorithmic)
• Range 0x54A8–0x994F: W-prefix stations (algorithmic)
• Range 0x9950–0x99B9: Legacy 3-letter calls (lookup table)

The EAS Station™ function pi_to_call_sign() in app_core/radio/demodulation.py implements this lookup.

Group Structure

Block A (bits 15-0): PI code
Block B (bits 15-0):
  bits 15-12: Group type (0-15)
  bit  11:    Version (0=A, 1=B)
  bit  10:    TP (Traffic Program)
  bits  9-5:  PTY (Program Type, 0-31)
  bits  4-0:  Group-type-dependent payload
Block C (bits 15-0): Group-type-dependent
Block D (bits 15-0): Group-type-dependent

4. Group Types Table

5. Decoded Group Details

Group 0A/0B — Program Service (PS) Name, Flags, Alternative Frequencies

Block B low bits:

• Bit 4: TA (Traffic Announcement)
• Bit 3: MS (Music/Speech)
• Bit 2: DI bit (Decoder Identification, address-dependent)
• Bits 1-0: Address (segment 0-3, selects 2 chars of PS and which DI bit)

Block C (0A only): Two AF codes (Method A, direct frequency encoding)

• Code 1-204: 87.6 + 0.1 × code MHz
• Code 224-249: Method A count (next code = first AF, following = list)
• Code 250: Follow-on frequency indicator
• Code 205: Filler / not used

Block D: Two PS name characters at position address × 2 and address × 2 + 1

Decoder Identification (DI) bits by address:

Group 1A/1B — Programme Item Number + Slow Labeling Codes

Block C: Programme Item Number (PIN)

• Bits 15-11: Day of month (0=no PIN, 1-31)
• Bits 10-6: Hour (0-23)
• Bits 5-0: Minute (0-59)

Block B bits 4-2: Variant selector (0-7) for Block D interpretation:

Group 2A/2B — RadioText

Block B bits 4-0:

• Bit 4: A/B flag (toggle clears RT buffer)
• Bits 3-0: Text segment address (0-15)

2A: 4 chars per segment (Blocks C and D), max 64 chars
2B: 2 chars per segment (Block D only), max 32 chars

Carriage return (0x0D) terminates the displayed text.

Group 3A — ODA Application Identification

Registers an Open Data Application (ODA) for a specific group type.

• Block B bits 4-1: ODA group type (0-15)
• Block B bit 0: ODA version (A or B)
• Block C: 16-bit Application ID (AID)
• Block D: Application-specific message

Common AIDs:

• 0x4BD7: RDS-TMC (Traffic Message Channel)
• 0xCD46: RT+ (Radio Text Plus, song title/artist tagging)

Group 4A — Clock Time and Date

• Block B bits 1-0 + Block C bits 15-1: MJD (Modified Julian Day, 17 bits)
• Block C bit 0: Hour MSB
• Block D bits 15-12: Hour LSBs
• Block D bits 11-6: Minute (0-59)
• Block D bit 5: Local offset sign (0=+, 1=-)
• Block D bits 4-0: Local offset in half-hours

MJD to calendar conversion uses the algorithm from EN 50067 Annex G.

Group 5A/5B — Transparent Data Channel (TDC)

Carries arbitrary byte streams on up to 32 channels.

• Block B bits 4-0: Channel address (0-31)
• 5A: 4 bytes from Blocks C and D
• 5B: 2 bytes from Block D only

Group 6A/6B — In-House Applications

Proprietary data for use by the broadcaster's own equipment.

• Block B bits 4-0: In-house application data (5 bits)
• 6A: Blocks C and D carry additional 32 bits
• 6B: Block D carries 16 bits (Block C = PI of other network)

Group 7A — Radio Paging

Used for alphanumeric paging services. EAS Station™ logs these groups at DEBUG level but does not decode paging data.

Group 8A — Traffic Message Channel (TMC / ALERT-C)

ALERT-C encoded traffic messages. EAS Station™ flags the presence of TMC (tmc_present=True) but does not implement full ALERT-C decoding, which requires a location database.

Group 9A — Emergency Warning System (EWS) ⚠️

This is the most important group for EAS Station™ operation.

EWS is the RBDS mechanism for broadcasting emergency alerts directly in the FM signal, complementing SAME/EAS audio alerts.

• Block B bits 4-0: EWS channel number (0-31)
• Block C: EWS message word 1 (16 bits, broadcaster-defined format)
• Block D: EWS message word 2 (16 bits, broadcaster-defined format)

EWS channel assignments and message formats are defined by the Emergency Alert System coordinator for each country/region. See Section 10 for the relationship to SAME/EAS.

Group 10A/10B — Program Type Name (PTYN)

Provides an 8-character name further describing the PTY.

10A: 4 chars per segment (2 segments: 0 and 1)

• Block B bit 4: A/B flag
• Block B bit 0: Segment (0 or 1)
• Blocks C, D: 4 characters

10B: 2 chars per segment

• Block B bit 4: A/B flag
• Block B bit 0: Segment (0 or 1)
• Block D only: 2 characters

Group 14A/14B — Enhanced Other Networks (EON)

Carries information about other stations in a network for cross-network TA/TP linking.

14A (slow) — variant in Block B bits 3-0:

Block B bit 4: TP of the other network
Block C: PI code of the other network

14B (fast) — for rapid TA updates:

• Block B bit 4: TP of other network
• Block B bit 3: TA of other network
• Block C: PI code of other network

Group 15B — Fast Switching Information

Repeats critical flags from Group 0A at higher frequency for fast TA/TP response.

• Block B bit 4: TA
• Block B bit 3: MS
• Block B bit 2: DI bit (same address scheme as Group 0)
• Block B bits 1-0: Address
• Block D: PS name chars (same as Group 0A Block D)
• TP comes from Block B bit 10 (same as all groups)

6. Programme Type Codes (PTY)

7. Language Codes (Annex J)

Language codes are carried in Group 1A/1B, variant 0, Block D bits 7-0.

See RBDS_LANGUAGE_CODES in app_core/radio/demodulation.py for the complete table.

8. PI Code Structure

For US stations the 16-bit PI code encodes the call sign:

• Algorithmic K stations (PI 0x1000–0x54A7): PI - 0x1000 maps to base-26 encoding of letters A-Z for positions 1-3 of the call sign.
• Algorithmic W stations (PI 0x54A8–0x994F): PI - 0x54A8 maps similarly.
• Legacy 3-letter calls (PI 0x9950–0x99B9): Direct lookup table per NRSC-4 Annex D.3.

PI codes outside the US ranges (< 0x1000 or 0x9A00–0xFFFF) are European/other-country codes and are displayed as hex without a call sign.

9. EWS Integration with EAS

Group 9A carries the Emergency Warning System data, which is the in-band RBDS complement to EAS/SAME audio alerts.

Relationship to SAME:

• SAME (Specific Area Message Encoding) is the audio-layer protocol used by EAS. It consists of a digital header broadcast as AFSK before the alert audio.
• EWS Group 9A carries the same alert information in the RBDS data stream, allowing receivers without audio-path monitoring to detect and decode alerts purely from the FM subcarrier.
• The EWS channel number (bits 4-0 of Block B) identifies which of up to 32 alert channels is active.
• Blocks C and D carry two 16-bit words whose format is defined by the alert coordinator (often mirroring key SAME fields like event code and FIPS location).

EAS Station™ handling:

• When Group 9A is received, ews_channel, ews_message_c, and ews_message_d are populated in RBDSData.
• The audio monitoring UI displays an alert-style panel (red background) when EWS data is present.
• The fields are propagated through metadata['rbds_ews_channel'] etc. for use by monitoring logic.

10. Alternative Frequencies (Method A)

When a broadcaster transmits on multiple frequencies (e.g., translators, network affiliates), Group 0A Block C carries alternative frequency codes.

Method A encoding:

• Each byte is an AF code.
• Codes 1–204: Direct frequency. freq_MHz = 87.6 + 0.1 × code
• Code 224–249: Count byte. Value = number of following AF codes. The next code is the first AF.
• Code 205: Filler (ignore).
• Codes 250–255: Special (follow-on, regional variant, etc.)

EAS Station™ implements the simple case (direct codes 1-204 only). Count bytes (224-249) and special codes are skipped. The decoded frequencies are available as rbds_af_list in the metadata.

11. How EAS Station™ Implements This

Where the code lives

Decoded fields and metadata keys

Running the tests

python -m pytest tests/test_rbds_demodulation.py -q

All RBDS decoder tests are in tests/test_rbds_demodulation.py.