Field Guide · term

Also known as: DMR sync words, DMR sync patterns, BS-Voice sync

The DMR sync patterns are the nine fixed 48-bit words that open the sync field of every DMR burst, telling the decoder both where a burst begins and what role it plays.1 Before a receiver can read a slot type or a payload it must lock onto one of these words in the demodulated dibit stream — exactly the job of frame synchronization.2 The nine words are defined in ETSI TS 102 361-1 §9.1.1 and split by source (Base Station, Mobile Station, Direct Mode) and traffic type (voice, data, reverse channel).

payload half · 98 b SYNC · 48 bits payload half · 98 b BS-Voice 0x755FD7DF75F7 XOR 0xAAAAAAAAAAAA ↓ BS-Data 0xDFF57D75DF5D polarity flip = dibit + 2 mod 4
Each burst is framed by a central 48-bit sync word; the nine words are closed under the discriminator-polarity flip, so a spectrum-inverted data sync arrives byte-identical to a clean voice sync.

The nine sync words

Each word is 48 bits, expressed as 24 dibits MSB-first. GopherTrunk stores them in internal/radio/dmr/sync.go as the package-level AllSyncs slice, in the stable order below.

Name Hex (low 48 bits) Role
BS-Voice 0x755FD7DF75F7 Base station outbound, voice
BS-Data 0xDFF57D75DF5D Base station outbound, data / control
MS-Voice 0x7F7D5DD57DFD Mobile station, voice
MS-Data 0xD5D7F77FD757 Mobile station, data
MS-RC 0x77D55F7DFD77 Mobile station, reverse channel
DM-Voice-TS1 0x5D577F7757FF Direct mode voice, slot 1
DM-Voice-TS2 0x7DFFD5F55D5F Direct mode voice, slot 2
DM-Data-TS1 0xF7FDD5DDFD55 Direct mode data, slot 1
DM-Data-TS2 0xD7557F5FF7F5 Direct mode data, slot 2

A SyncDetector slides a 24-dibit window across the stream and, at each position, counts dibit mismatches against every pattern; a hit is declared where the best match falls within the configured tolerance (default 4 of 24). The words are chosen for sharp autocorrelation, so a genuine alignment stands clear of noise even a few dB into the channel.

The polarity-flip closure

The most consequential property of the set is that it is closed under a whole-alphabet polarity flip. A conjugated-I/Q or spectrum-inverted front end negates the FM-discriminator output, mapping +3 ↔ −3 and +1 ↔ −1 — in dibit space, adding 2 mod 4 to every dibit, which is exactly XOR-ing the 48-bit word with 0xAAAAAAAAAAAA. Apply that to BS-Voice (0x755FD7DF75F7) and the result is 0xDFF57D75DF5D — BS-Data, bit-for-bit. Every voice word maps to its data twin and back, the flip being its own inverse (2 + 2 ≡ 0 mod 4).

The practical consequence, recorded as GopherTrunk issue #264 (the RTL-SDR Blog V4 / R828D path), is that a sync match alone cannot resolve polarity: an inverted data burst fires the detector as a clean voice sync and vice versa. GopherTrunk therefore treats sync detection as polarity-agnostic and resolves the ambiguity downstream — the Tier II / III Process adapters hand each burst to the decoder at both candidate polarities (CandidatePolarities = {0, 2}, RotateBurstDibits), and the slot-type Hamming(20,8), BPTC(196,96), and CSBK CRC together act as the arbiter, dropping the wrong polarity with no state change.

Relevance to SDR

The sync patterns are the front door of GopherTrunk’s DMR decoder. internal/radio/dmr/sync.go holds the canonical dibit forms, the AllSyncs ordering callers index against, and the SyncDetector that reports each hit’s stream position and matched pattern. Voice decoding uses only the four voice syncs to anchor burst A of a superframe, then locates bursts B–F by TDMA cadence; control decoding slices a burst on every BS-Data / MS-Data hit. Getting both the 24-dibit patterns and the polarity closure right is what lets everything downstream decode a real, possibly inverted, off-air DMR signal.

Sources

  1. Digital mobile radio — Wikipedia, on the DMR standard whose bursts these sync words frame. ↩

  2. Frame synchronization — Wikipedia, on locating frame boundaries with a fixed sync sequence. ↩

See also