Protocol Decoders, Part 8: The Legacy Family — EDACS, LTR & MPT-1327

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Part 8 of Protocol Decoders. Parts 2–7 built up the modern digital control channel — typed PDUs, strong FEC, an explicit grant message with a talkgroup and a frequency. This episode goes the other direction, to the three legacy families still on the air: EDACS, LTR and MPT-1327. They predate the “one clean digital CC” idea, and each solves the trunking problem with what the 1970s–80s had on hand. Understanding them is also a reminder for the Mercury hunt ahead — an unknown emitter might not be modern at all.

TL;DR: EDACS runs a continuous 9600-baud GFSK control channel of 40-bit Control Channel Words (CCWs), each protected by a shortened BCH(40,28,2). LTR has no central control channel at all — every repeater transmits a 41-bit status word at 300 bps under the voice, and the scanner follows calls by watching all of them. MPT-1327 uses 1200-baud FFSK carrying 64-bit codewords (38 info + 26 BCH check). All three land in the same engine through events.KindGrant, but the decode front-ends could hardly be more different.

Key takeaways

  • EDACS is the closest to modern: a dedicated fast digital CC with a per-word BCH and a clean Command opcode enum.
  • LTR is distributed trunking — there is no control channel; call-follow means reading every repeater’s sub-audible status word.
  • MPT-1327 is FFSK tone signalling: 64-bit codewords, a GoToChannel (GTC) grant, and BCH left to the caller.
  • All three converge on the same trunking.Grant the engine consumes — the legacy weirdness is quarantined in the decoders, not the engine.

Cheat sheet

System Control scheme Unit Grant message Where
EDACS continuous 9600-baud GFSK CC 40-bit CCW GroupVoiceGrant internal/radio/edacs/
LTR no central CC — per-repeater 41-bit status word @ 300 bps active Status (F-bit) internal/radio/ltr/
MPT-1327 continuous 1200-baud FFSK CC 64-bit codeword GoToChannel (GTC) internal/radio/mpt1327/

In this post

  • What “legacy trunking” means — three answers to one problem.
  • EDACS — the fast digital CC and the CCW.
  • LTR — distributed trunking with no control channel.
  • MPT-1327 — FFSK codewords and the GTC grant.
  • The common denominator — how all three feed one engine.

Three answers to one problem

Every trunked system answers the same question: when a user keys up, which frequency does everyone’s radio jump to? Modern digital systems (P25, DMR, NXDN, TETRA) answer it with a dedicated digital control channel streaming typed, FEC-protected PDUs — the world Parts 2–7 lived in. The legacy family answers it three different, older ways, and GopherTrunk keeps each one in its own package with the same overall shape: a wire parser, an opcode/field layer, a band-plan resolver, and a control.go state machine that emits cc.locked and events.KindGrant.

EDACS 9600-baud GFSK CC 40-bit CCW · BCH(40,28,2) LTR no central CC 41-bit status @ 300 bps / repeater MPT-1327 1200-baud FFSK CC 64-bit codeword · GTC grant trunking.Grant events.KindGrant → engine
Three unrelated control schemes, one convergence point: each decoder emits the same protocol-neutral grant the trunking engine already knows how to follow.

EDACS: the fast digital CC

EDACS (Enhanced Digital Access Communications System, the GE-Marc / Ericsson lineage) is the most “modern-feeling” of the three. It runs a continuous 9600-baud GFSK control channel over which 40-bit Control Channel Words flow, each prefaced by a 24-bit sync pattern and protected by a shortened BCH(40,28,2) code. A CCW packs four fields — {Command, Status, Address, LCN, Aux} — and the Command nibble is a clean opcode enum:

// internal/radio/edacs/opcodes.go (shape)
const (
    CmdIdle            Command = 0x0
    CmdGroupVoiceGrant Command = 0x1
    CmdProVoiceGrant   Command = 0x2 // EDACS ProVoice (digital voice)
    CmdDataGrant       Command = 0x4
    CmdSystemID        Command = 0x5
    CmdAdjacentSite    Command = 0x6
    // ...
)

A voice grant is a CCW whose command is CmdGroupVoiceGrant (or the digital CmdProVoiceGrant); AsGroupVoiceGrant lifts the talkgroup out of Address and resolves the LCN (Logical Channel Number) through the operator’s band plan to a frequency. The encrypted and emergency flags are the low two bits of the Status nibble. The BCH decode is an opt-in (SetBCHMode, wired to an edacs_bch_mode YAML key) because per the canonical open reference it’s the only on-wire FEC — an earlier package comment claiming an interleaved Reed-Solomon layer above the BCH was simply wrong, and the code says so. Honesty about what’s real matters more than a tidy-sounding claim; the same instinct runs through the whole codebase.

What’s not wired is honest too: EDACS ProVoice / Aegis digital voice uses a proprietary AMBE-derived vocoder GopherTrunk doesn’t decode, so a CmdProVoiceGrant gets you the grant metadata but not (yet) the audio.

LTR: trunking with no control channel

LTR (Logic Trunked Radio, E.F. Johnson, 1970s) is the outlier that makes the whole “control channel” abstraction wobble, because it doesn’t have one. Instead, every repeater in the system continuously transmits its own 41-bit status word at 300 bps, riding under the in-band voice. There is no central coordinator; a scanner follows a call by watching every repeater’s status word and tuning to whichever one currently announces the talkgroup of interest.

// internal/radio/ltr/status.go (shape)
type Status struct {
    Sync    bool
    Area    uint8  // 5-bit — disambiguates co-channel LTR systems
    Group   bool   // the "F-bit": 1 = active call for GroupID on this repeater
    Channel uint8  // 4-bit physical channel (1..20)
    Home    uint8  // 5-bit home-repeater number for the active group
    GroupID uint16 // 8-bit talkgroup (1..250)
    Free    uint8  // 5-bit free-repeater hint (for handoff)
    FCS     uint16 // 12-bit frame check
}

func (s Status) IsActive() bool { return s.Group && s.GroupID != 0 }

The IsActive test — F-bit set and a non-zero group — is the entire “is this a grant?” logic. When it’s true, the per-repeater state machine republishes the status as events.KindGrant; the first well-formed status from a repeater also fires a one-shot cc.locked so the hunter can confirm it’s tuned to a real LTR site. Because noise can occasionally pass a bare sync test, IsWellFormed gates on the fixed-range fields (channel 1..20, home 1..20) before the state machine trusts a word — a small but important guard when your “control channel” is a 41-bit sub-audible word with a 12-bit check.

How that principle shaped the Go code

LTR forces the same abstraction every other protocol uses even though it violates the abstraction’s premise. The engine only understands “a grant arrived on some frequency for some talkgroup.” LTR has no grant message — it has a state (this repeater is active for this group right now). The decoder’s job is to translate that state into the engine’s event vocabulary, and it does so without the engine ever learning that LTR is special. That’s the payoff of the publish-a-neutral-Grant contract from Part 1: a protocol with a completely alien topology bolts on as one more control.go, and the Trunking Engine is none the wiser.

MPT-1327: FFSK codewords

MPT-1327 (the 1988 UK Code of Practice, still running taxi, transport and utility fleets across Europe, Australia and beyond) sits between the two. It has a proper dedicated control channel like EDACS, but the physical layer is 1200-baud FFSK — audio-frequency-shift keying with 1200/1800 Hz mark and space, the same tone family as classic POCSAG. It carries 64-bit codewords back-to-back: 38 information bits plus a 26-bit BCH(63,38)-derived check folded into the unit.

The interesting decode detail is that the message type lives in the upper 4 bits of the 17-bit Function field, the spec’s “Address Categorisation” subfield:

// internal/radio/mpt1327/opcodes.go (shape)
const (
    KindAloha    CodewordKind = 0x1 // ALH  — control-channel idle
    KindAhoy     CodewordKind = 0x2 // AHY  — paging / inquiry
    KindAhoyChan CodewordKind = 0x3 // AHYC — broadcast / system info
    KindGoToChan CodewordKind = 0x4 // GTC  — voice grant ("go to channel")
)

func (c Codeword) Kind() CodewordKind { return CodewordKind((c.Function >> 13) & 0xF) }

The grant is a GTC (“Go To Channel”): AsGoToChannel reads the assigned channel number out of the lower 13 bits of Function and the called party from Prefix + Ident. The state machine locks on the first valid Aloha (the idle “I am a control channel” beacon) or AHYC broadcast, then republishes GTCs as events.KindGrant with Protocol="mpt1327". As with LTR, the demodulator (the 1200-baud FFSK front-end) and the BCH(63,38) correction are honest deferrals — the codeword parser assumes clean, error-corrected bits arrive from upstream.

message richness → LTR implicit F-bit state MPT-1327 GTC channel number EDACS CCW cmd + TG + LCN P25 / digital typed TSBK: TG+src+freq
Grant "richness" rises across eras: LTR conveys an implicit active state, MPT-1327 a bare channel number, EDACS a structured command, and modern digital a fully typed PDU with source and frequency.

The common denominator

For all their differences at the wire, the three legacy decoders converge exactly where the modern ones do: each emits a trunking.Grant on events.KindGrant, and each control.go follows the shared idiom — lock on the first credible frame, keep a band-plan Resolver to turn a channel/LCN into Hz, and gate on a strict-validation flag so a noisy word doesn’t spawn a phantom call. The engine that records the resulting call has no idea whether the grant came from a P25 TSBK, a TETRA D-CONNECT, an EDACS CCW, an LTR F-bit, or an MPT-1327 GTC. That is the entire point of decoupling the decoder from the engine, and it’s why adding a 40-year-old protocol is a decoder-package problem, never an engine problem.

Where this goes next

Part 9 leaves trunking behind entirely for the non-trunked world: conventional (fixed-frequency) decode, wideband Phase 2, DMR LCN mapping, and the symbol-scope diagnostic that lets you see a modulation before you trust a decode. For the standards themselves, the EDACS, LTR and MPT-1327 reference pages go deeper on each air interface.

FAQ

Why does LTR not have a control channel? Because it’s distributed trunking — an intentionally decentralised 1970s design where every repeater advertises its own state. Each transmits a 41-bit status word under the voice at 300 bps, and any radio (or scanner) reconstructs the system’s state by listening to all of them. There is no central coordinator to point a “control channel” at.

What FEC does EDACS use on the control channel? A shortened BCH(40,28,2) per 40-bit Control Channel Word — and, per the canonical open reference, that is the only on-wire FEC layer. GopherTrunk decodes it as an opt-in (edacs_bch_mode); there is no additional Reed-Solomon stage above it, despite older documentation to the contrary.

What is an MPT-1327 GTC? “Go To Channel” — the voice grant. It’s an address codeword whose Address Categorisation subfield is KindGoToChan, carrying the assigned channel number in the low 13 bits of the Function field. AsGoToChannel decodes it and the state machine republishes it as a trunking.Grant.

Do these legacy protocols decode voice in GopherTrunk? The trunking follow-along (control-channel decode, grants, retune) works; the voice side varies. Analog FM voice records fine, but EDACS ProVoice / Aegis digital voice uses a proprietary AMBE-derived vocoder that isn’t decoded — an honest deferral noted right in the package docs.

Series navigation

Part 8 of 12 · ← Part 7: TETRA · Next → Part 9: Conventional, Wideband & the Symbol Scope

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