---
layout: page
title: MDC1200 / Motorola Signaling
description: Decoded Motorola MDC1200 FFSK pipeline — DSP frontend, events bus, SQLite log, REST endpoint, web panel
nav_group: Reference
---

# MDC1200 / Motorola Signaling

GopherTrunk decodes **MDC1200** ("Motorola Data Communications") —
the analog in-band data burst Motorola two-way radios key at the
start (and optionally the end) of a transmission. It carries the
radio's **unit ID** (ANI — automatic number identification) plus
emergency, status, call-alert, radio-check and selective-call
signaling on otherwise-analog conventional VHF / UHF voice
channels. On a system that is just FM voice, MDC1200 is what tells
you *which* radio is talking and surfaces emergency / status
events.

Added in response to [#438](https://github.com/MattCheramie/GopherTrunk/issues/438).

## Modulation

MDC1200 is a **1200-baud FFSK** burst using the CCIR tones
**mark = 1200 Hz** (binary 1) and **space = 1800 Hz** (binary 0),
carried inside the narrowband-FM voice channel — the same
modulation class GopherTrunk already demodulates for MPT 1327, so
the DSP frontend reuses `internal/dsp/demod.FFSK`. Unlike APRS the
line code is plain **NRZ** (no NRZI differential decode).

## Pipeline

```
IQ chunks (Fs Hz, complex64)
  → FM demod (internal/dsp/demod.FM)
  → real resampler to 9600 Hz (1200 baud × 8 oversample)
  → FFSK tone discriminator (mark 1200 Hz / space 1800 Hz)
  → Mueller-Müller symbol-timing recovery (8 sps → 1 sample/symbol)
  → DC-tracking NRZ slicer
  → 40-bit sync framer (internal/radio/mdc1200/receiver)
  → op/arg/unit-ID parse + CRC-16 check (internal/radio/mdc1200)
  → events.KindMDC1200Message on the bus
  → storage.MDC1200Log → mdc1200_log SQLite table
  → GET /api/v1/mdc1200/messages → /mdc1200 web panel
```

The frame layout, after the 40-bit sync word `0x07 09 2A 44 6F`
(most-significant bit first), is 112 payload bits column-interleaved
over a 16×7 grid. De-interleaved and packed LSB-first, the header
bytes are:

| Bytes      | Meaning                                            |
|------------|----------------------------------------------------|
| `data[0]`  | op (operation code)                                |
| `data[1]`  | arg (operation argument)                           |
| `data[2:4]`| unit ID (big-endian)                               |
| `data[4:6]`| CRC-16 of `data[0:4]` (little-endian on the wire)  |
| `data[6:]` | redundancy (over-the-air FEC; not yet exploited)   |

The CRC is CRC-16/CCITT with reflected in/out, polynomial 0x1021,
initial value 0x0000 and final XOR 0xFFFF. The sync hunt tolerates
a few bit errors and accepts the bit-complemented sync word so a
flipped FM discriminator (inverted tone sense) still decodes.

## Operations

The decoder resolves a human label for the common Motorola CPS
opcodes (PTT ID / ANI, emergency, status, radio check, call alert /
page, selective call, radio inhibit / enable, remote monitor). The
op/arg table is best-effort and intentionally non-exhaustive — many
vendor-specific and extended opcodes exist; unrecognised pairs
surface the raw op/arg so nothing is silently dropped. The unit ID
and CRC are always decoded regardless of the label.

**Double packets** (extended two-block messages, op `0x35` / `0x55`)
are framed as two consecutive 112-bit blocks; the second block's
header bytes are attached but its vendor-specific payload
interpretation is left to a follow-up.

## Configuration

Each entry pins one SDR to a conventional analog voice channel:

```yaml
mdc1200:
  channels:
    - serial: "vhf-antenna"
      frequency_hz: 154_000_000   # the analog voice channel to monitor
      drop_bad_crc: false         # true to drop CRC-failed bursts
```

Leave `drop_bad_crc` false to see CRC-failed bursts on the panel
(flagged with `crc_ok=false` and dimmed); flip it on for noisy
channels.

## What's surfaced

- **Bus event** — `events.KindMDC1200Message`, payload
  `storage.MDC1200Message`.
- **Storage** — the `mdc1200_log` SQLite table (op, arg, unit ID,
  operation, body, raw hex, crc_ok), indexed by time and unit ID.
- **REST** — `GET /api/v1/mdc1200/messages?limit=N` (default 200,
  max 5000); 503 when the daemon runs without `storage.path`.
- **Web** — the `/mdc1200` panel polls every 5 s, tinting emergency
  bursts red and dimming CRC failures.

## Licensing note

The MDC1200 protocol facts implemented here (sync word, interleave
geometry, CRC parameters, opcode semantics) are public protocol
details. This is a clean-room Go implementation; no third-party
decoder source — including the GPL-licensed reference libraries —
is incorporated, keeping the decoder under the project's Apache-2.0
license.