Also known as: MDC-1200, MDC1200
MDC-1200 (Motorola Data Communications) is an in-band signalling system that sends a short 1200 bps FFSK data burst over an otherwise analog FM voice channel. It conveys a transmitting radio’s unit ID (PTT ID / ANI), plus emergency and status signalling.12 “In-band” means the data rides in the same audio passband as speech, so no separate data channel or digital modulation is needed — the radio simply prepends (and optionally appends) a brief data tone-burst to an ordinary FM transmission.
Overview
When a user keys up, an MDC-equipped radio can send a brief data burst at the start (and/or end) of the transmission identifying the unit — giving an analog system some of the radio-ID visibility that digital systems such as DMR and P25 carry natively. The most common use is PTT-ID / ANI (automatic number identification): the dispatcher’s console displays which radio is talking. Beyond simple ID, MDC-1200 supports emergency signalling (a distress button that flags the unit at the console), Stat-Alert status messages, remote radio check, and remote inhibit/monitor functions, all encoded as different message types in the same burst format.
Technical characteristics
| Property | Value |
|---|---|
| Modulation | 1200 bps FFSK (AFSK: 1200/1800 Hz tones) |
| Channel | Rides over analog FM voice (in-band) |
| Carries | Unit ID (ANI/PTT-ID), emergency, status, radio check |
| Burst | Sync + framed data words |
| Error control | Parity and repetition of the data words |
How it works
MDC-1200 is a form of FFSK — fast frequency-shift keying — which is simply audio FSK clocked so that each bit is a whole number of tone cycles, keeping the burst short and the tone phase continuous. Two audio tones (nominally 1200 Hz and 1800 Hz) encode the bit values, transmitted through the radio’s normal FM audio path just like an AFSK packet tone. A burst begins with a synchronisation pattern the receiver correlates against to find bit timing, followed by data words carrying the message type, the unit’s ID, and target/argument fields. Because there is no return channel and the burst is very short, robustness comes from repeating the data words and checking parity rather than from a heavy error-correcting code — a decoder accepts an ID only when the repeats agree.
History
Motorola developed MDC-1200 for conventional and trunked analog systems as a lightweight data-signalling layer, decades before affordable digital land-mobile radio.1 It became a de-facto standard fleet feature on Motorola conventional channels, widely deployed by public-safety and business users who wanted unit identification and an emergency button without moving to a digital system. Many of those channels remain in service, so MDC bursts are still common on VHF/UHF land-mobile frequencies.
Deployment
MDC-1200 is used on analog FM land-mobile systems that need unit identification and emergency/status signalling — public-safety conventional channels, utilities, transportation, and business fleets. It frequently appears alongside CTCSS/DCS sub-audible squelch, since MDC handles identity while the tone squelch handles channel access.
Decoding it with GopherTrunk
MDC-1200 is in scope for GopherTrunk. On an analog FM channel the decoder detects the FFSK burst at the head (and tail) of a transmission, correlates the sync, recovers the 1200 bps tones, and reads the message type and unit ID after confirming the repeated data words agree. There is no encryption or vocoder involved, so the ID and emergency/ status flags decode cleanly. See the MDC1200 decoder page.