MDC signaling includes a number of features: unit ID, status buttons, emergency button, selective inhibit, radio check, and selective calling. These features are programmable and could be used in any combination desired by the user. They are typically incorporated in high-end analog FMFrequency Modulation commercial and public safety radios made by Motorola and other manufacturers. In addition to Motorola, at least two other companies make compatible base station decoders for MDC-1200.
Push to talk/Unit ID
Many MDC-1200 systems utilize the unit ID option. With each push-to-talk press, the radio sends a data burst identifying the sending radio. Unit IDs are decoded as unique hexadecimal four-digit numbers. Every radio would have a unique four-digit ID, (for example: 0423 or 5990).
Unit ID can be sent as leading or trailing a voice message. In the leading option, the data burst is sent at the moment a user presses the radio's push-to-talk button. An option can be set to make the radio's speaker emit a tone for the length of the unit ID data, (about 1-1.5 seconds). This reminds a user to wait until the data has been sent before talking. The leading unit ID takes slightly more air time (is longer) than a trailing ID because of a header tone and the need to delay the data burst to allow time for CTCSS decoders and voting comparators to open an audio path to the decoder. A default delay is defined with the unit ID option. To adjust for time delay variations in each individual system, radios can be programmed to delay the sending of a radio's unit ID data by up to hundreds of milliseconds within a range. In the trailing option, the data packet is sent at the moment the microphone button is released. This avoids timing issues because the audio path to the base station is already open.
(audio files from wikipedia): https://en.wikipedia.org/wiki/File:Trucking_w_leading_PTT_ID_using_MDC1200.ogg <-----Push to talk with data muting, taken from tts software.
https://en.wikipedia.org/wiki/File:TRucking_leading_PTT_ID_MDC1200_non-Motorola_recvr.ogg <----- Push to talk without data muting, same TTS.
- Hobby Level Software