This page will serve as a gathering place for information relating to the MDC1200 Signalling Protocol.
General specifications of MDC1200 are as follows:
|Modulation||1200 baud, 1200 and 1800 Hz carriers,
MSK 1% MER on frequency, no fading; 12 dB SINAD
|Noise Fading||less than one per ten years|
|Cross Code Falsing||one false in one million transmissions|
|Data Operated Squelch Falsing||one false per fifteen minutes of received audio|
AKA Stat-Alert, it is a 1200 bit per second digital signalling method which has been specifically designed for high reliability in the land-mobile radio environment.
A Stat-Alert message packet contains 32 information bits. Of these bits, 16 are addresses used to identify particular individuals. Current products support over 13,000 possible different addresses in a fleet. Of the 16 remaining bits, 8 are used to define the particular function of the packet, ie. PTT ID, CallAlert, etc. The remaining 8 bits are used to transfer optional information. To the basic 32 bit information message, additional bits are appended for error detection. These error detection bits are mathematically derived from the information bits, and if received incorrectly, will cause the entire message to be rejected. This provides a very high degree of message falsing protection. Further capability is added to provide burst error correction.
Several message packets may be combined to provide higher tier features. Simple PTT ID or Radio Check requires only one packet. Selective Calling requires two, since the ID of the destination as well as the source are both transferred over the air.
The entire amount of time required to transmit a PTT ID message is 173mS.
A long quest to figure out the inner workings of MDC-1200 was helped by the US Patent and Trademark Office online search.
The following patents seem to relate to signaling and features of MDC-600. MDC-1200 seems to be similar in protocol although the modulation is 1200 baud MSK rather than 600 baud BPSK.
US Patents 4,457,005 4,517,561 4,590,473 4,517,669 contains information about the word synchronization sequence.
The book "An Introduction to Error Correcting Codes" by Shu Lin, Prentice Hall, 1970, is listed as a reference in these patents and was very useful in determining the convolutional encoding for error correction and other aspects of the data encoding. Many other Motorola patents refer to this text.
The key to picking apart MDC-1200 was seeing that the modulated signal did not make sense as regular MSK encoding. There was no obvious alternating 1-0-1-0... bit synchronization pattern at the beginning of the data stream. For some reason, MDC-1200 exclusive-ors successive data bits just before they go to the modulator. The modulated tones then only carry the information of "this bit is different from the previous bit" or "this bit is the same as the previous bit". Thus the alternating bit synchronization pattern is transmitted as a constant tone. This XOR scheme seems to remove some information from the signal. For example, if there is a single bit receive error somewhere in the message, the recovered polarity of all successive bits is the opposite of what was intended. It is not obvious yet how an MDC-1200 decoder can recover from this.
Once the reverse XOR process is done on the received bits, the word synchronization pattern can be seen and the data and CRC can be found interleaved with the error correction bits.