What is DSC?

Digital Selective Calling

(DSC) is a facility that is intended to automate many of the procedures that most of us carry out by voice. The bit that gets people excited is that DSC can transmit the electronic equivalent of a distress call, automatically and complete with the boat's position. But DSC's not just for emergencies: it can also speed up and simplify ordinary boat-to-boat calls and remove some of the audio pollution that clogs Channel 16.

When DSC was first introduced to small craft VHF, back in the closing years of the twentieth century, DSC controllers tended to be separate boxes. Now, almost all new radios have a DSC controller built in.

Like the controls of the radios themselves, the controls, displays and menu systems of DSC controllers vary, but the general principal is that every radio has a unique 9 digit maritime mobile service identity (MMSI) number. To call another vessel, you tell your DSC controller that you want to make a routine call, the MMSI of the vessel you want to contact and the working channel you would like to use. It then uses Channel 70 to transmit that information in the form of a very short burst of digital code.

The DSC controller on board your target receives the message, recognises that it's being called and then sounds an alarm.

Assuming the recipient is prepared to take your call, he acknowledges it by pressing the apprpriate button on its DSC controller and both radios switch automatically to the working channel you selected. All the business of calling on Channel 16 and changing channels is removed: you just have to pick up yhe microphone and get on with your call.

The distress button provides a short cut into a special section of the DSC controllers menu, in which, instead of selecting who you want to call, you specify the nature of the distress situation. Assuming your DSC controller is connected to a GPS set, it then broadcasts a distress message, complete with your position and a one-word description of what is wrong, which will appear on the screen of every other DSC controller within range. Your GPS set requires an NMEA 0183 interface and connecting it to your radio is a simple DIY job.