Module 4 - Channels & Frequencies
Channel 70
What you will learn
Separate DSC signalling on Channel 70 from the voice channel used for the follow-up communication.
Channel 70 carries Digital Selective Calling signals, not speech. A DSC alert or call establishes the digital contact; the required voice traffic follows on Channel 16 or an accepted working channel.
A DSC set uses Channel 70 to send and receive digital calls identified by MMSI. The digital call can carry a priority and, for some call types, propose a channel for the radiotelephony exchange. Speaking into the microphone while Channel 70 is selected does not turn that channel into a voice route.
For distress, MCA guidance pairs the DSC distress alert with the voice distress call and message on Channel 16 after acknowledgement or after about fifteen seconds. For a known-MMSI routine inter-ship call, the digital exchange proposes an inter-ship channel and the conversation starts there after acknowledgement.
Urgency and safety routes have their own digital-to-voice sequence. Do not reduce every DSC call to 'send on 70, speak on 16'; read the accepted or instructed channel and follow the procedure for the actual priority.
Follow a routine DSC call into voice
You know another vessel's MMSI and want to discuss a routine meeting point.
1. Select
Make an Individual DSC call with routine priority and propose an appropriate inter-ship channel.
2. Wait
Let the called radio receive and acknowledge the digital call; do not try to speak on Channel 70.
3. Talk
Use the accepted inter-ship channel for the brief voice conversation, then restore the required watch.
Sense check: The display should show the accepted voice channel before the microphone is used.
Common mistake or limitation
- Trying to conduct the follow-up voice exchange on Channel 70.
- Assuming every DSC call is followed by voice on Channel 16 instead of reading the accepted working channel.
- Treating the digital alert as a replacement for the required distress voice message.
Recap
- Channel 70 is for DSC digital signalling only.
- Distress voice follow-up is on Channel 16.
- Routine, urgency and safety traffic may move to an accepted or nominated working channel.
Optional quick check
Section 2 of 9
A routine DSC call is acknowledged with an inter-ship channel accepted. Where should the voice conversation begin?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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