Module 8 - Digital Selective Calling
What DSC Does
What you will learn
Explain what a VHF DSC call sends, what remains for voice, and why Channel 70 is never a voice channel.
Digital Selective Calling is an alerting and call-establishment layer. It identifies stations and communicates a call type or priority before the necessary voice traffic continues elsewhere.
A VHF DSC set transmits digital signalling on Channel 70. Depending on the call, that digital packet can identify the caller and addressee, state a priority, propose a working channel and include position and UTC data. The alert makes the right set ring; it does not carry the full spoken situation, assistance request or conversation.
After a routine individual call is acknowledged, voice begins on the accepted working channel. After a distress alert, the vessel sends the Mayday call and message on Channel 16 on receipt of a DSC acknowledgement or after about fifteen seconds. Urgency and safety use their own published follow-up routes. No part of those voice messages belongs on Channel 70.
DSC reduces the need to call repeatedly by voice, but it does not guarantee reception or response. Range, aerial, power, correct MMSI, current position input, chosen call type and the receiving station's equipment all still matter. Keep the required listening watch and be ready to use the correct voice procedure.
| Layer | Carries | Does not replace |
|---|---|---|
| DSC on Channel 70 | Digital identity, call category and selected data | The full voice call or message |
| Voice channel | Situation, assistance, intentions and conversation | Correct digital selection where DSC is used |
| Listening watch | The reply, coordination and changing instructions | Active incident management |
Read DSC as an envelope, not the whole message
A routine DSC call from another yacht rings and displays a proposed inter-ship channel.
1. Read the envelope
Check the caller identity, routine category and proposed voice channel before accepting.
2. Acknowledge digitally
Accept only when the identity and channel make sense for the intended contact.
3. Continue by voice
Move to the accepted channel, identify the station and conduct the short voice exchange there.
Sense check: If the display says distress rather than routine, do not treat it as an ordinary incoming call: cease interference and listen on Channel 16.
Common mistake or limitation
- Speaking on Channel 70 after the digital alert.
- Assuming a DSC alert contains every fact the receiver needs.
- Treating the alarm as proof that another station has received and acted on the call.
Recap
- Channel 70 carries DSC digital signalling only.
- DSC establishes or alerts; the appropriate voice procedure still follows.
- Identity, equipment, range, power and watchkeeping remain part of the system.
Optional quick check
Section 1 of 9
What belongs on VHF Channel 70?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Digital Selective Calling
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