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Old 04 August 2018, 17:53   #1
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Voice on CH70

I turned on scanning on my VHF.
It locked onto Ch70 with routine voice traffic from Falmouth Coastgaurd. I'm not sure if 70 was one of the channels within the scan.

I thought Ch70 was data only...

Can anyone shed any light? Don't be afraid of going technical with me - I feel I really should be able to work it out, but haven't done so yet!
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Old 04 August 2018, 18:04   #2
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70 is data only.
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Old 05 August 2018, 17:56   #3
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70 is data only.
so under what circumstances might I be hearing voice on it?
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Old 05 August 2018, 18:39   #4
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so under what circumstances might I be hearing voice on it?

Iíd get your radio manual out and work through the scan set up and DSC part carefully... my guess is there is something about how/what your set displays that youíve misinterpreted.
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Old 06 August 2018, 04:14   #5
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Can you select channel 70 as a channel on your VHF? I am nowhere near a VHF at the mo but I don't think it should even appear as a channel you can select.
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Old 06 August 2018, 04:20   #6
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I can select chan 70 on my Cobra handheld and the icom fixed set in the RIB !!
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Old 06 August 2018, 04:21   #7
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I can select chan 70 on my Cobra handheld and the icom fixed set in the RIB !!
But I assume you can't transmit on it?
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Old 06 August 2018, 04:57   #8
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Just swiched them both onto chan 70 and tried transmitting.... Nothing happens... I will monitor 70 when next at the coast though....
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Old 06 August 2018, 04:59   #9
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Just awiched them both on and they dont transmit or receive each other on 70....
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Old 06 August 2018, 16:24   #10
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Interesting a none dsc set I have has ch70 labelled as “dsc”
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Old 06 August 2018, 17:49   #11
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Interesting a none dsc set I have has ch70 labelled as “dsc”

Ch70 (156.525 simplex) will appear on all marine radio's (regardless of DSC capability) as it is part of the international channel plan ..............

Ch70 is designated as the DSC distress calling channel ........... look upon it as a ch16 data only channel.
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Old 07 August 2018, 02:45   #12
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Ch70 (156.525 simplex) will appear on all marine radio's (regardless of DSC capability) as it is part of the international channel plan ..............

Ch70 is designated as the DSC distress calling channel ........... look upon it as a ch16 data only channel.
Just found it odd that a the channel appears in the list despite it being unusable?

Guess the programming is done in bulk, shame it doesnít have M1 or M2 programmed, European spec set I think.

If ch70 is simplex does a dsc set transmit on 70 at any other time or only when a dsc alert/message is sent? Or does data transmit on ch70 regardless of the ch the radio is set to? Iíve never used the selective element of dsc to ring someone so unsure of its practical operation.
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Old 07 August 2018, 03:05   #13
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If ch70 is simplex does a dsc set transmit on 70 at any other time or only when a dsc alert/message is sent? Or does data transmit on ch70 regardless of the ch the radio is set to? I’ve never used the selective element of dsc to ring someone so unsure of its practical operation.
I think the simplex description is legacy: It's more likely half-duplex.

If you want to send a Mayday, or call up another vessel via DSC, your DSC set will automatically send a short burst of data (with clever error correction built in) on Ch70. Consider it digital hailing where the data is spewed out at about twice the speed of an old fashioned Telex machine.

The intended recipient(s) then acknowledge this hail/mayday on CH70 with a corresponding burst of data. Both sets then switch to the required/chosen channel & continue the same as good old VHF.

Within the burst of data it can include the latest known position & the nature of any emergency.
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Old 07 August 2018, 03:52   #14
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Hdav, yes 70 is used for ALL DSC messages not just distress (the same as 16 is used for calling and agreeing a working channel too)
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Old 07 August 2018, 06:31   #15
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But as a user you don’t need to select channel 70? The set does this automatically and sends the data? Or does ch70 need to be monitored manually like voice on ch16 to “listen for data”

ie if set is on 16 it’s also listening for data on ch70 and will receives the data be it a call or distress while still monitoring ch16 for voice?

So selecting ch70 is effectively redundant on a dsc or none dsc set as ch70 is always monitored by the sets that receive the data burst and will switch over automatically when sending?
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Old 07 August 2018, 06:33   #16
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Originally Posted by HDAV View Post



Guess the programming is done in bulk, shame it doesnít have M1 or M2 programmed, European spec set I think.

.
If youíve got a Garmin set there is a way of easily adding M1 and M2 by going into a programming mode.
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Old 07 August 2018, 06:41   #17
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[QUOTE=HDAV;779205]
ie if set is on 16 itís also listening for data on ch70 and will receives the data be it a call or distress while still monitoring ch16 for voice?
QUOTE]

Correct. It monitors 70 continuously in the background, regardless of channel settings.

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So selecting ch70 is effectively redundant on a dsc or none dsc set as ch70 is always monitored by the sets that receive the data burst and will switch over automatically when sending?
Correct. The only use I can think of is to listen (for diagnostic purposes) to see if a DSC set is sending out any busrst of data on 70. Not tried it, but probably will now that I have both fixed & handheld DSC.
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Old 07 August 2018, 07:45   #18
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If you’ve got a Garmin set there is a way of easily adding M1 and M2 by going into a programming mode.
Standard hx370 seen it on some icoms too software needed I think

Seems odd ch70 isn’t just missing from the list on a none dsc, just have 69-71 plenty of other channels don’t appear by default I guess before dsc it was a channel but now for marine it’s inoperable for voice
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Old 07 August 2018, 16:40   #19
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Standard hx370 seen it on some icoms too software needed I think

Seems odd ch70 isn’t just missing from the list on a none dsc, just have 69-71 plenty of other channels don’t appear by default I guess before dsc it was a channel but now for marine it’s inoperable for voice
Ch 70 is essentially part of the specification for type approval (which used to be IIRC RTD1251 & Base stations were RTD 1250) .........

The method used for scanning Ch70 is quite complex, but clever enough to appear transparent to the user.
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Old 07 August 2018, 17:21   #20
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I think the simplex description is legacy: It's more likely half-duplex.
Its simplex. No legacy. Just straightforward simplex. No half duplex.


I may be wrong - but the data burst is TINY - I doubt you'll audibly hear it. Its transmitted at 1200Baud and there isn't that much data to transmit
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