Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 07-04-2015, 01:09   #1
Jd1
Registered User

Join Date: Aug 2011
Location: Victoria, BC
Boat: Catalina 36 MKII
Posts: 1,095
M802 - Trying to understand DSC

I am trying to wrap my mind around the DSC thing and would like to bounce a few thoughts off of you.

It is my understanding that there are two DSC receivers in the M802. One being the emergency receiver that uses the DERA antenna and scans 6 pre-assigned frequencies no matter what the M802 is doing. Then there is the DSC Watch system in the 802 that uses the main antenna and is only active in DSC Watch mode.

What happens if a 'routine' call comes in on one of the distress frequencies monitored by the secondary (emergency) DSC system in the 802? As this is not an 'emergency' classed DSC call, will it be ignored by the 802 or will the 802 alert me to an incoming 'routine' call ? Note that I am NOT in DSC Watch mode - assume I was just listening to some random channel.

The next thing I would like to figure out is how to test that the secondary receiver is actually working. Based on my tests so far, a DSC 'Test' call is transmitted on the main antenna and the acknowledgment seems to also be received via the main antenna. How then does one verify that the secondary receiver that scans the 6 emergency frequencies is actually working correctly ?

It seems that it would be ideal if all DSC calling can happen on the emergency frequencies using the secondary receiver so that no matter what I do on the 802, a DSC call (even a routine call) can always get through to me no matter what I am doing.

Thoughts ? Corrections ?
__________________

__________________
Jd1 is offline   Reply With Quote
Old 07-04-2015, 12:45   #2
Registered User
 
ka4wja's Avatar

Join Date: May 2007
Location: Florida
Boat: Catalina 470
Posts: 2,033
Re: M802 - trying to understand DSC

Jd1,
You're not alone in your confusion....which is why I made the videos....so that I wouldn't have to spend hour after hour typing answers...


Seriously, if you watch the videos, almost all of the info you need to effectively use the M-802, and understand HF-DSC are there!!!
And, its accurate....(except, for once, I think I said 8290 instead of 8291.0 in my narration, omitting the 1....but the radio showed 8291.0 show nobody has ever noticed my narration error!)

https://www.youtube.com/playlist?lis...ga2zYuPozhUXZX



I will spend a few brief minutes answering your specifics here, in red....
But, in a nutshell, you are over-thinking things, a bit...
Quote:
Originally Posted by Jd1 View Post
It is my understanding that there are two DSC receivers in the M802.
Well, actually not exactly the case...

Two receivers, yes...
--- One is the DSC receiver (with its own antenna) which is separate, dedicated to scanning only six DSC freqs (2187.5, 4207.5, 6312, 8414.4, 12577, and 16804.5khz), with a 400hz - 500hz rec bandwidth, and only receives 100-baud FSK signals, sent to the DSP module for decoding and display....

--- The other is the "main receiver", connected to the "main" antenna, and is used for everything else (including scanning other DSC freqs you may have programmed in, when in the "DSC Watch" mode)...
(Note that this "main receiver" is not a second DSC receiver, but the "main receiver" that CAN be used for DSC reception on the freqs you select/program....BUT...
But, in actual real-world use, this is NOT done by anyone other than experimenters like me!!! I tried to convince some sailors that it would be cool to use it for "routine" signaling, and I even reprogrammed mine with six new freqs from the GMDSS DSC freq list, and wrote a nice thread about all of it....but nobody else thought so, so in the real world, the M-802's DSC Watch Mode is unlikely to be of any use to you...)

One being the emergency receiver that uses the DERA antenna and scans 6 pre-assigned frequencies no matter what the M802 is doing. Then there is the DSC Watch system in the 802 that uses the main antenna and is only active in DSC Watch mode.
See immediately above for clarifications....


What happens if a 'routine' call comes in on one of the distress frequencies monitored by the secondary (emergency) DSC system in the 802?
First off you may be under the false impression that these six GMDSS DSC Safety and Calling Frequencies are exclusively for "Distress" signaling....but they are not...
They are specifically for "Safety and Calling", including Distress, Urgency, Safety and Routine....

When the MF/HF-DSC system was designed about 23 years ago, it was anticipated that there would be a lot of "routine" traffic being sent on HF radio, and that public correspondence stations and vessels contacting them, would need other freqs to signal each other and/or signal other vessels for "routine" traffic as well as "routine" public correspondence traffic....
So, provision was made in the GMDSS DSC system to handle that....but that never materialized in any significant way, and today there are no public correspondence stations that use anything other than the six GMDSS DSC Safety and Calling freqs...and few (if any) vessels and private coast stations that do otherwise either...
So, for the most part....use of "other" DSC freqs is almost non-existent....


As this is not an 'emergency' classed DSC call, will it be ignored by the 802 or will the 802 alert me to an incoming 'routine' call ? Note that I am NOT in DSC Watch mode - assume I was just listening to some random channel.
It should receive the call normally. (as the DSC receiver is not a "Distress" receiver..)

You ask a good question here, although I doubt this is ever going to come up for you, in the real world....here's my experience...

I have received many replies to my "routine" calls, from shore stations or other boats, but I have never been the first recipient to a "routine" call from someone else (when NOT in DSC Watch mode).....but assume it has worked for the others, as they replied to me....
But, perhaps one of those that I have sent a "routine" call to, with an M-802, will chime-in here and confirm...



The next thing I would like to figure out is how to test that the secondary receiver is actually working. Based on my tests so far, a DSC 'Test' call is transmitted on the main antenna and the acknowledgment seems to also be received via the main antenna.
While waiting for the ACK from a "Safety", "Urgency", or "Distress" call, I believe my M-802's main receiver is monitoring the corresponding SSB Voice freq (or the one I specified as my traffic channel, in my DSC Call)....so the DSC ACK is coming from the DSC rec, not the main receiver...
I know this is the case for "Distress" DSC calls, but assume the radio is using the DSC receiver for "Urgency", "Safety" and "Routine" as well, but simply mutes the main receiver...and assumed the same for "test" calls / waiting for the ACK from a "Test" call....I assume that the ACK is coming from the DSC receiver (but I have NOT spoken with Icom Japan to confirm this....)
This makes sense, as why have a "test" call feature that didn't actually test the DSC receiver??
(and this IS in the GMDSS rules for GMDSS compliant equipment, but the M-802 was never submitted for GMDSS compliancy...so, we haven't an exact absolute answer here...)


Be advised that speaking with Icom USA about this will be MOSTLY USELESS, and a BIG waste of your time!!!
It was myself that finally got Icom Japan to tell Icom USA that the manual was incorrect, in that the "default" for a "simple distress" was in fact 8414.5khz and not 2187.5khz, and that even when a DSC call was sent on 8414.5khz, the manual stated that the radio would standby on 2182khz!!
Icom USA reps, as late as 2013, were still advising folks using WRONG info....yes, you read that correctly, the manufacturers' USA reps / tech support / etc. were using bad info to teach/advise/troubleshoot their own radios!!


How then does one verify that the secondary receiver that scans the 6 emergency frequencies is actually working correctly ?
Again, understand that they are not "emergency" freqs, but "safety and calling" freqs....and we can assume that Icom designed the "test" call to "test" both the transmitter and receivers....and if you wish to verify things, send a "test" call (or two) to the USCG or other shore station, or to another vessel, and try receiving the ACK both with the DSC antenna connected and with it disconnected....and you will know 100% where the ACK is being received from, and that the DSC receiver is working, as well as your DSC rec antenna....(and understand that it is the antenna, and its coax, that I would be questioning, not the receiver itself!!)

It seems that it would be ideal if all DSC calling can happen on the emergency frequencies using the secondary receiver so that no matter what I do on the 802, a DSC call (even a routine call) can always get through to me no matter what I am doing.
This is what is being done....and I think the reason you are having trouble "wrapping your mind around the DSC thing", is two-fold....
a) misunderstanding that the six GMDSS DSC Safety and Calling Freqs are "emergency only" freqs, which they are not...
b) the horrible Icom M-802 manual!!!


Thoughts ? Corrections ?

If you radio works, you get test call responses, etc. then the rest is really just an intellectual exercise....as since everything is working as it should, there is no issue to worry about...


I hope the above helps...

Fair winds...

John
__________________

__________________
John, KA4WJA
s/v Annie Laurie, WDB6927
MMSI# 366933110
ka4wja is offline   Reply With Quote
Old 07-04-2015, 13:58   #3
Jd1
Registered User

Join Date: Aug 2011
Location: Victoria, BC
Boat: Catalina 36 MKII
Posts: 1,095
Re: M802 - trying to understand DSC

Quote:
Originally Posted by ka4wja View Post
Jd1,
You're not alone in your confusion....which is why I made the videos....so that I wouldn't have to spend hour after hour typing answers...


Seriously, if you watch the videos, almost all of the info you need to effectively use the M-802, and understand HF-DSC are there!!!
And, its accurate....(except, for once, I think I said 8290 instead of 8291.0 in my narration, omitting the 1....but the radio showed 8291.0 show nobody has ever noticed my narration error!)

https://www.youtube.com/playlist?lis...ga2zYuPozhUXZX



I will spend a few brief minutes answering your specifics here, in red....
But, in a nutshell, you are over-thinking things, a bit...



If you radio works, you get test call responses, etc. then the rest is really just an intellectual exercise....as since everything is working as it should, there is no issue to worry about...


I hope the above helps...

Fair winds...

John
John, I have watched a lot of your videos (THANKS!) though not all.
The radio works and I DID receive test call responses but they were not consistent - every once in a while I would get an ack but it wasn't reliable enough to do the antenna disconnect test to my satisfaction. I did disconnect my DERA during an ack reception but it did NOT interrupt the ack. This is what makes me think that the ack is received with the regular antenna rather than the DERA.
Re the 'intellectual exercise' comment - I could start a long rambling thread here about what all is wrong in the world but suffice it to say that IMHO it is not possible to understand system workings properly if one accepts everything at face value. No, I do not need to know EVERYTHING but as a user of the M802 I feel a need to know the basic operation and knowing which receiver is used for what is (IMHO) important to understand. The secondary receiver is directly interacting with me and how can I understand it's function and indeed evaluate if it is working or not, if I have no clue what it is supposed to do.
Unfortunately, most of the world these days works on the principle of 'it seems to work, why should I be concerned with HOW it works'. The magic black box syndrome. It is foolish to accept, especially as a sailor, a magic black box without understanding to some degree what makes it tick. Take the example of a chartplotter, a magic black box for most of us. If it stopped working suddenly I would hope that most of us have the wherewithal to check the fuse (as an example). Anyway, I am rambling ....
Back to the issue at hand - I understand that the 6 frequencies are general use frequencies as indeed any frequency is. A frequency by itself does not limit anything. When I was saying 'emergency use', I am saying that the secondary receiver scans the 6 frequencies, evaluates what it receives and only if it sees a valid emergency transmission does it take over the main radio.
So having clarified this a little bit, I am wondering if the secondary receiver will recognize a 'routine' DSC call to my MMSI and activate the main receiver for me to deal with that call. Alternatively, will it only activate the main radio if it receives a distress, urgency or a safety call ? That scenario would mean I still need to go into DSC watch mode to receive routine DSC calls directed at my MMSI whereas in the first scenario I do not need to be in DSC watch mode in order for somebody to hail me via DSC.
The even stranger thing is that a 'test' call is classified as a 'safety' call and thus should be received by the secondary receiver.

OK, here is a thought - anybody near Victoria, BC or within about 50 miles willing to do a bunch of radio tests with me to check out DSC operation ? If I am not in DSC watch mode but my radio notifies me of an incoming DSC call then I think I can assume that the secondary receiver works. If this does not work then I still do not know if my secondary receiver is working - it could be defective or it could be programmed to not react to anything other than emergency DSC broadcasts.

The test acknowledgements received were from WLO at night on 8 MHz but as I mentioned were not reliable - is there maybe a closer station to Victoria, BC that auto acknowledges test calls?
__________________
Jd1 is offline   Reply With Quote
Old 08-04-2015, 13:41   #4
Registered User
 
ka4wja's Avatar

Join Date: May 2007
Location: Florida
Boat: Catalina 470
Posts: 2,033
Re: M802 - trying to understand DSC

Jd1,
It was not my intention for my words to be taken in the context that "if it works, don't worry about how/why"....NOT at all!
So, please accept my apologies if that is how it came off!

My main point was directed at everyone else that might be reading this....
I've had problems in the past delving into deep descriptions and esoteric flow charts, etc., when all most wanted was a simple "press this button, and it does this.....then press this button, and it does THIS...."

So, while I agree that it IS important to verify that everything that you paid for and may be planning on using as a safety device, etc., IS working properly....and in order to do that, you must know what everything does / is supposed to do....
But, for most that wish to operate the radio, and/or just be able to send a DSC Distress Call when needed, they have little interest in learning all the why's and how's.....so, I've modified my way of posting / explaining things...


The problem here is that you really DO want to know the details, and I was using my new, more casual, approach....
Again, sorry about that!


I just got word yesterday that my favorite uncle (my Mom's big brother) had passed, and in addition to some business clients I need to work with this next week, I'm going to be tied up with some family matters....
So, I won't be available to do on-air DSC testing for a couple weeks....
BUT...

But, I think this might be a winner of an idea??

You're in Victoria BC, and I'm in S. FL...about 2800 miles away, over the whole continent of N. Amer....but there should be no reason that we couldn't do some on-air testing, probably in the late afternoon on 12mhz (or possibly even 16mhz) or early evening on 8mhz, when our schedules permit??
(if you have a listen to WLO on 16, 12 and 8, in late afternoon and early evening, and see what reception you have...and let me know...I'm about 600 miles further away, and only have 150 watts on-board, but with DSC we should be able to make things work just fine!)



A few specifics in red...
Quote:
Originally Posted by Jd1 View Post
Back to the issue at hand - I understand that the 6 frequencies are general use frequencies as indeed any frequency is.
Just a clarification....
These six frequencies ARE specifically designated by the ITU as GMDSS DSC Safety and Calling frequencies, and are not designated as maritime "general use" freqs....
Just wanted all to 100% clear on that!

A frequency by itself does not limit anything. When I was saying 'emergency use', I am saying that the secondary receiver scans the 6 frequencies, evaluates what it receives and only if it sees a valid emergency transmission does it take over the main radio.
There are many "all ships" HF-DSC calls that I have received over the years, that have been received by the DSC receiver (what you call the "secondary receiver"), usually when I'm on 20m, or just puttering around working on boat projects and listening to some guy's QSO, etc. on either ham or marine bands....
The radio beeps, and the display changes to show I have an incoming DSC message....but the main receiver is still receiving the same QSO, net, etc. and it is up to me to press the button to go to the "traffic frequency" designated in the DSC message...
So, with an "All Ships" DSC call (one that could be any category except "Distress"), the M-802 beeps and the display changes....but the radio is not taken over, per se, as it is when receiving a "Distress" Call...

I have received only TWO HF-DSC "Distress" Calls and ONE HF-DSC "Distress Relay" Call, in all the years I've had the M-802....
The most recent of which was last summer (?), when I on 20m and sitting right in front of the radio, the DSC Alarm went off (startling me!) and I proceeded with the correct GMDSS procedure...
{I immediately noted the MMSI# and position info, and switched to the "traffic channel" (12290khz), where I heard NMN (USCG in Chesapeake, VA) calling the vessel in question...(and I assume that they sent out a DSC ACK to them)
They continued to call them over and over again, every 30 - 45 seconds, during the next 5 minutes or so...

This went on for about 5 minutes, during that time I turned on my chartplotter and pulled up the charts for that location, and it turned out that the location that was sent in that DSC Distress call (according to my Navionics charts) was 100 - 200 yards inland/ashore, about 1.5 miles up the river in Suriname's main port....(making me think it was either a vessel docked there, or most likely a guy in a radio shop working on a radio and forgot it was on an antenna, rather than a dummy load)

After about another 5 minutes (about 10 minutes since the DSC Distress call was sent), NMN had only called them once more....
So, I called NMN, on 12290khz, with my vessel name and location in S. FL....they immediately responded to my call....I told them that I also had received the DSC Distress call, and I confirmed with them the MMSI# and position info...
When I mentioned up the river in Suriname, and according to my electronic charts on-shore by a couple hundred yards, they were impressed, and said they didn't have charts that detailed....they thanked me, and asked me if I would standby that channel for a few minutes while they called again....
I did as they asked, and they called a few more times, but no response was heard....
I cleared with them, and went back to 20m...

So, in a Distress Call reception, the radio is taken over....but only partially taken over with "safety", "urgency" calls....
BUT...

But, with "routine" calls??
I don't know....as I don't know if the "all ships" calls I've received were "routine" or "safety"....

And, I've never received a "test" call initially, only received responses to my "test" calls...



So having clarified this a little bit, I am wondering if the secondary receiver will recognize a 'routine' DSC call to my MMSI and activate the main receiver for me to deal with that call. Alternatively, will it only activate the main radio if it receives a distress, urgency or a safety call ?
Again, you pose a good question....and one that we can test to prove...
(but again, my assumption is that "routine" calls that come in on one of the six GMDSS DSC safety and Calling freqs, on the DSC receiver, WILL register on the radios display and a beep will sound, just like a "safety" call...but, there is no alarm like there would be with a "Distress" call...)


That scenario would mean I still need to go into DSC watch mode to receive routine DSC calls directed at my MMSI whereas in the first scenario I do not need to be in DSC watch mode in order for somebody to hail me via DSC.
The even stranger thing is that a 'test' call is classified as a 'safety' call and thus should be received by the secondary receiver.


OK, here is a thought - anybody near Victoria, BC or within about 50 miles willing to do a bunch of radio tests with me to check out DSC operation ? If I am not in DSC watch mode but my radio notifies me of an incoming DSC call then I think I can assume that the secondary receiver works. If this does not work then I still do not know if my secondary receiver is working - it could be defective or it could be programmed to not react to anything other than emergency DSC broadcasts.

The test acknowledgements received were from WLO at night on 8 MHz but as I mentioned were not reliable - is there maybe a closer station to Victoria, BC that auto acknowledges test calls?
You can try NMC, USCG in Pt. Reyes, CA on ONLY 4207.5khz DSC, for their automated test call response...
They are about 700 - 750 miles south of you....and 4mhz propagation should be good for you from 2 hours after sunset, to sunrise....

Or, if you can wait a few weeks, we can give it a try from my boat, here in S. FL...

I'm pretty sure that WLO's automated DSC response is done only from their DSC receivers in Mobile, AL....
But, you may wish to see if WLO has remote DSC receivers at KLB (their remote site in Seattle, which has 4 remote HF-SSB receivers and 4 remote HF-SSB transmitters, which they control from Mobile, AL)

I hope this helps...

Fair winds...

John
__________________
John, KA4WJA
s/v Annie Laurie, WDB6927
MMSI# 366933110
ka4wja is offline   Reply With Quote
Old 10-04-2015, 15:20   #5
Jd1
Registered User

Join Date: Aug 2011
Location: Victoria, BC
Boat: Catalina 36 MKII
Posts: 1,095
Re: M802 - trying to understand DSC

John, my condolences! Let me know when you are available again and we will see if we can arrange things.
I finally got a test ack from Pt. Reyes the other night - I had tried numerous times at different times in the night. WLO is very hit and miss with mostly 'miss' recently - possibly because of my makeshift antenna.
Pt. Reyes came in very strong last night and I was able to confirm one thing - I could operate completely without the DERA and received the ack just fine .... the more I dig the more confusing it gets
Take care and don't worry about the 'dumbing down' thing ... unfortunately it seems that the more complicated life gets the less people want to know what makes things go
__________________
Jd1 is offline   Reply With Quote
Reply

Tags
dsc

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
M802, M402, NMEA, GPS (DSC), Problems Wotname Marine Electronics 6 01-05-2016 10:01
Looking for a fixed VHF *without* DSC (or ability to disable DSC) susswein Marine Electronics 31 10-11-2014 13:18
ICOM M802 DSC antenna? Pa La O La Marine Electronics 45 28-02-2013 11:50
trying to understand ssb sailor12 Marine Electronics 12 20-03-2011 10:46



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 05:12.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.