Use of DSC alerts?

The graphic WAS funny

  • I DISAGREE

    Votes: 0 0.0%

  • I agree

    Votes: 0 0.0%
  • Total voters
    0
[ QUOTE ]

Thanks Solitaire i did'nt dream it would create
such flak for you,

[/ QUOTE ]

No worries Tony, I've been around long enough and seen most things. It's the way things go I just smile and move on! I have a box of pins! /forums/images/graemlins/grin.gif
 
FM or Phase Mod..really really geeky....

Good post from Julian..explains the concepts really well.
I'll use that site next time I do a radio course.

Most radios these days use what is known as Phase Modulation.
It comes out the same as FM, but is in fact classed as a different modulation method...from Memory G3E..wheras FM is F3E.
Sets are much easier to manufacture with this technique.

To get true Frequency modulation it is necc to apply the processed audio directly to the master oscillator to get a small shift in frequency.
This is usually done with a varicap diode, which when placed in paralell with the main oscillator tuned circuit would result in a small capacitance being applied, when the varicap was biased with the audio derived voltage, causing a shift in the main oscillator frequency usually limited to about +/- 4khz.
In the early days these varicaps were sometimes unstable and very prone to being overdriven with audio, even with limiter circuits in place,sometimes causing frequency error on fault condition.

A better method was developed using a phase modulation technique.
This shifts the phase of the signal ie the relationship between each cycle of the carrier, rather than the frequency.
The beauty of the system is that phase mod can be applied virtually anywhere on the RF driver chain, leaving the master oscillator completely stable.
This results generally in much cheaper manufacturing and easier setup.

Just thought someone may be interested.
End of geekiness.

Steve.
 
Re: Further with digital??

Erm, I don't think so ... not quite ... its a fire and wait system - the call needs to be acknoledged by the CG or full DSC enabled Ship (coastal stuff is usually the coastguard!) ... no acknowledgement and it will transmit again ... at least I think it does, just for the Urgency & Emergency calls ... Not sure of the time delay either. 121 calls assume you manually take care of lack of response ...

Couldn't be a packet acknoledgement as per computer networks as it is a broadcast (1 to many) transmission ... and some might only get part of the message (on the edge of the range or other DSC interferance) whilst others get the whole lot - it'd be there for ever and a day trying to get the complete message to a unit that is right on the edge of the range....
 
I'd been wondering what those pains were in the middle of my back today...and I thought it was arthritis..I'm relieved.
No personal offence ever intended David...it's the RYA's system that I object to.

Steve.
 
Re: FM or Phase Mod..really really geeky....

Hmm ... we need a [geek] [/geek] markup code don't we ! /forums/images/graemlins/laugh.gif
 
Re: Further with digital??

[ QUOTE ]
no acknowledgement and it will transmit again ... at least I think it does,

[/ QUOTE ]

Correct, and will do so approx every 4 minutes until acknowledged, or some other "external" influence stops it!
 
Re: Further with digital??

[ QUOTE ]
or some other "external" influence stops it!

[/ QUOTE ]
That'll be the "Emergency Sledge Hammer" then ... I knew I had it next to the set for some reason
 
Re: Further with digital??

Solitaire,

On a standard set the ack has to be sent manually.
Do you know if the CG have anything smart which handles acks automatically,and flags status up on a screen..or is their system manual too?
Also do they have anything that will correct errors on the system when used at distance?
Can they and if so, to what sort of degree are they able to interrogate the DSC PROM in our radios once the MMSI number is transmitted to them?
It would be interesting to know this and relevant to the DSC at distance sub thread.

Steve.
 
Re: Further with digital??

[ QUOTE ]
On a standard set the ack has to be sent manually.

[/ QUOTE ]

I was referring to the fact that a in the event of your Distress Alert not being acknowledged, then the set will automatically send the alert again at approx imately 4 minute intervals.

Class D sets - the most common in leisure vessels cannot acknowledge a DSC alert!


[ QUOTE ]
Do you know if the CG have anything smart which handles acks automatically,and flags status up on a screen..or is their system manual too?

[/ QUOTE ]

To my knowledge the alert comes into the CG and is centrally "highlighted". It appears on a computer screen with the same data as we see on our screens. It does not automatically (or at least it didn't when I was last in Solent Coast Guards control room) automatically assign a vessel's name to the MMSI number. I did speak to them recently and they confirmed that they used the ITU's MARS database to source the vessels name and owner. There is currently no method that simply displays all the vessels details. Cost!!


[ QUOTE ]
Also do they have anything that will correct errors on the system when used at distance?
Can they and if so, to what sort of degree are they able to interrogate the DSC PROM in our radios once the MMSI number is transmitted to them?
It would be interesting to know this and relevant to the DSC at distance sub thread.


[/ QUOTE ]

I think the answer is no - but why don't you call your local CG and ask?
 
Re: Further with digital??

OK David that pretty much answers my question.
I'm just trying to gain a better technical understanding of the system.
I know how it works, but not what they do with it at the CG end.
It is pretty much as I imagined it would be. Therefore the system is manual throiughout..the only auto part of it is the repeating alert transmission until acked by a shore station, SOLAS vessel or cancelled by the user.

Many thanks

Steve.
 
Re: Further with digital??

I think that about sums it up. Phew I reckon we've sort of met in the middle here. /forums/images/graemlins/grin.gif
 
Re: Further with digital??

Warning! Geek mode is ON!
DSC transmissions DO include a very basic error detection mechanism, I know this as I'm busy building a radio to do it.

So far in this discussion, everyone is correct, but it depends on which circumstances are occuring (typical).

Both DSC and voice are transmitted at the same RF level and so travel an equal distance. If the Rx on the far end responds to a squelch only, then it would open at the same rx level in both transmission modes. However.... there is no need to use a squelch in DSC mode as the signal can be recovered out of the recieved signal, in this situation, it is possible for a very good DSC modem to recover a valid DSC transmission at a signal level where normal voice would be un-intelligible. But as the next sequence is usually to goto a voice mode to pass the message, this "advantage" is actually of little real value.

Take a look at ITU-R 493-11 for the DSC standard, ETSI also have test spec which should give you some info, and its free.

There is no way that I can that the CG can interrogate for any additional info in your radio, besides that specified in the standard.
 
You must log in or register to reply here.

Share this page

Top