Digital modes offsets

DrTeeth

Hi chaps,

Could somebody please explain the purpose of using offsets in the first place rather than just txing on the dial frequency? Doesn't it create a problem for spotting as for it to work there would have to be an offset standard?

There must be clearly some advantage to this as FRS has made it easy in the latest release(s) to use one, but **** if I can see it.

TIA for any clarification

Guy

Mike NN9DD

I assume You are talking about when you click a spot and they are up the band a bit. PSK uses .070 and JT uses .076 as you are likely aware. When you are on that frequency and click on the waterfall you are transmitting a tone a few hundred to a few thousand hertz up the band depending on where on the water fall you click. It is more of a waterfall position then an offset. I hope that helps Mike N9DFD

DrTeeth

I am afraid it does not help; I'm still as puzzled as ever. Some who know me would say they are not surprised, but I digress.

Why just not transmit on the carrier frequency and be done with it rather than having several different offsets to contend with?

SSDR recently introduced an adjustable offset with a default value of 1500 Hz as a default. "Why and why was that value chosen" is my question to which I have not either managed to get an answer or find any info on my web site trawling.

73

Gary L. Robinson

digital sound card modes are essentially tones modulating a SSB (or other analog voice modes AM,FM). All digital modes have their own bandwidth - some 31hz wide and others as large as 3000hz. To maximize the effeciency of the transmitted mode they need to be offset to the "sweet spot" where they modulate the SSB signal to get the most effective output and NOT cut off any of the tones, etc. Think of a SSB voice transmission. They are best utilized by using equalization on both input audio and output audio to make it more effective for each of us to hear what the other is saying. If you try a bunch of digital modes and carefully try different offsets on transmit looking at the power out you will see that IF you go too low or too high the power out will drop off. Henceforth each mode generally has it's own "sweet spot" where transmit and receive generally works better. As I said, sound card digi modes are mostly just tones modulating a SSB signal and that is why most digi program waterfalls are 3-4khz wide.

DrTeeth

Hi Gary,

Thanks for that explanation. It has cleared the fog somewhat.

With kindest regards from a cold, damp and rainy London.

es 73.

Gary L. Robinson

BTW, another question you also might ask is why SSB frequencies are NOT the CENTER frequency of the transmitted signal and NOT the carrier freq which isn't used hihi

DrTeeth

An excellent point well made sir. But hand on a minute...how about having a new system of modulation where the carrier frequency *is* used? So what if some spectrum was wasted having the same information were transmitted on either site? It could take off as all the 'offset confusion' would be eliminated.

George KF2T

We could call that new mode "CW " :-)

Michael Coslo

Since there is no carrier present un a single sideband signal,  if you are on say 10.00 MHz and transmitting your signal @ 1000 Hz USB, you add the 1000 hz to the 10 MHz, and you have the frequency you are transmitting on. LSB, you would be subtracting. 

Cal Spreitzer - N3CAL

The offset is very handy while operating JT-65/JT-9 (DIGU).   Normally I have the Flex bandwidth set to 5khz to see the entire JT65/JT9 spectrum within WSJT-X.  When the band is crowded and there are very strong signals within that 5khz I can narrow the flex bandwidth down to 300hz and move that 300hz up and down the WSJT-x screen to where I want to operate using just the offset slider.  It blocks out all the unwanted signals.  It's great when you want to park on a freq and call CQ . 

Cal/N3CAL

Dave - W6OVP

We could call that new mode "CW " :-)

But we would still need to use an "offset" in our receiver in order to "decode" it. <ggg>.

DrTeeth

Cal, ISTR that somebody stated about FLDigi, it could have been the author himself, that all the necessary filtering is done by the program, and that doing what you suggested had a deleterious effect on performance

Tim - W4TME

The DIGx offsets are used to center the preset filters and click-tuned signals in the middle of the preset filters.  DIgital mode programs operate at an offset from the Slice "dial" frequency.  Most of the programs have a feature or a macro to change the radio's VFO so that the signal of interest is at a fixed offset (aka the "sweet spot" for Fldigi).  Using these features with a user defined offset makes centering a signal of interest in a filter very easy allowing for quick and easy selection of narrow filters. 

Tim - W4TME

Incorrect.  Here is why.  With a wide RX filter a strong signal in the passband can capture the AGC and reduce the audio gain making a weak signal disappear.  Using a narrow RX filter eliminates that possibility.  This is absolutely necessary in a crowded band situation like Field Day.

Ken - NM9P

Besides, the filter skirts in SSDR seem to be much sharper than those in FLDigi.

Mark Erbaugh

Are direct sampling radios somewhat immune to AGC effects from strong in-band signals?

Tim - W4TME

If you mean inside the slice RX filter, the answer is the AGC will be captured.  If you are talking about a strong signal outside of the slice RX passband filter, those signals do not capture the AGC.  However large signals outside of the RX passband may actually be inside the RX filter due to the offending signal's transmitter or PA having bad phase noise characteristics, splattering due to an over-driven mic input or CW key clicks, as theses are real signals.