DStar DV Sensitivity vs. Analog Sensitivity

In the Spring 2008 issue of the TAPR Status register I have written an article on DStar DV Sensitivity vs. Analog Sensitivity. Here is a link to the article, it begins on page 4.

73,
Mark N5RFX

 

No Worky

The link to the D-Star site is not working.

ftp://ftp.tapr.org/psr/psr105.pdf

Fixed it for you.

 

From the Article:

"Conclusion
Trading 2 dB of sensitivity for a 17dB increase in nearly noise free reception is an advantage of DStar over analog FM. When weak signal reception is necessary, the analog signal will provide better performance."

Since EMCOMM most often happens under less than ideal conditions, this conclusion supports the retention of analog infrastructure. Digital Voice can be a SUPPLEMENT to existing analog systems but, IMHO, should not be the system of last resort.

 

The problem is, most hams are used to operating with a fair amount of noise on the RX signal. It seems wrong to compare a D* signal to a less than fully quieted FM signal, and imply that D* has any sort of advantage.

Certainly, a signal that is 12dB sinad can be copied by most hams. Signals stronger than that become even easier to copy. The FM doesn't have to be full quieting to be really decent copy.

A more interesting test would be a mobile flutter test, where signals are dropping below -122 for a few milliseconds at a time. FM handles this situation well, I haven't heard what happens to D*.

Joe

 

There seems to be two camps of operators, those that prefer full quieting and those to don't seem to mind noisy signals. What I have seen in most DStar comparisons is the noisy analog signal vs. the quiet digital signal. The fact is that a digital voice signal can provide a larger range of full quieting audio, than can an analog signal. The analog signal however will outperform the digital voice signal under weak signal conditions.

I am looking at how to set this up in the lab. I do know from adjusting the attenuator on the signal generator that resync is not instantaneous when a signal fades below threshold and then returns to a level that is above threshold. I saw this when I was in the San Fransisco Bay area driving around mountains, hills and canyons. The operating conditions were much different than what I have experienced in the DFW area.

Thanks

73,
Mark N5RFX

 

Yes, I prefer a full quieted signal, but I don't need one. Most people seem to equate 30+dB of quieting to "full quieting" but it's not. That level probably occurs at about .6-.7 uV of signal on most radios, or around -110 dBm or so. Below that, most people will start to notice the noise, or perhaps comment about it, but still they have a good copyable signal.

There ought to be a way to switch in attenuation, or simply a fast antenna switch that could operate in 10-80ms or so. If D* has a hard time recuperating from this type of flutter, it might limit its usefulness. A 10 to 100ms gap in speech will usually be filled in by the brain, thats why simple mobile flutter on FM isn't such an issue. If D* has a long period to resync, this would further limit its usefulness in real world weak signal operations.

Joe