As anyone who was in the WPX RTTY contest this past weekend knows, anything that can help reduce the unnecessary interference from overly wide transmissions is more than welcome.
Although Andy's work showed clearly that shaped AFSK could be very spectrally efficient, most of his testing was done with a K3 in AFSK mode *with the AFSK filter engaged*. Elecraft are already head and shoulders above the other manufacturers by including and AFSK filter and an automatic audio gain control (ALC) which makes it nearly impossible to generate "wide" AFSK even if the user engages "all knobs to the right". This is not the case with AFSK on other transceivers - *particularly* when operated with audio connected to the microphone input of a rig designed for a dynamic mic. Most dynamic mic rigs expect *only* 5 mV of audio drive to the mic jack and will distort significantly when drive with more than 50 mV - something that no adjustment of the mic gain can resolve since the mic preamp is a fixed gain stage *before* the mic gain control. In spite of this, many manufacturers of low cost audio interfaces continue to sell noting but mic cables and their documentation continues to instruct users to turn the sound card level *all the way up*! When one overdrives a mic stage - here's what the spectrum looks like: http://www.subich.com/BAD_AFSK.jpg. In this case audio was being fed to the "mod in" pin on an Icom ACC jack but at more than 700 mV on an input rated for 100 mV RMS it was still much too hot. One can see the AFSK signal, the second harmonic, the third harmonic, "suppressed" carrier and the incompletely suppressed lower sideband. As bad as it is, this capture does not show the horrendous effects of hum evident on many AFSK signals either. While it is great that Elecraft give us the tools to make sure we are stinking up the band but until all rigs include AFSK filtering ahead of the modulator, I would much rather deal with the modest "key clicks" from some of the traditional transceivers than the 3 - 5 KHz wide mass of hum and harmonics from an improperly operated AFSK transmitter - and I suspect anyone who has had to deal with the interference from an improperly set-up AFSK rig will agree. 73, ... Joe, W4TV On 2/13/2013 7:06 PM, Mark n2qt wrote:
I had missed that this wasn’t discussed on this reflector, so first a little history At the end of December, on the RTTY reflector, Andy K0SM/2 produced measurements showing that the use of FSK, which had been the ‘gold standard’ for generating RTTY was actually not as spectrally efficient as correctly adjusted, shaped AFSK generated RTTY. This was shown in his write-up posted at http://www.frontiernet.net/~aflowers/k3rtty/k3rtty.html Note that this does NOT indicate that there was any sort of problem with the K3, just that there was a chance to be better. Other manufacturers also have this same chance to improve. A little later, Ed, W0YK, also known as P49X in many contests, announced that he had successfully used experimental DSP firmware from Elecraft in the BARTG rtty contest. “The K3 DSP firmware engineer coded a wave-shaping filter that narrows the transmit bandwidth similar to what their AFSK filter provides. The resulting spectrum image looks like the better, i.e., narrowest, of the images in K0SM's recent work.” Elecraft then allowed Ed to share this firmware with others who were interested in trying it, which is how I got it. As anyone who was in the WPX RTTY contest this past weekend knows, anything that can help reduce the unnecessary interference from overly wide transmissions is more than welcome. It is great that Elecraft has reacted so quickly to this opportunity. (Would we expect anything less from these guys??) Mark N2QT/WE4M From: Stephen Prior Sent: Wednesday, February 13, 2013 3:22 AM To: Mark Cc: [email protected] Subject: Re: [Elecraft] K3 Reduced BW FSK Mark, You will have to enlighten us (well, me anyway) - just what pre-beta software are you referring to? 73 Stephen G4SJP On 13 February 2013 04:10, Mark <[email protected]> wrote: Just a report that I used the pre beta low bandwidth fsk software on both k3s during the WPX RTTY contest. Made over 1900 low power contacts, with no apparent problems (As expected). As close as some stations felt comfortable operating next to me, it looks like it helps. With the ever increasing popularity of rtty contesting, improvements like this helps us all. Mark. N2QT/WE4M ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
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