On Sat, May 1, 2010 at 11:59 AM, Anony Mous <[email protected]> wrote: > Uh, no. If I'm tuned to 14.230 MHz on the radio, and I'm receiving a FAX > signal that is AFSK between 1500 Hz and 2300 Hz, then the transmission is, > in fact, on 14.230 Mhz - the "center" (1900 Hz, or 14.2319 MHz) is not only > *not* where the radio is tuned, if it *was* tuned there (you'd have to > offset the fax modem to do it), then if the operator of the transmitter were > to speak up and tell you "that was a fax of the important document you asked > for", you wouldn't be able to understand them, because you're incorrectly > tuned. Furthermore, if you were to send a fax back, you typically want the > radio on 14.230 as well; otherwise, you have to operate split. > > The correct frequency IS the frequency the radio is tuned to. Not whatever > fldigi calculates as the modem center. This stuff is AFSK. Note the AF.
Uh, no. What you have described is the convention used by amateur radio operators, not what really happens physically. Here is an example. We have 50 stations operation PSK on 20m. All have their radios tuned to 14,070 kHz. None of them are interfering with each other. By your reckoning, they are all on the same frequency because they have tuned the big knob on their transceiver until the number on the display reads 14,070 kHz. OTOH, if each of them uses a frequency counter to measure the carrier frequency they are radiating, they are each going to get a different number, something between 14,070 and 14,073 kHz. So the actual RF radiated is the sum of the low IF signal injected at the mic jack, e.g. 1.500 kHz, and the dial frequency of the radio, e.g. 14,070 kHz. You would hold that is 14,070 kHz and I would hold that it is 14,071.5 kHz. > There are numerous reasons to receive signals at the tones they were > designed to be sent at. If you have external gear of almost any kind, RTTY, > packet, SSTV, FAX, etc, they're going to be expecting very specific AF > tones. The tones you'll get when your radio is tuned appropriately, and when > the signal falls on the AF indications on the waterfall that are the design > points for the mode - for FAX, that's 1500 hz and 2300 hz. For SSTV, 1200, > 1500...2300 Hz. For RTTY, etc. That is the problem of the receiver, not what goes out over the air. To me all the following are the same: Dial low IF (tone) 14,070 3.0 14,071 2.0 14,072 1.0 As they all produce a carrier at 14,073 kHz. The operator then choses the proper receiver injection frequency (dial setting) to produce an IF output that the demodulator expects. Again, that is the *operator's* problem. What I want to know is the actual frequency of the emitted RF as that is what the FCC or anyone else is going to care about. Even fldigi pays homage to this concept. That is what the QSY button does. It retunes the radio to produce the IF signal at the same frequency no matter what. >> Fldigi does calculate and display the >> actual center frequency only in the log window. > > It's a misleading number at best. The correct frequency is the one the radio > is telling you. Au contraire! What fldigi reports is that actual carrier frequency, the frequency one would measure with a frequency counter connected to the transmitter output! > For optimum operations, the waterfall should be set to AF, > the modems should be at the design points, and the frequency display is > misleading and/or duplicates the radio display, consumes UI space that could > be better used for something useful, and should probably be disabled as a > direct consequence. Speaking of any situation where we are not just > entertaining ourselves, but are working in circumstances where correct > operation may have social or human consequences. There is no benefit to > fldigi's frequency display when it is duplicating the radio's; there is no > benefit when it is displaying a misleading "center frequency," either. If > you were using it to preview fldigi memories, that I can see. Otherwise... I > have yet to be presented with a point in its favor. You actually believe what you wrote there, right? You aren't just yanking my chain? If not, please tell me the proper modem design point for PSK31, Olivia, DominoEX, JT65, etc. One of the things I used to do was set up VSATs. The frequency we were assigned was the center of the transmitted signal, not the frequency of the LO injection. As far as I can discern, the only place where "dial frequency" is "transmit frequency" is amateur radio. Everyone else uses the center of the transmission. Regardless, I would be quite happy with the display being the start-frequency or the mark-frequency in an *FSK (2-FSK, m-FSK, ISK). And I know that the display in fldigi is the frequency sent to the rig. But if you think about it, the value sent to the PLL or DDS in the radio is NOT the display on the front of the rig. The value in the VFO is actually the sum (difference) between the VFO injection frequency and the IF frequency. In the case of fldigi we are dealing with an IF of typically between 0kHz and 3kHz. (OK, call it an IF frequency of 1.5kHz.) That is what the frequency readout should display (in my world). This is why I suggested it as an option. I want the display in fldigi to best reflect the physical transmission, not an arbitrary value agreed upon because that was what made the calculation easier 50 years ago. And I agree with Rick, K9AO, 100% on one thing: this is an education issue. Operators need to know precisely where their signal is going. That is why I am asking for the option to display a center frequency as an option in addition to the dial frequency of the radio. The dial frequency of the radio is already displayed on the radio. No need to repeat it on fldigi (except for troubleshooting purposes). Here is another way to look at it. When I see a 500Hz blank spot in my spectrum display, I want to be able to tell my buddy to transmit in the middle of that blank spot, not have to calculate a bunch of offsets in my head. -- 73 de Brian, WB6RQN/J79BPL _______________________________________________ fldigi-alpha mailing list [email protected] https://lists.berlios.de/mailman/listinfo/fldigi-alpha
