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Why is accurate tuning important on a HF WEFAX signal?

simpilo

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Well it is important especialy fo rimages like SSTV and WEFAX for example. I know some of you are new to it and need some insight as to why we need to be finicky about tuning radios to the proper frequency as far as digital modes are concerned.

For one thing your image will come out like this
06-02-2019_031227[1].jpg

Secondly the sync detection may not work correctly and no image will be recorded. Some apps will still decode if you disable or switch off auto detection but still the image won't come out right.

A radio tuned correctly with a slight bit of slant.
06-02-2019_032452.jpg

I hope I helped newcomers to digital decoding understand why we need to be accurate in tuning. Other modes like NAVTEX also require a accurate tune.
 

spongella

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Also, slant on FAX images can be adjusted in some software programs like FLDigi.

A good read is Just The FAX by Steve Handler. Thanks for the info simpilo.
 

GB46

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I've noticed when using MultiPSK for both Fax and NavTex that it doesn't matter too much about tuning accuracy, as long as I'm approximately on frequency and click on the peak to the right in MultiPSK's spectrum display. The separation between the lower and upper frequencies of the signal will remain the same. My receiver has fine-tuning, but it's guess work, as the decimal values of fine-tuning are not displayed, just the integers.

For example, I have a NavTex station in one of my presets at 8416 kHz, while the assigned frequency is actually 8416.5. I can't tell where I'm going if I try to fine-tune for that frequency, but the signal still decodes perfectly, even if I wind up at the next full kHz plus or minus, as long as the signal is still audible and I click on that all-important right peak. Same holds true for Fax, where the software also lets me straighten out a slanted image, or shift it to the left or right if necessary.

Drift, of course, would be a different matter. MultiPSK's AFC and Lock functions can keep up with a slight drift on the receiver's part, but obviously not if it drifts rapidly way off frequency. I haven't had a receiver like that since my vacuum tube days. The SP-600 was the only stable vacuum tube radio I ever owned. In a way, despite its enormous footprint and power consumption, I wish I still had it today.
 

simpilo

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I've noticed when using MultiPSK for both Fax and NavTex that it doesn't matter too much about tuning accuracy, as long as I'm approximately on frequency and click on the peak to the right in MultiPSK's spectrum display. The separation between the lower and upper frequencies of the signal will remain the same. My receiver has fine-tuning, but it's guess work, as the decimal values of fine-tuning are not displayed, just the integers.

For example, I have a NavTex station in one of my presets at 8416 kHz, while the assigned frequency is actually 8416.5. I can't tell where I'm going if I try to fine-tune for that frequency, but the signal still decodes perfectly, even if I wind up at the next full kHz plus or minus, as long as the signal is still audible and I click on that all-important right peak. Same holds true for Fax, where the software also lets me straighten out a slanted image, or shift it to the left or right if necessary.

Drift, of course, would be a different matter. MultiPSK's AFC and Lock functions can keep up with a slight drift on the receiver's part, but obviously not if it drifts rapidly way off frequency. I haven't had a receiver like that since my vacuum tube days. The SP-600 was the only stable vacuum tube radio I ever owned. In a way, despite its enormous footprint and power consumption, I wish I still had it today.
Depends on which software decoder used. Most decoding software built for windows platform have a AFC. Perhaps i should've said using apps like Wolphi HF Weather Fax for Marine needs accurate tuning. Easily obtained tuning the BFO or clarifier knob until the continuous tone rides on the third line in it's audio spectrum FFT display. Droid Navtex can decode it off frequency a bit. Droid Navtex decodes much better with the radio tuned to 2100hz
 

GB46

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Depends on which software decoder used. Most decoding software built for windows platform have a AFC.
True, and MultiPSK is the only one I've tried that works for me. It's free, and is fairly easy to set up, although it has its drawbacks. It takes forever to launch on my system, which is not particularly fast to begin with. Also, the screen is way too crowded. I'd prefer to have all those modes displayed in a dropdown menu rather than as buttons on the main screen, where they use up valuable screen space on my netbook's 10-inch screen.

Of course, the software is aimed primarily at hams, so that the transmit controls are also on screen, but since I only monitor the stations, it sure would be nice if I could hide the transmit functions. This would allow for a taller spectrum display, along with more room for the message text and fax images.
 

simpilo

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4317.9khz USB 0200/1400 GOES IR TROPICAL SATELLITE IMAGE (NEW ORLEANS) Decoded before this posting.
I tune to center frequency then shift the BFO watching the TONE move on top of the third line. I find I have a better signal. There is noise on this image.. My antenna isn't optimal for 60m utilities. It works. Much better image than the first satellite image I posted. Slight slant but not enough of a slant to be concerned about.
 

Attachments

db_gain

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For black and white fax you want to use the bw decoder if available, for ir images you want the greyscale decoder.
 

ka3jjz

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GB46

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MultiPSK is not totally free; some of the modes work for only a few minutes then shut down, but that's not true of all of them.
That applies to certain features within the free modes, too, but luckily none of those features matter to me.

For the last couple of days I couldn't use MultiPSK, because my computer's mike input suddenly stopped picking up any audio from the receiver's line-out jack. I plugged my headphones into that line-out jack and was able to hear the audio from the radio, so I assumed (wrongly) that something in the computer was amiss. After going through all of the computer's audio settings, the built-in mike could be heard, but nothing that was input via the mike jack. Today my internal light bulb finally lit up and I tested the patch cord with a multimeter. There was no continuity between the center conductors at either end, so I'm off to buy a new patch cord. Man, it's been several years now of equipment failures, even though I always handle my stuff with extreme care.
 

spongella

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Few years ago I paid for the registered version of MultiPSK, well worth the forty dollars I paid back then. As KA3JJZ mentioned some of the options on MultiPSK will only work for a few minutes, then you'd have to pay for the registered version to get full-time use of certain modes.
 

majoco

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Personally I wouldn't use a microphone to capture the audio at all - hopefully your radio has a headphone socket which you can connect to the 'line in' socket on your computer - this prevents all the room noises affecting the reading. A'line out' socket on the receiver is even better as it's (not usually) controlled by the volume control and it's the right level for the computer input. Often it had better fidelity and less noise as the signal has not had to go through the audio amplifier.
 

GB46

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Personally I wouldn't use a microphone to capture the audio at all - hopefully your radio has a headphone socket which you can connect to the 'line in' socket on your computer - this prevents all the room noises affecting the reading. A'line out' socket on the receiver is even better as it's (not usually) controlled by the volume control and it's the right level for the computer input. Often it had better fidelity and less noise as the signal has not had to go through the audio amplifier.
I'm not sure if you were responding to my post, but I wasn't trying to use an actual microphone. The microphone I was referring to was the built-in mike in the laptop's lid; I was just using that to see if the microphone was working, as the line-in jack, which I mistakenly called a mike jack, won't work unless the microphone function is enabled in the audio settings. The built-in mike is automatically disabled if anything is plugged into the line-in jack. Anyway, a faulty patch cord was the culprit, and a new one solved the problem. It could have been worse, because a broken connection at the line-in jack or elsewhere would have been nearly impossible to repair in this tiny laptop.
 

majoco

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Way back when you had to build your HF Fax decoder with a modem using XR2206 RTTY IC's you tuned the decoder to 1500 and 2300Hz, the 1500Hz is black and the 2300Hz white. Your HF receiver had to be very stable and you had to make sure that the BFO was right on frequency too. My first computer, a Commodore 64, had a modified RTTY dot matrix printer that did a reasonable job but keeping the picture in sync was the hard part - everything had to be well warmed up to keep the 120 lpm from slipping or slanting as was all in "real time" - no storage in the C64! Thank goodness for modern software!
 

SDRPlayer

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wow! i still have my '64 Martin and i never considered any sort of HF decoding. As kids we only thought of games. I'm wondering if i should dig her out and give it a go for laughs and old time sakes?
 

GB46

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My first computer was a Radio Shack TRS-80 with a "big" (!) 8kb of RAM, for which I bought another 8kb as an extension. This was in 1978. There was obviously no possibility of FAX decoding with that computer! During the 80s I bought a little standalone code reader with a one-line, scrolling display; again, no FAX, but it could decode RTTY and CW using the receiver's audio output. The only thing I recall having decoded with that was KCNA, North Korea's central news agency in 45-baud RTTY, and VAI, Vancouver Coast Guard Radio in CW. VAI has since been decommisioned. I doubt if KCNA still broadcasts in RTTY; no great loss, as I can still hear their propaganda in AM mode on the international broadcast bands.
 

ka3jjz

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KCNA is still on the air, but very rarely, and uses a 600 baud format that a lot of inexpensive decoders can't handle. No great loss, really, but it's cool if you happen to catch them on the air. I would imagine it would make for entertaining reading...Mike
 

GB46

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KCNA is still on the air, but very rarely, and uses a 600 baud format that a lot of inexpensive decoders can't handle. No great loss, really, but it's cool if you happen to catch them on the air. I would imagine it would make for entertaining reading.
It would probably have everything I hear from them in AM mode, except for their pompous, militaristic sounding music, much of which has a distinctly Russian flavor (no surprise there). :rolleyes:
 
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