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Author Topic: Getting started with SSTV  (Read 9973 times)
WAVEYDIPOLE
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« on: May 16, 2014, 07:20:40 AM »

Well the other day I discovered this thing called SSTV so I hooked up the 'tape' output on my DX390 to the Line In of the soundcard on the computer and installed MMSSTV. I tuned in to 14.230MHz and started picking up signals. Some were quite weak and faded in out out. Occasionally I picked up what seemed like a strong signals. Of course, I expected that the images would be grainy, but MMSSTV seems to decode only bits an pieces of images before it resets itself. The auto start often seems to 'kick in' rather late. The upshot is that the whole thing is rather unreliable.

Interestingly I have found the same with text modes such as PSK31. I can't get a reliable stream of characters, it just gets very garbled.

Do I need to use a TNC interface? For SSTV, does MMSSTV need tweaking somehow? I'm also told that a DSP box can improve digital signals. Would this help? I have been advised to see what I can do to improve my antenna and although I am very limited in terms of what I can put up, I'm also looking at getting an ATU and setting up a loft antenna of some sort. However, any further advice would be appreciated.

Thanks.
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G4IJE
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« Reply #1 on: May 16, 2014, 11:30:14 AM »

MMSSTV works superbly straight "out-of-the-box" with everything at default settings. It does not need a TNC - and couldn't use one even if you wanted to try it. Any old soundcard should be fine too, so I would have to suspect shortcomings in your receiver. You do need to be tuned accurately to the signal - ideally within 100Hz. I am not familiar with the DX390 but if I have found the right thing on Google it looks like a general "shortwave" receiver. Do you have to adjust a BFO setting to resolve SSB? It would be important to set that exactly as you would to resolve a USB* signal. Some of those type of receivers that I have played with are a little unstable and if you get any kind of "warble" on SSB signals that might degrade your SSTV reception.

73, Paul G4IJE.

* As in Upper Sideband, not Universal Serial Bus ;-)
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WAVEYDIPOLE
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« Reply #2 on: May 16, 2014, 12:12:28 PM »

Thanks Paul. I made a mistake and actually meant the DX394. I also have a DX390 and got the two models mixed up. The fine tune on the DX394 is in 100hz steps and you can select between LSB and USB and even two CW modes. I have been using USB mode as I understand that is the mode one is supposed to use? As you say, the DX390 does not have the facility to select LSB or USB and relies instead on the BFO control which is trickier. The DX394 receiver has had several mods done to it to improve its performance, including the SSB mod which transformed SSB receiption from being almost unintelligable to something that is actually very clear and much better than my DX390 or the Tecsun PL660.

Interesting that a TNC would not work. I based my assumption on the information I found here:
http://www.essexham.co.uk/sstv-the-basics
As far as I can tell, the TNC appears to be just a soundcard to USB converter anyway with perhaps some gain control added although some of them combine CAT facilities which I don't require.

If MMSSTV should work with 'any old soundcard' then clearly the problem lies with the original signal, and hence the receiver or antanna. I do also have a high quality external sound card which is likely to be much lower noise than the cheapo one built into the PC, so I might give that a try.

Incidentally, I tried out Easypal on 14.233, but so far no pictures. I did notice a cuirious thing in that the waterfall would every so often display a filename embedded in the white noise which looked kind of surreal!
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G4IJE
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« Reply #3 on: May 16, 2014, 12:38:22 PM »

I was thinking of the original meaning of TNC - Terminal Node Controller - which is what we used to use for AX.25 packet radio (some still do!) and that was based on just two tones (1200Hz and 2200Hz if memory serves) which would not be any use for SSTV. Nowadays I guess the term is sometimes used to me a sophisticated sound card interface which is used for a variety of modes. I have used one of those $5 (or £5) USB sound card dongles with MMSSTV and they work great, so a high-quality sound card is certainly not essential. To receive SSTV you only need a connection like the one you have, from the receiver audio to the sound card input. The fancy interfaces usually offer transformer isolation of the TX audio to prevent problems caused by ground loops.

Writing that last sentence got me thinking about whether you could possibly have some kind of ground loop causing hum on the received audio. Depending on your sound card and PC OS, you should be able to monitor the receiver audio through your PC speakers. Does it sound clean and free from hum or other noises?

Yes, you are right about using USB mode on 14.230MHz.

I have toyed just a little with EasyPal and have seen those callsigns in the waterfall display. If you're getting those you may not be too far from receiving pictures.

73, Paul G4IJE.
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WAVEYDIPOLE
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« Reply #4 on: May 16, 2014, 02:33:42 PM »

Well that make sense. The older TNC stuff that I have seen is usually serial port connected and related to packet radio. I guess the term TNC has been ported over to newer USB technology, although its actually different connectivity and usage.

I can monitor the sound through my PC speakers. I did have a cable that produced a hum which turned out to be a ground fault. Somewhere along its length, the shield was broken and I was getting no conductivity from end to end. The inner core was fine. I ended up throwing it away and constructing another one. With the one I currently have, I am not getting any hum. I did discover that I get interferance from Powerplugs on 14.400 upwards to about 17.000. I can't hear it on 14.230, but that is still quite close. Could this interferance be a factor even although I cant audibly hear it?

I noted a reference to transformer isolation on the ZLP website. I presume a 600ohm 1:1 transformer would suffice for such a purpose.

Yes, I noticed callsigns in the watefall as well but the the two I took particular not off had the letters WAV so I assumed it was a reference to a .wav file... But I also saw callsigs. Unfortunately no pictures.

I was playing with PSK31 tonight and received a few good signals using Digipan and Fldigi. The latter seemed to be better as I could focus on one signal. Was interesting to receive signals from various parts of europe. Once that particularly fascinated me was from DK0WCY as its astronomy related, which is an interest myself and my wife share and which puts out signals in PSK31 and CW and Fldigi had no problem decoding either.

This at least demonstrates that I can receive digital signals on my setup, but maybe images requires a better signal?

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G4IJE
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« Reply #5 on: May 17, 2014, 04:48:20 AM »

MMSSTV does a good job on weak, noisy signals, but you do need a moderately strong signal to get anything approaching a "noise free" picture. Isolating the audio should not be necessary on receive, but as you say, a 600 Ohm 1:1 transformer would do the job. Interference that you can't hear should not be an issue.

Keep at it and I'm sure you'll get some good results. Activity tends to be much higher at the weekends (contest QRM permitting) although so far today conditions on 20m seem to be rather poor.

73, Paul G4IJE.
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KC9YTJ
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« Reply #6 on: May 17, 2014, 05:04:02 AM »

Sounds like the signals you are hearing on 14.233 are Hybrid mode SSTV.  Check the EasyPal documentation for setup instructions.
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NA4IT
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« Reply #7 on: May 17, 2014, 06:11:34 AM »

If you are on 14.233USB, it IS EasyPal...

See http://www.kc1cs.com/
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WAVEYDIPOLE
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« Reply #8 on: May 20, 2014, 08:45:52 AM »

Well it would be nice to get noise free pictures but for now it would be nice to get a complete one! I'm listening on both 14.230 and 14.233, the latter, of course, being Easypal. I have only managed to capture one image on 14.230 over several hours listening so far and that one was very poor.... Getting something via Easypal would be even better as the pictures are digital and noise free.

On advice I have picked up an ATU and I will also add a balun at the feed to my 30ft (10m) random wire antenna. Although the coax feed to the radio is properly grounded via an earth rod, the antenna currently feeds directly into the co-ax which is not ideal. I have also picked up an MFJ748B for a relatively modest price to experiment with.



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KC9YTJ
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« Reply #9 on: May 20, 2014, 10:34:36 AM »

Did you look into setting up hybrid mode in EasyPal?

The short digital transmissions aren't the picture.  They're a URL pointing to where the picture is stored on the Internet.  Almost all, if not in fact all the pics I've seen on 233 are hybrid mode transmissions
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WAVEYDIPOLE
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« Reply #10 on: May 21, 2014, 02:53:59 PM »

Well I picked up quite a few images tonight via MSSTV on 14.230. I actually had to tune to 14.2298 to get the best and most consistent reception. I am in the UK and most were from Europe (Italy, Spain, Slovenia, UK and Portugal) but I even got a partial one from Japan! Not sure whether the balun made the difference or whether we have good propagation tonight.

Unfortunately I got nothing usefull on Easypal on 14.233 though. I did pick up a conversation at one point but no call signs or pictures. I did have a look at Hybrid mode, but I'm still not sure how to set it up as although I found at least 3 documentation links, they describe the options but do not not really explain how to use the program. Is it sufficient just to select 'Hybrid' from the Hybrid menu or is there something else I must do? I'm also a little confused about the sound card setup as the check mark does not stay on 'Sound Card' when I select it. It always goes back to 'Use Com Port (PTT CAT)'. One of the documentation pages I found actually says to use the VOX option, but this does not seem to relate to the sound card either?

Incidentally, I read that Easypal uses DRM encoding, but it was my understanding that to receive DRM you have to use a downconverter to convert the IF frequency to 12kHz. Do you have to use a downconverted for Easypal, or can you just output the audio into the PC sound card?




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N0SYA
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« Reply #11 on: May 23, 2014, 07:27:10 AM »

Easy is soundcard and uses parts of drm to do its magic.
For decoding easy you simply pass the rx audio to the soundcard and tell easy what soundcard to use if more than one is seen by the pc.
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If you have a clumsy child, you make them wear a helmet. If you have death prone children, you keep a few clones of them in your lab.
WAVEYDIPOLE
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« Reply #12 on: May 24, 2014, 04:51:43 AM »

Thanks N0SYA. I found some more documentation that explained things a bit better and I think I've sorted the sound card setup and I beleive it is using my soundcard, otherwise I would presumably not be getting call signs in the waterfall.

I also have 'Hybrid' checked under the Hybrid menu, but still nothing unfortunately. It does say that the audio levels are critical, but the signals - when received - are short and infrequent bursts of activity so that it is difficult to do any meaningful trial and error tuning. Is there a trick or method to getting the audio levels right?
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N0SYA
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« Reply #13 on: May 24, 2014, 07:30:00 AM »

As to audio levels easy is forgiving, as long as there is some shown on the easy level meter you should be ok. On the hybrid mode, you're connected to the web when a hybrid sig is rx'd, right?
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If you have a clumsy child, you make them wear a helmet. If you have death prone children, you keep a few clones of them in your lab.
NK7Z
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« Reply #14 on: May 24, 2014, 09:44:25 AM »

As an FYI there is a support group for MMSTV on Yahoo if you are not aware of that...  It is located at:
http://groups.yahoo.com/neo/groups/MM-SSTV/info
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Thanks,
Dave
Amateur Radio: RFI help, Reviews, Setup information, and more...
https://www.nk7z.net
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