Thanks for the information Chris. Very helpful and much appreciated. Happy Easter.
73, Tony, AD1Y --- In [email protected], "Chris Danis" <[EMAIL PROTECTED]> wrote: > > Hi all, > > My last message seemed to be modestly well-received, and since then I've > received / seen a number of questions regarding decoding JT65 signals, so > here's my stab at addressing those. Remember that I'm a "newbie" myself: I > haven't even been playing around for WSJT for a whole week! Certainly don't > hesitate to correct me if I say anything wrong, or to ask further questions. > > Anatomy of a JT65A signal > Although JT65A is always a MFSK mode, there are two fundamental types of > JT65A signals: the "full text" ones used to send CQs, exchange callsigns and > signal reports, or any arbitrary text, and the "shorthand" signals used to > send RO, RRR, and 73. Both kinds of messages have a "sync" tone and other > tones -- in the case of full text messages, many other tones, and in the > case of shorthand messages, just one other tone. However, the sync tone is > always the tone lowest in frequency, and in my experience with HF operation, > almost always between 1200 and 1300Hz (assuming you are tuned to the same > frequency as the transmitting station). (For the sake of completeness, I > should also note here that only one tone is transmitted at any given time, > and that transitions between tones are done in a phase-continuous manner.) > > Note on full text messages: arbitrary text can be, at most, 13 characters. > For those of you who have noticed that e.g. "K3UK N2YYZ FN12 OOO" is quite > a bit longer than 13 characters, JT65 has a special compression scheme for > common message components, like "CQ", "QRZ", "OOO", and callsigns, which it > uses in the standard exchanges. > > Decoding basics > Start up WSJT. Make sure "Freeze" is unchecked. Examine the SpecJT > window. You should see a long green bar at the top of your passband. This > green bar represents the frequency range that WSJT will search for signals > when it attempts a decode. That's right -- it attempts to decode anything > it can find in that range (as long as the sync tone falls within the green > bar, it should "just work"). This is a feature designed to make > compensating for EME Doppler easy. If there's a signal that you can hear, > or even that you cannot hear, WSJT should decode it. However, you will > probably need to increase your "Recording Settings" "line in"/"microphone" > slider to more than you use for other digital modes. The WSJT documentation > states that the "RX noise" figure should be around 0dB while you are > receiving just noise. I usually don't pump things up this high, but I do > make sure it's above -10dB (the associated status indication turns red if > this condition isn't met). If you can't use your soundcard's mixer to get > things this high, the rightmost slider at the bottom of the SpecJT window is > "Digital gain" and can increase audio levels more. > > (Aside: here's a point where I would appreciate feedback from others who > have been successful. How high do you usually turn things up? Am I wrong > here?) > > Also remember that you will not decode what you just heard until the full > minute has passed (I said 52 seconds in my last message; a more accurate > figure is something like 55 seconds). This is just due to the nature of the > protocol: each JT65A transmission is sent as a single FEC'd block, which is > one of the things that gives it the weak-signal abilities for which it is > known. > > More "advanced" decoding > There are a lot of knobs and buttons to play with on the WSJT decoder. I'll > start with the more important ones... > > Clicking in the SpecJT window (or on the red line plot in the main window) > will set the clicked frequency as the frequency of the sync tone. I'll > reiterate that -- when you click on something in the waterfall, it should > always be the leftmost sync tone! Once you've given WSJT a frequency value > this way, you can check "Freeze", which locks the search for the sync tone > down to within "Tol" Hz of the frequency you selected. You can vary Tol by > left or right-clicking on it. Watch the length of the green bar in the > SpecJT window change as you do this. > > (Cool tip: in the cases where multiple stations have both transmitted in the > same timeslot (with a slight frequency difference), I have often been able > to decode both transmissions by setting Tol to 10, clicking one sync tone, > decoding, and then clicking the other and decoding.) > > Once you've tuned a station, you should leave "Freeze" on. It gives you a > little more sensitivity, (I think) decreases decode times slightly, and also > makes the shorthand signals (RO/RRR/73) decode without question marks. Once > you've clicked on the sync tone of a station, SpecJT also shows you "helper" > red ticks on the frequency scale, that indicate where tones for RO, RRR, and > 73 will be seen (from left to right). > > "NB" is what you'd expect -- some sort of noise blanking. I've left it on, > but I'm not sure if it's actually necessary. "Zap" somehow attempts to > "zap" away receiver birdies. I usually keep this off. Enabling either > probably reduces sensitivity slightly. > > There's another feature of WSJT that is useful for very weak signals -- so > weak that they can't be decoded in one RX period. It has two slots (one for > the even slot, one for the odd -- I think; not sure on this) where it stores > averages of past signals. These are displayed in the two-line textbox below > the main textbox. Thus it is possible to monitor a "dead" channel for a few > RX periods, and suddenly see a signal "pop out of the noise" once you've > done enough averaging. To use this effectively, though, you must > occasionally clear the averaging buckets and allow new data to accumulate. > That's what the "Clear Avg" button is for. (I haven't actually decoded an > otherwise-unreadable signal with this functionality myself, but maybe one > day.) > > > I hope this helps, and that it explains more than it confuses. > > best & 73, > -chris N2YYZ >
