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#11
July 7th 05, 12:34 AM
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Michael Coslo wrote:
wrote: Michael Coslo wrote: wrote: Dee Flint wrote: Another method is of course to increase the ERP. So we can put up a directional antenna. And an amplifier. Better be a skweeky clean one! But now we're certainly a long way from simple, and many present day rigs and users wouldn't be able to participate in the fun. But all that is doable. After all, Morse Code has a big S/N advantage over even SSB voice, yet hams manage to make SSB contacts on HF. Morse Code simply does better under marginal conditions. Or perhaps we should say that its margins are lower. I wonder if any experiments have been performed by Amateurs along these lines to find a practical limit to how many different phase angles can be accommodated. Sounds like fun. The problem is in two parts: First there's the accuracy of the hardware, which is probably pretty good using modern parts and methods. But second is the distortion of the RF path, which is not under our control. If, say, the ionosphere causes the phase to wander a couple of degrees each way, modes like BPSK and QPSK may still work, but "256PSK" will be full of errors. Note how there are times when PSK31, even if the signals are loud, won't work in QPSK mode but will work in BPSK mode. That's not because of narrower bandwidth or some hardware or software change. It's because the path is introducing so much phase distortion (a form of noise) that the distortion exceeds the QPSK demodulation criteria. W0EX observed path-induced phase distortion that was so high that PSK31 wouldn't work even in BPSK mode, yet the PSK31 carrier could be heard clearly and seen easily on the waterfall. As K0HB points out, the 300 baud limit only applies in the CW/data subbands. If you can stuff "TV" into a reasonable bandwidth, it can be sent in the voice/image subbands. However, receiving it may be another matter... The baud rate was chosen because it is around the level that a good typist can type at. But it can be changed PSK100 baud easily, just sacrifice a bit of bandwidth. Exactly. The principle is what matters. The problem is that the transmitter and receiver must be very linear to avoid IMD products causing trouble. How can we do it? Bandwidth is directly related to baud rate. Only if "all else is equal". The trick is to make the tradeoff somewhere else. The familiar "56K" modem trades off S/N rather than bandwidth. It's not a complex subject at all. You've probably heard the old engineering adage: "You can have it fast, good or cheap. Choose any two" Same with light bulbs: Bright, long lasting, or cheap. I'd say "bright, long lasting, or efficient". Maybe, but you ought to see people choke when I tell them what their replacement bulb in a data projector is! Bright, long lasting and .5 kilobucks. pick out two... All Shannon's Theorem does is equate fast to data rate, good to S/N, and cheap to bandwidth. There's also the factor of error rate. In the above simplified discussion I assumed the same error rate for all cases. Obviously there are some situations where a higher error rate is tolerable. Error-correction can help, but error correction carries its own overhead, slowing down data rate. That it will. PSK *is* a delicate mode, and gets more delicate the more phase shifts in use. It's only delicate to certain kinds of disturbance. PSK has been the mode of choice for deep space communications for over 40 years because of its performance in a Gausssian-noise environment. Interestingly enough, it works pretty well at frequencies where there is more bandwidth available Don't forget that filters can cause phase distortion! A dramatic example of the effect of errors can be seen on TV. Conventional analog NTSC-type TV shows "errors" as "snow" and sometimes even loss of sync. But you can still watch a "snowy" picture. Digital TV methods often show errors as pixelation or complete loss - you see *nothing*. That brings up a useful analogy. When I got started in video, the portable work was being done with the old 3/4 inch U-Matic tapes. As time progressed, we shifted to formats like Betacam, S-VHS and the like. Eventually some interesting formats such as Hi-8 came out. The Hi-8 had a pretty decent video quality to it, and looked like it was going to revolutionize things. But there was a problem. I'm sure you are familiar with the way that modern video lays the tracks down on the tape - the record heads are at an angle, and there are at least two of them. Helical scan. Goes back at least 45 years... This way the tape is "striped", with the video laid on at an angle to provide more linear space with which to write the image. This was videotaping's bandwidth "cheat". And it works fairly well. What it does is to increase the effective tape speed. But the needed bandwidth didn't go away, and there was tremendous demand to make the tape smaller and more slender. So the fix was to lay the heads at an even greater angle, so as to compensate for the smaller tape. What may have been a 70 degree angle for the 3/4 inch tape might be 45 degrees for 1/2 inch tapes, and perhaps 22 degrees for the 8mm tapes. (note: approximate angles) A dropout - a fairly common thing - on U-Matic would just make a little white dot on the screen. On the 1/2 inch tapes, an entire line might be lost. On th 8mm tapes the entire signal might go away for a little bit. This is because that missing oxide will be cutting across a lot more of those stripes on the tape as it got smaller. At the same time, the tape media has improved, as have the heads and transports. But HF is still HF. 73 de Jim, N2EY |
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