KV9U wrote: > If I understand it correctly, the raised cosine pulses tend to be > more efficient with power, reduce the crest factor (Pactor 2 is under > 1.5), and perhaps make it easier to have a cleaner signal.
Raised cosine is, above all, less bandwidth greedy. > Just for clarification I have a question: Is QAM modulation a form of > ASK? It would seem so to me but I am not sure. Otherwise, what other > modulation forms fall into the ASK category? It may be seen as that. Just depend on what abstraction you make to reach that conclusion. FSK is a form of complementary ASK of two carriers....which is bad is pure ASK because one state is pure signal and another pure garbage (noise, etc). QAM can be seen as an ASK of four phases at a fixed amplitude. Using two quadrature modulators, you create four states keying them with (1,1) (1,-1), (-1, -1) and (-1,1). 1 is the same phase, -1, reversed phase. Combine them and you get a constellation with points every 45 degrees. off the XY axis. > Although the SSTV modes are not automatically adaptive, there is a > limited choice of number of tones, but for the most part I believe > that they have found 16QAM to be about all you can get to work well > on many HF circuits, particularly on the lower frequencies. It depends on the signal to noise ratios. There is a video presentation of Doug Smith on Georgia Tech about Digital Voice which is pretty illustrative. Some Googling should find it. It shows the constellations and the effects of noise on it. The more complex the constellation, the less distance there is between constellation points, and so, less leeway for noise before confusing the decoder. > I wonder how a 4-QAM mode compares to say a 4-PSK mode when up > against the ionosphere? Should be about the same... > There must have been a reason that DRM uses QAM instead of PSK? Any > thoughts on why? When you need a modulator for 64QAM for the MSC, it is rather easy to create 4QAM with the four extreme points of the 64QAM constellations. > In terms of coding, it would be very interesting is to compare two > multitone modems, perhaps a 2 tone and an 8 tone (similar to pactor 2 > and 3) and have one with R-S and one with Viterbi and see if there > is any difference on various circuits. Pactor II and III use both Viterbi decoding and block encoding with interleaving... That is not the test that needs to be done. The difference between P2 and P3, is that P3 stays with the most robust and capable constallation, 4DPSK, and starts deploying carriers using it. The coding tricks are about the same. What I don't know so far is how does it distribute the traffic among the carriers. Jose, CO2JA
