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.
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? 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. I wonder how a 4-QAM mode compares to say a 4-PSK mode when up against the ionosphere? There must have been a reason that DRM uses QAM instead of PSK? Any thoughts on why? 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. Does anyone have information on this already? 73, Rick, KV9U Jose_Angel Amador Fundora wrote: >> >>We know from Pactor 2, that a raised cosine shaped pulse is likely a >>very good basic waveform. >> >> > >That is for saving bandwidth, mostly. It might allow better decoding, as well. > > >>Pactor 3 is runs an occupied bandwidth of about 2.4 kHz, but raw speed >>is over 2700 bps. Instead of 2 tones, P3 uses up to 18, separated by 120 >>Hz and modulated at 100 baud DBPSK or DQPSK. >> >>SCS has some fairly detailed data on Pactor 3 at: >> >>http://www.scs-ptc.com/download/PACTOR-III-Protocol.pdf >> >>I wish someone could explain why we can not have a sound card mode that >>is roughly the same as Pactor 2 at least. Even if there was no ARQ at first. >> >> > >I don't know if the least complex of it all is ARQ...most likely, the rest is >harder to implement. > >Yes. A key requirement is having the highest distance between constellation >points to >have an edge against the noise (or QRM). That's why, in DRM, the FAC uses >4QAM, as it allows to send the reduced but very important info it conveys. But >the MSC must use 64QAM, because the amount of data to be sent does not allow >otherwise in the least bandwidth. > > > >>What happens if you use a multitone DPSK? It seems to a non-engineering >>person like myself, that a lot of what P2 and P3 are made up of are >>really a series of PSK100 or PSK200 tones (carriers). >>Isn't Q15X25 a similar modulation scheme? It even runs at 83.33 baud >>rather than a minimum of 100 baud such as P2. >> >>Why did it not work as well as P modes? >> >>Or is it because it has no coding such as Reed-Solomon block coding or >>Viterbi convolutional coding? >> >> > >Certainly...all those tricks add up, and most likely, in a non proportional >way...I cannot assure it by heart, but is very likely. One of the gains of the >code used in pactor modes is >using convolutional encoding with Viterbi decoding. The Viterbi decoder, >knowing the history >of what has been sent, as the convolutionally coded stream depends on what has >been sent >previously, makes a soft decode of what is the most likely symbol transmitted. >RS coding, after deinterleaving, on the other side, may allow to recover erors >WITHOUT retransmission, which may save more "bandwidth" than what is wasted on >the FEC overhead. > >Also, P2 and P3 avoid the edges of the channel to have the least amplitude and >delay differences between carriers. That's why a "reduced" version of Q15X25 >is being more succesful >in holding the link. > > > >>73, >> >>Rick, KV9U >> >> > >73, > >Jose, CO2JA > > > > >______________ ______________ ______________ ______________ >Correo enviado por ElectroMAIL. Facultad Eléctrica. CUJAE Dominio: >electrica.cujae.edu.cu > > > > > > > > >
