--- KV9U <[EMAIL PROTECTED]> wrote: > I used to operate Amtor, Pactor I (and Clover II) a > decade or two ago, > when we had the old Aplink and later Winlink > systems. But at the time I > did not have a good idea of how deep into the noise > these modes could > work. Is it possible for some of you folks who have > Pactor (II and III > as well, if you have that) to determine how deep > into the noise they > will work until the link slows to unusable levels > and compare it with > the newer sound card modes that can handle the same > conditions? > > I do know that when I used to try and connect on > Clover II with Ray > Petit, W7GHM, we would have what I would call today > a moderately weak > signal and not have any throughput, yet now it would > easily be able to > print with Olivia/MFSK16/DominoEX/FEC, etc. Amtor > and Pactor would quite > working even though you could still hear the tones. > One of the numbers I > have heard is about -5 S/N where the Pactor modes > fail. On the other > hand, I recall SCS making the claim that it could > operate much lower in > the noise than that. > > Real world testing like some others have been doing > with DominoEX et al > would be very helpful. > > 73, > > Rick, KV9U
Actually, with bad S/N ratios, thruput suffers a lot. I have been checking at http://www.scs-ptc.com/pactor.html before sending this reply, and there is some info, but I cannot find the graph comparing several modes up to P-II, that I think was published in QST in the late 90s. P-I with memory ARQ holds the link down to -5 dB (in a 3 kHz bandwidth, SNR in pactor´s bandwidth is BETTER than that). P-II holds the link down to -18 dB, but that´s all, thruput is almost nil. P-III shines at some 20 dB SNR, because with more complex constellations, the distance between the constellation points diminishes, and is less noise tolerant. Interleaving, compression and convolutional encoding with Viterbi decoding all that helps a lot, but there is a big distance between "telegraphy" and "telepathy"...that is, digging for signals down into the noise. After all, the most succesful newer digital modulation schemes are using some of the tricks that have made P-II and P-III successful, but there are physical limits that cannot be violated. Specially some forms of FEC, block codes and interleaving are helpful to avoid signal degradation due to QRM, fading and multipath, and data recovery without repeats thanks to block codes. But it carries a price tag, the more noise and QRM resistant a mode is, generally implies more latency, that is, makes it less "keyboard friendly". 73 de Jose, CO2JA __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
