Jose, Thanks for your comments and sorry I missed you yesterday.
> I have doubts on this department. If the peak amplitudes are the same, > as may be happening with the audio tests, the decoding on those > conditions should be equally valid. The audio for each mode was normalized during the path simulation tests so I would assume the peak amplitudes were the same. > Vojtech points out that MT63 is more sensitive to distortions which are > pretty common with some not so careful operators. I'm glad Vojtech mentioned this. I think some may be overdriving their rigs unintentionally with MT63 by not taking the peak power ratio into account. > I did not find any details on my references, but seemingly interleaving > is done both in the time and frequency domains, so there is more chance > for MT63 to get the message thru, specially with long interleave. Certainly seems that way -- hard mode to beat when it comes to static crash resistance. Thanks for all Jose... Tony, K2MO ----- Original Message ----- From: "Jose A. Amador" <[EMAIL PROTECTED]> To: <digitalradio@yahoogroups.com> Sent: Saturday, October 11, 2008 7:43 PM Subject: Re: [digitalradio] Contestia 1K vs MT63? > Tony wrote: > >> Patrick, >> >> I get the same minimum SNR for Contestia but can squeeze -8db out of >> MT63 when using DM780 and IZ8BLY. > > Yesterday I had no luck with DM780 while monitoring Tony's QSO's on > 14106. Of course, I have not calibrated DM780, so that is no surprise. > Propagation was not good, but MultiPSK did fairly well. > >> The threshold difference shows on-air as well as under controlled >> conditions and so it would seem that the best way to get the most out of >> MT63 is to use software that decodes deeper into the noise. > > No doubt... > >> The 10-to-1 peak-to-average power ratio is an excellent point and >> it's obvious that Contestia will put more RF into the air on average. >> There's no doubt in my mind that the Contestia 16-1K will do better most >> of the time. > > I have doubts on this department. If the peak amplitudes are the same, > as may be happening with the audio tests, the decoding on those > conditions should be equally valid. Of course, Vojtech points out that > MT63 is more sensitive to distortions which are pretty common with some > not so careful operators. Fast attack slow decay ALC could have a chance > of correcting some of the overloads after the transmit IF, but the rest > of the chain, from the audio input to the IF should receive proper audio > levels. I have seen rebel cases of distorted PSK-31 when people closes > the mic gain and distortion remains... because the early stages of the > transceiver are already overloaded, and I had a real bad luck when > explaining that to the other operator. People should know their way > around... > >> On the other hand, it does not seem to recover from the complete >> drop-outs that occur during deep fading or with lightning static the way >> MT63 does. > > I was browsing my references this afternoon (local) and I I decided not > to send a reply, since it seemed that Vojtech had a good point and was > not worth arguing about it. Nevertheless, I wonder how the degradating > effect of -30 to -20 dB IMD, the usually accepted values when adding > that to the channel noise. > > Even more when I read that Contestia was devised with a flat envelope on > mind (nonlinearity does not affect it) and uses about the same > Walsh-Hadamard code. > > But it _might_ mean, conversely, that Contestia is more power greedy, an > important consideration for emergency operation. > >> I've tested this theory by removing short 1-to-3 second segments of the >> signal at random intervals and the mode continues to print despite the >> missing 'chunks'. As you say, this could be due to the difference in >> modulation speeds. Is there an alternative mode that I can test that >> might have similar characteristics? > > I did not find any details on my references, but seemingly interleaving > is done both in the time and frequency domains, so there is more chance > for MT63 to get the message thru, specially with long interleave. On the > other hand, if someone "pulls the carpet" (heavy doppler) there is a > risk that MT63 will fail strepitously with bits falling on the wrong > bins while a mode with less, wider frequency "bins" (like Contestia or > Olivia) will really shine. > > I had really a low esteem for MT-63, but it had been hard to make a > MT-63 QSO before Tony started the present tests campaign. It just > happens that each mode should be used according to its most promiment > strengths. > > I still have low steem for Chip-64... 8-) > > 73, > > Jose, CO2JA > > > > >
