Hello Vojtech, This problem of crest factor and linearity exist too with the Video ID (the one you see appear on the waterfall).
If I send "Contesta 16-50", due to the high number of carriers (CMT-Hell) I am going to have a crest factor equal to 1/(square root of n) so very small, so the mean power transmitted will be small. If I increase too much the output power, the signal will saturate, each carrier will create harmonics and the ID will be not readable. So the mic gain must be adjusted on the main mode (Contestia in the example) and not on the Video ID. 73 Patrick ----- Original Message ----- From: Vojtech Bubnik To: digitalradio@yahoogroups.com Sent: Saturday, September 15, 2007 4:49 PM Subject: [digitalradio] Re: The decline of Olivia and DominoEX Hello Patrick. Thanks for your explanation. Do I understand your S/N figures well? Let's say I am using a 100 watts transceiver. If transmitting > * MT63 à 10 bauds: - 5 dB and 100 wpm, 1000 Hz bandwidth, crest factor 0.1 then I will be actually able to send energy equivalent only to 10 watts CW. If transmitting > * Contestia "16-1K": "Fast" 16 tones, bandwidth=1000 Hz, speed=62.5 bauds, 78.2 wpm, lowest S/N =-9 dB, crest factor 0.76 then I will be actually able to send energy equivalent to 76 watts CW. Then I am losing another 10 * log10(76/10) = 8.8dB and I may say, that with the same 100W transceiver MT63 will perform 12.8dB worse at 100 wpm than Contestia at 78.2 wpm? It sounds pretty bad for MT63. I think there is another issue with ODFM of high number of tones. It is highly sensitive to linearity of the whole transmitting and receiving chain. If not linear, the tones will mix together, which will further decrease sensitivity. In my opinion, MT63 was a useful experiment, that has shown a dead way. 73, Vojtech OK1IAK --- In digitalradio@yahoogroups.com, "Patrick Lindecker" <[EMAIL PROTECTED]> wrote: > > Hello Votjech, > > Yes you are right the crest factor in MT63 is very low (mean power/ crest power = 0.1 (OFDM general problem) for 0.76 in Olivia... > > Other problems: due to the big number of carriers (64) you need a good calibration of the sound card. Otherwise, you decode nothing. > The minimum S/N is not very good either and the latency time is big. > > If Olivia is too slow, another better option is to transmit in Contestia. For about S/N, it is only about 1 dB under Olivia but the speed is twice. > > Example: > * MT63 à 10 bauds: - 5 dB and 100 wpm, 1000 Hz bandwidth > * Contestia "16-1K": "Fast" 16 tones, bandwidth=1000 Hz, speed=62.5 bauds, 78.2 wpm, lowest S/N =-9 dB, > No problem of crest factor (0.76), small latency time and no problem of sound card calibration. > It's available in Mixw, Multipsk and perhaps Fldigi. > > 73 > Patrick > > > ----- Original Message ----- > From: Vojtech Bubnik > To: digitalradio@yahoogroups.com > Sent: Friday, September 14, 2007 12:45 PM > Subject: [digitalradio] Re: The decline of Olivia and DominoEX > > > > Question - what's so special about MT63 - where / when is it used? > > From my point of view, MT63 has high number of carriers, which implies > low crest factor - the effective transmitted power will be much lower > than of single tone mode like Olivia, if you make sure PA is not > overdriven by peaks where the amplitude maxima of multiple carriers meet. > > Some argue that the mode is fast, but are not there Olivia submodes > with the same throughput? > > 73, Vojtech OK1IAK >
