--- In digitalradio@yahoogroups.com, "Andrew O'Brien" <andrewob...@...> wrote: > > The replies to Ralph's question about audio levels appear to be sound advice > and certainly in keeping with what has been advised since sound card digital > modes burst upon the scene. I wonder how accurate it is though? I have > seen a few serious hams argue that "no ALC" is not really the case, that > some ALC can be OK. I have also seen mention that the no ALC issue applies > to some modes (like PSK) but not to others like (JT65A). I also wonder > about the half-power advice that some advise. With my homebrewed interface, > I could never get much above 40 watts before some ALC began to show. When I > switched to a commerical interface with good isolation (Microkeyer by > Microham) I can almost always get 100 watts output without any ALC action. > I have not received any negative reports about my signal . If I run 100 > watts SSB for phone contacts, why would I not want to do the same for > digital modes assuming the signal was "clean" ? . Yes, I would agree I > should not run 100 watts if communication was possible with less power, but > I don't think a brief PSK CQ at 100 watts is going to do much more harm to > my finals than a 3 minute ragchew at 50 watts, phone . Right ? > > Comments ? > -- > Andy K3UK
Andy, There are some interesting figures on this web site, http://f1ult.free.fr/DIGIMODES/MULTIPSK/digimodesF6CTE_en, about some of the technical details of the different digital modes. One interesting specification is the average to peak power ratio of the waveform. For PSK the number is 0.79. This means that increasing the average output power level above 79 watts will begin to show some clipping on a transmitter designed for 100 watts output peak. The average to peak ratio is a statistical average over some period of time. There may be some short periods of time when the peak signal exceeds this ratio and begins to exhibit some objectionable distortion that may cause broadening of the spectrum, increased IMD and reduced readability. For the FSK modes such as RTTY and for some of the MFSK modes the average to peak ratio is 1.0. For these modes you can transmit at the full power rating of the transmitter without causing any distortion of the signal. The effect of ALC action on the linearity of the signal will depend on the design of the ALC. If the time constant of the ALC is fast enough to follow the envelope of the signal then it effectively produces non-linear compression of the signal which causes distortion of the waveform. If the time constant is long then the ocassional peaks will cause some gain reduction in the TX chain but it will be mostly linear for everything less than those ocassional peaks. The error rate vs SNR for digital modes has a very steep curve with a sharp cutoff of readability only for constant steady state noise conditions. For HF conditions with QSB of 20 dB to 30 dB and static crashes, a 1 dB difference in average SNR may only mean the difference of 10% (or less) error rate or readability. So increasing the power from 40 to 60 watts (1.76 dB) would cause only marginal difference in the readibility. This is my somewhat simplistic understanding of the subject. Some of the figures given here are only qualitative examples based on my experience with testing digital communications system error rates under standardized channel noise and fading conditions. I made some PSK contacts last week runing 1/2 watt of power on 30m and 20m. Now I feel a little guilty about running 25 - 50 watts the rest of the time. Ed WB6YTE
