| porównanie skuteczności FT8 i CW |
| Witaj Piotr. Znając Twoją dociekliwość to pewnie przewertowałeś już cały internet. Fajnie zagadnienie opisane jest m.in tutaj: http://kf6hi.net/radio/SNR.html, ale pewnie to już znasz. Nie wiem jak można określić "na słuch" poziom sygnału cw. Przy niezakłóconym paśmie jest możliwość zdekodowania sygnału cw na podstawie zmiany brzmienia szumu. Ten sam sygnał w obecności zakłóceń będzie już niezauważony. Podobnie z sygnałem kilka dB powyżej szumu. Bardzo ładnie to widać na przykładzie zawodów cq160, gdzie pół Europy gnieździ się w pierwszych 15kHz pasma, żeby mieć nadzieję na "zrobienie" stacji JA. Przed zawodami jest to możliwe, w czasie zawodów graniczy z cudem. Fragment z podanej wyżej strony: Looking at Figure 2 we see for casual CW operation the signal can be 11 dB lower or -5 dB SNR for the same intelligibility as casual SSB. But what does -5 db mean? Doesn't that mean the signal is below the noise? How can you hear that! Remember this is relative to a 2500 Hz bandwidth. You would normally click in a 500 Hz filter and now the -5dB becomes 1.8dB. That's good enough to copy CW. Still too noisy for you? click in the 250 Hz filter and now you have SNR=4.8dB. Bogusław sp7ivo Dopisane: Conclusions The AMSAT ZRO Tests, in which several hundred amateurs participated in a controlled experiment over more than seven years, established that many good operators, approximately the top quartile of test participants, were able to copy by ear a sequence of five random digits at a key-down SNR of –3.6 dB in a noise bandwidth of 100 Hz, with a few (4%) able to reach –6.6 dB. The median participant required a SNR of –0.6 dB. Given the test conditions, these findings are considered to be accurate plus/minus approximately 3 dB. A study of the W2RS 144 MHz EME log from 1985 to 1995, when the author operated with 150W output to a single Yagi antenna, yielded fairly comparable results. The W2RS EME log also shows that in prearranged schedules, when operators know what they are listening for, contacts were completed with SNRs at least 3 dB lower than was possible in random operation. Only four stations could be worked on random, out of 37 worked in total. For a good weak-signal operator in a prearranged EME schedule, copy by ear down to –6 or –7 dB key-down SNR in a 100 Hz bandwidth, equivalent to –23 or –24 dB average at 2.5 kHz, would not be unreasonable to expect (again, plus/minus approximately 3 dB). Signal-processing techniques developed by SM5BSZ and WB9UWA may be able to improve upon the performance of the unaided ear by as much as 2-3 dB, depending upon the characteristics of the received signals. |