In n-ary FSK, if all tones in the ensemble have identical maximum magnitudes, then isn't it true that the maximum bandwidth will be identical that of binary (2-tone) FSK with a shift whose value is difference in frequency between the highest and lowest tones in the ensemble?
73, Dave, AA6YQ --- In digitalradio@yahoogroups.com, "expeditionradio" <expeditionra...@...> wrote: > > Hi Andy, > > There is no simple universal relationship between > the "shift" and the transmitted "signal bandwidth", > because there are so many factors other than "shift" > that contribute to the bandwidth of an "FSK" signal: > > 1. Symbol rate > 2. Shape of waveform > 3. Symbol transition point > 4. Filtering > 5. Number of tone frequencies > 6. Transmitter chain > 7. Other factors related to modulation process > 8. Noise > 9. Transmitter oscillator spectral purity > 10. Definition of bandwidth > > This is an especially complex calculation for > multiple frequency FSK signals, commonly > 4-ary FSK, 8-ary FSK, 16-ary FSK, 32-ary FSK etc. > where the number of shift frequencies is greater > than 2, or the number of carriers is greater than 1. > > The FCC rule says "maximum frequency shift of > 1 kilohertz between mark and space." But, > that FCC rule was written in the old days when > common ham RTTY was Frequency Shift Keyed between > only 2 frequencies, technically described by > "mark and space". However, in modern multiple > tone frequency shift techniques, with binary > symbols there is no such thing as "mark and space". > Thus, the rule became inapplicable to the new > multiple frequency shifting keying modes. > > When the FCC was asked to convert from "shift > limit" to "bandwidth limit", the FCC refused, > and at the same time, FCC said it had chosen not to > limit bandwidth because it is important for > ham radio to have the freedom to innovate and > develop new techniques. > > Thus, the mark and space shift limit became a > mere footnote in history that largely does not > affect most modern digital techniques used in > ham radio today. > > If you wish to delve into the finer math points > of relationship between bandwidth and shift, may > I suggest reading Section 6 (starting on page 37) > of this fine document: > "Necessary Bandwidth and Spectral Properties of > Digital Modulation" by David J. Cohen: > http://www.its.bldrdoc.gov/pub/ntia-rpt/84-168/84-168.pdf > > 73 Bonnie KQ6XA > > > Andy K3UK wrote > > -Bonnie, can you explain to this bozo what the > > difference between a "shift" restriction and > > "bandwidth" restriction would be? My brain > > viewed them to be the same, that is that a > > 170Hz shift would be roughly that amount > > of Hz wide at the usual ham speed. > > >