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.
> >
>
