On Nov 22, 2013, at 7:40 AM, Ben Antanaitis - WB2RHM wrote:
> IF it ain't broke, don't fix it. Put the super speed/data volume 2.8KHz
> signals in the band segments where 2.8KHz BW is already the 'standard'.
Ben,
While I too believe that the ARRL petition has no merit, if the petition passes
FCC's rule making, the situation is really not that much more dire than it
already is today. Unless of course if sub-bands stop being segregated by modes
and bandwidths.
Personally, I really don't care if a 3 kHz wide signal is QRMing a 2.2 kHz wide
signal in the subbands that 2.2 kHz signals already use. My concern is that a
3 kHz signal is allowed to intermingle with a 300 Hz signal.
Today, we already have 2.2 kHz signals intermingling with 300 Hz wide signals.
As happens often nowadays, when you answer an RTTY CQ, a loud Bzzzzt comes back
at you instead.
Not all the Bzzzzt are 2.2 kHz wide either, some of them are only 500 Hz wide,
but at a high symbol rate; that is why they sound like that -- you hear a
similar sound with fast RTTY or PSK125, for example.
Perhaps I can use this opportunity to explain the symbol rate stuff (the gist
of ARRL's petition) to the general audience of the reflector, and how it
relates to HF propagation.
Symbol rate (a.k.a. baud rate) is NOT directly related to occupied bandwidth.
Please don't make that mistake in your RM comments. Your comments might
otherwise be diminished if the ARRL lawyer convinces the Commissioners you
don't understand the technical issues.
Again, let me repeat -- Symbol Rate is not directly related to bandwidth.
In general, it is directly related to bandwidth only when 1 symbol is equal to
1 bit -- as in the case of BPSK31 and amateur RTTY.
(For lack of better term, I use "amateur RTTY" to mean "2-tone 170 Hz shift FSK
at 45.45 baud, with Baudot encoding" -- that, or my more affectionate term
"steam RTTY" for it :-).
"Symbol Rate" is just a fancy way of saying how many times you switch
modulation in each second. And it is expressed in units of "baud" (symbols per
second).
MFSK16, DominoEX and Olivia for example uses many tones (a weird number like 18
tones for DominoEX, even) to achieve better performance. These signals occupy
500 Hz or more (but not for the sake of higher data rates).
If you look at DominoEX's and MFSK16's philosophy (both created by ZL1BPU) they
are designed with the understanding of HF channels by purposely making the
symbol rate very *slow.* And to make up for it, so it can achieve a practical
typing rate, more tones are used (thus, more symbols), and that therefore uses
more bandwidth.
The reason why a lower symbol rate is desirable is that when HF conditions
become poor, the RF signal becomes smeared in time (visualize a rectangular
pulse whose rise and fall times no longer have sharp skirts). This smearing
comes both from multipath and (more so) from what is called Doppler Spreading
by the ionosphere.
To overcome the smearing, you would want your symbols to change at a rate that
is *slower* than the smearing. Guys like ZL1BPU and G3PLX (the creator of
Amtor and PSK31) really do understand the ionospheric effects.
This is contrary, for example to the ARRL petition claim that higher symbol
rate somehow is more "modern" and by implication, more useful. It is neither.
Thus, in the case of MFSK16, DominoEX and Olivia, we have the case of actually
quite slow symbol rates (much slower than RTTY), but the signals have
bandwidths a that are wider than RTTY, and they also, unsurprisingly, beats
RTTY through poorer propagation conditions (RTTY standards were created before
HF propagation became more understood).
The modes above are the correct way of applying symbol rate tradeoffs under HF
propagation (i.e., go to lower baud rates). And that is why I have been saying
that there is no *need* to have higher symbol rates.
You can already, today, keep the symbol rate at 45.45 baud and create a 128
tone FSK signal that occupies many kHz of bandwidth. There are no rules
against that (since the symbol rate is under 1000 baud). You are only limited
by bandwidth rules from using something like that.
The relaxation of Symbol Rates only serves to allow certain modulation methods,
which are not legal today, that happens to also be very wide.
73
Chen, W7AY
_______________________________________________
RTTY mailing list
RTTY@contesting.com
http://lists.contesting.com/mailman/listinfo/rtty
|