[UK-CONTEST] RTTY Bandwidth

Mon Jul 25 11:21:55 PDT 2011

Chris

Baud rate, shift and propagation effects is a  very interesting subject 
in its own right. I agree 75 is often counter productive at HF 
especially on the lower bands. Having a narrower shift is a trade off. 
Less receive bandwidth reduces significantly the probability of QRM and 
substantially increases the probability of finding a clear frequency to 
run on. A prime example would be SSB going to CW.

Narrower shift means for those few FSK decoders that decode each tone 
separately and combine  them become less effective against frequency 
selective QSB. By the way the popular MMTTY is not able to copy on a 
single tone signal which is what happens with frequency selective QSB. 
However if we want to get more run frequencies per KHz reducing shift is 
the obvious way to go similar to 2m FM went from 25KHz to 12.5KHz 
channels to reduce congestion.

Did you know we may transmit FSK (or AFSK) but most decoders receive 
each tone through a very narrow filter (circa 50Hz) then compare the 
amplitude between the 2 tones to get the 1s and 0s. If the TXs used well 
filtered differential ASK (Amplitude Shift Keying) with 170Hz shift it 
is possible to interlace 2 RTTY transmissions 85Hz apart and not suffer 
QRM. This is because DASK has significantly less energy between the mark 
and space tones than FSK. Now that would produce some interesting 
arguments about whose frequency it is.... And it is not difficult to 
generate either using PC software and audio into the rig, i.e replace 
AFSK with AASK.

73 David G3YYD

On 25/07/2011 17:19, Christopher Plummer wrote:
> David,
>
> This is all very well, however, real band conditions include all sorts 
> of odd birdies, and overlapping signals, so the theory breaks down.  
> Incidentally 45.45Baud RTTY rate with 170Hz shift is about the 
> optimum.  Just looking at the repeat rate working at 75Baud generally 
> means the contact rate is about the same as 45.45Baud.  As regards 
> narrower shift, the same applies, error rates are much higher and the 
> effects of interference are much worse.
>
> If you take your points to the limit we would use extra slow rates 
> with less than 10Hz shift, just like the US submarines.  We would then 
> decide a 90 minute contest session on who actually manages more than 
> one contact.....
>
> From experience its all been done before.
>
> Chris
> BARTG Awards Manager
>
> > Date: Mon, 25 Jul 2011 16:19:08 +0000
> > From: g3yyd at btinternet.com
> > To: UK-Contest at contesting.com
> > Subject: [UK-CONTEST] RTTY Bandwidth
> >
> > Rob
> >
> > RTTY occupied bandwidth is like CW bandwidth depends how hard the 
> keying
> > is and how strong the signal is. With little to no filtering then the
> > bandwidth will be measured in KHz for a strong signal. A strong signal
> > with good TX filtering will occupy about 400Hz of bandwidth. Then it
> > depends how good the RX filtering and performance is. In high activity
> > RTTY contests I normally use RX bandwidth of 250Hz at -6dB with around
> > 350Hz at -60dB.
> >
> > With a +/-200Hz TX signal bandwidth and a RX +/- 150Hz (about -30dB 
> down
> > side of filter then PC software filtering. I can copy the next signal
> > QRM free at about 350Hz spacing between signal centre frequencies.
> > Before you think I have got my maths wrong remember it is TX to RX that
> > counts and not TX to TX adjacent TX clicks can overlap. Of course if 
> you
> > are using a FT897 with SSB filters you will have a problem, both in
> > filtering and dynamic range of the RX.
> >
> > Some TX signals are a lot wider and therefore 350Hz spacing is not
> > possible for all TX/RX pairings. Penalty points for transmitting wide
> > signals would be an excellent idea and with SDR technology easy to
> > provide proof. With today's very stable rigs and software RTTY could
> > amend rules to a narrower shift, 85Hz?, has to be used. Or could go to
> > Minimum Shift Keying (MSK) at baud rate/2, i.e just under 23Hz. 
> Reducing
> > the shift to 85Hz would reduce occupied bandwidth and adjacent channel
> > spacing comes down by 85Hz.
> >
> > Incidentally the minimum occupied bandwidth for a RTTY FSK signal 
> can be
> > very small compared to current practice. Minimum shift of baud rate/2
> > Hz coupled with optimum TX filtering gives occupied bandwidth of shift
> > plus baud rate in Hz. For 45.45 baud this is 22.73+45.45=68.2Hz say
> > 70Hz. Allow a bit extra for signal spacing could have 10 run 
> frequencies
> > per KHz instead of less than 3 for the current practice. 75 Baud would
> > occupy 102.5Hz with a bit extra say 8 per 10KHz.
> >
> > Of course it will never happen as it is always easier to preserve the
> > status quo. In the "RadCom Letter" subject heading on the reflector 
> much
> > has been written about contesters pushing the technology envelope so 
> may
> > be that is what we should do. We want (need?) more run frequencies per
> > KHz so why not change the RTTY shift to a much smaller one? Given the
> > nature of contesting, it would have to be a rule rather than just a few
> > changing to their contest disadvantage.
> >
> > 73 David G3YYD
> >
> > On 25/07/2011 14:14, G4LMW wrote:
> > > It is worth mentioning that the main reason that PSK was 
> introduced was to
> > > help alleviate the overcrowding that was experienced with the RTTY 
> ONLY
> > > contests.
> > >
> > > Also, should there be a move to 75 baud, then each signal will 
> occupy 370Hz
> > > instead of 270Hz (happy to be corrected on my exact numbers).
> > >
> > > So, RTTY ONLY with 75 baud would be worse that ever!
> > >
> > > Regards
> > > Rob, G4LMW
> > >
> > > ----- Original Message -----
> > > From: "john reynolds"<g3rsd at btinternet.com>
> > > To:<uk-contest at contesting.com>
> > > Sent: Monday, July 25, 2011 2:57 PM
> > > Subject: [UK-CONTEST] UK Contest data modes in CC
> > >
> > >
> > >> I believe CC was intended to encourage and train operators in all 
> modes of
> > >> on air communications used in contesting.
> > >>
> > >> Before the CC's I had never used Ry or PSK.
> > >>
> > >> Probably because of my age,it took me a long time to configure 
> and use
> > >> Data Modes and it was thanks to other local Amateurs that I am now
> > >> comfortable with all modes.
> > >>
> > >> I do not think DATA should be dropped from these Contests, but it 
> could be
> > >> modified!!
> > >>
> > >> May I suggest that on alternate Months RTTY only is used and it 
> be left to
> > >> the individual to use whatever speed they wish to achieve maximum 
> points.
> > >>
> > >> The following Month PSK would be the mode and here again speed is 
> left to
> > >> the individual.
> > >>
> > >> May I also point out that a close "Observer" who has suffered 
> Contesting
> > >> for Years considers that the "SHOUTING" mode should be dropped 
> ...........
> > >> it is most unsociable around the house!!
> > >>
> > >> John (JR) G3RSD
> > >> _______________________________________________
> > >> UK-Contest mailing list
> > >> UK-Contest at contesting.com
> > >> http://lists.contesting.com/mailman/listinfo/uk-contest
> > >>
> > >
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