[SFDXA] Are You Frequency Confused with RTTY?

Wed Mar 7 12:02:41 EST 2012

Hi Bill,
You've complicated simplicity itself! But discussion on this is indeed 
needed. 

Ham-RTTY, that is, 170 Hz shift 45.45 baud signaling has an occupied 
bandwidth of just under 250 Hz no matter what way you generate the 
signal. See the definitions and methods of computing  occupied bandwidth 
for the various modes are in US Title 47 Code of Federal Regulations: 
2.201 -- 2.202, as well as in the international recommendations (ITU-R 
Recommendation SM.1138, 1995), which Part 97 references.  By "Terman" I 
presume that you means Caron's rules for determining approximate 
bandwidth, which is 2x(deviation + mod. freq). In ham-RTTY the deviation 
is +/- 85 Hz, and the modulation frequency is the 45.45 Hz baud rate, or 
2X(85+45.45) = 261 Hz. The FCC/ITU-R "occupied bandwidth" formula gives 
250 Hz. Close enough. This agrees with Terman and others. 

I don't care how you generate your ham-RTTY, but to do it in the way 
that complies with FCC/NTIA desires on the 60m band channels you need to 
use recipe I gave below so that you can monitor for upper SSB voice. We 
can discuss the reason if you want. Its really simple.

"Official Observers" will observe complete compliance.  An OO will not 
be able to tell whether you generated ham-RTTY using "FSK", or "AFSK" 
using upper or lower SSB.  The OO will see only the two tones separated 
by 170 Hz and occupying a necessary bandwidth (see the FCC definition of 
necessary bandwidth) of 250 Hz. I don't know where the bogus focus on 23 
Hz comes from. I plan to operate under the PACTOR-III data example. 

The "FCC bandwidth" for data in the 60m band channels is 2,800 Hz or 
less. In their example,   PACTOR-III occupies about 2,200 Hz in its high 
speed mode, but less than 1,000 Hz in the lowest speed mode. Their 
second example PSK31 occupies less than 62 Hz.  Both are, in the words 
of the FCC "examples of permitted data modes". The ham-RTTY we know and 
love occupies 250 Hz which is certainly "2,800 Hz or less". So no problem.

It really is that simple. One thing that may help is to think of the SSB 
audio bandwidth (upper or lower, it doesn't matter) as the last IF in 
your receiver, because in "AFSK" that is exactly what happens!

73,
Kai, KE4PT

Bill wrote:
> Tell that to all the 1000's of  RTTY DXers, Contesters and the writers 
> and those commenting on the article. While this can be done, it is 
> complicated on a regular basis and not popular. Having been a digital 
> RTTY DXer for 32 years and having used some 10 different TNC's and at 
> least 15 different software programs, I'll stay with conventional 
> operations. Reading the article and from years of experience LSB, not 
> USB is  common practice on RTTY when using AFSK. USB is not 
> conventional in AFSK because of high/low tones. Too complicated a 
> discussion for most including me. You are welcome to run your station 
> in any way you like of course.
>
> To be specific on 60m, not very much digital activity on there at this 
> point. Cutting it close, as it is a very narrow range, is not for me. 
> Official Observers are out in force monitoring the strict rules for 
> 60m. There has been much discussion of 60m digital, on several lists I 
> follow. I do not plan to use any digital modes on 60m myself hi.
>
> Here's why, from W7AY just a couple of weeks ago. (more of my comments 
> below Chen's)
> "23 Hz shift MSK (Minimum Shift Keyed) will likely fit into the FCC passband.
> But check your output spectrum before you transmit 23 Hz shift FSK (Frequency Shift Keyed) on 60m.  Your FSK keying sidebands might make your bandwidth greater than what the FCC permits.  If you are using AFSK, *and* the AFSK waveform is wave shaped, you *might* be able to get away with it.  See the last two figures here 
>
> http://www.w7ay.net/site/Technical/K3/Content/digital.html
>
> to see what kind of keying sidebands are produced by a typical rig's FSK generator and what a wave shaped AFSK looks like.  Just eyeball the horizontal axis to match 23 Hz shift to get an idea.  Remember that the decay of the sidelobes remain the same since we are still talking 45.45 baud keying sidebands.  I.e., don't just squish the horizontal axes; instead, cut out a 147 Hz chunk at the center of the signal.
>
> Remember your Terman?  The bandwidth of an FM signal comes from two components.
>
> You can interoperate between MSK and 23 Hz shift RTTY by setting up the FSK transmitter for 23 Hz shift, but a true MSK modulator has the correct wave shaping to keep the signal narrow and the FSK modulator does not.  The MSK symbol baud rate is also related synchronized to the shift -- in addition to proper wave shaping of the amplitudes, that also keeps the transmitter's keying sidebands in check (that is an aspect of MSK that is shared with PSK31).
>
> Practically speaking, if you look at the aforementioned figures, placing the center of RTTY tones tones at exactly the middle of a 2.8 kHz channel on 60m should present very little QRM to the next channel, even if we use standard 170 Hz RTTY shifts.  But we are dealing with legal rules and regulations and not with mathematics.  Further, the FCC might be looking out for the primary users of the 60m band -- imagine if we spread out RTTY signals on the band like what happens during a heavy split pileup -- there is no way for the primary user to tell us to clam up when they need the spectrum, except to illegally interfere with the DX by transmitting commands over the DX station.
>
> Speaking of which, I did not come across any verbiage in the new rules prohibiting cross channel ("split") communications -- can we transmit on a PSK31 channel while receiving on a different channel? Other than being uncivil, that is.
>
> All that said, during selective fading conditions (read:NVIS), you will not be able to take advantage of an ATC anyway when you use a 23 Hz shift.  So there is less reason to use 60m.
>
> Now, if we use a very wide shift, and place the Mark and Space tones at the center of individual, adjacent 60m channels, you will get very nice frequency diversity to counter selective fading    (Disclaimer: don't do this, I am joking!)
> 73
> Chen, W7AY"
>
>
> As I mentioned too complex for me and probably others.
>
> A full read of the eham article and comments covers frequency readouts and why. I also only use FSK personally. On the Icom Pro III it is the way to get access to Twin PBT filters which are spectacular on RTTY, Particularly in RTTY DXing. Just my preferences.
>
> Life is too short for me to get around all the various software's, and reconfigure them. Since I run FSK I don't have to. PSK and other digital modes are a different story and AFSK is norm. Most are too slow and splatter sensitive for contesting and DXing. RTTY is used for those activities, simply because it is faster. Not character transfer faster, but on/off faster as in Full break in CW vs No break in or even semi.
>
> The article was a good reminder for those posting DX using USB frequencies and LSB frequencies and wondering why they have trouble locating posts on Telnet themselves. It shows them how to do it according to convention, that's all. I hope you enjoyed it.
>
> Once again you and all hams are welcome to set your station up, and run it in any way you like. My comments are to the Norms.
> 73,
> Bill Marx W2CQ
>
>
>   
>
> On 3/7/2012 10:06 AM, Kai Siwiak wrote:
>> Hi Bill
>> The main distinguishing feature of that article is that it is wrong 
>> because it assumes a specific set of tone frequency offsets from the 
>> dial frequency.
>>
>> For example:
>> To operate ham-RTTY (170 Hz shift, 45.45 baud) in the 60 m band channels 
>> the proper radio setting is upper SSB, radio dial set to the upper SSB 
>> dial frequency for that channel, and RTTY operated in "AFSK" mode. You 
>> would then select the two tone offsets as 1415 Hz and 1585 Hz.  Thus, 
>> fulfilling  the FCC/NTIA requirements that:
>> (1) you must monitor the channel for upper SSB voice transmissions, and
>> (2) the data signal must be centered in the 60m band channel.
>>
>> You can do this using the popular MMTTY software by editing the 
>> Mmtty.ini file so that the line "Mark= ..." includes 1415 Hz.
>> Then in the main panel choose MARK=1415, and "Rev".  The radio must be 
>> in upper SSB.
>> Since "Rev" is selected the MARK frequency is actually 1415+170 = 1585, 
>> while the SPACE frequency is actually 1415 with respect to the upper SSB 
>> radio dial frequency.
>> [Note: that is a nomenclature problem in MMTTY when the radio is set to 
>> upper SSB]
>> On the air, the two tones will be 1500 +/-85 Hz. above the published 60m 
>> band channel, or as required by the FCC and NTIA, dead center in the 
>> channel. Said another way, if you monitor the band with a spectrum 
>> analyzer you will find the two RTTY tone +/-85 Hz from the channel 
>> dead-center frequency - just what the FCC/NTIA want.
>>
>> You can operate RTTY this exact same way ("AFSK", upper SSB, "mark=1415" 
>> in MMTTY (Mark will actually be 1585), and "rev") on any ham band where 
>> data is permitted. Then your RTTY "spotting" frequency will be your 
>> upper SSB dial frequency plus 1585. That is the frequency that hams 
>> traditionally "spot". 
>>
>> Yes, its that easy to do RTTY, and to "know your RTTY frequency", and to 
>> set up for operation on the 60m band channels
>>
>> 73
>> Kai, KE4PT
>>
>>
>> Bill wrote:
>>     
>>> Frequency Readouts For RTTY...
>>>
>>> http://www.eham.net/articles/27625
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>> ______________________________________________________________
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>> SFDXA WebSite: http://qsl.net/k4fk
>> SFDXA Repeater 147.33+ 103.5 Tone
>> DX Net Wed 7:33 PM Repeater
>> To Post: mailto:SFDXA at mailman.qth.net
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