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Re: LF: RTTY Protocol

To: [email protected]
Subject: Re: LF: RTTY Protocol
From: g3zjo <[email protected]>
Date: Wed, 06 Mar 2013 12:18:40 +0000
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Hi Minto

On 06/03/2013 11:09, Minto Witteveen wrote:

Hmm might this be because ‘in the old days’  (amateur) RTTY was done with the rig in LSB mode using AFSK?

Yes, hence the confusion.

With mark at 2125 Hz and space at 2295 hz and mark being the idling position, when using LSB the actual transmitted frequencies would be ‘swapped’ (reversed)
So MTTY seems to assume rig at USB, while all (all?) others assume LSB…

No the opposite MMTTY Assumes (Demands) (Instructs) LSB.

All others that I have tested use the modern convention of USB for Digi Modes and the Mark is high

Still, Stefan and I seem to do RTTY wrong (i.e. amateur reversed, as in AFSK on USB).  I just checked my PIC code and I have mark at high (DDS_BASE + 85 Hz) and space at low (DDS_BASE).

This is the  same as fldigi

This then gets swapped around by fldigi because I listen (via websdr) in USB… This explains why fldigi decodes me (and Stefan) just right.

No swap it is already correct.?

This is of course based on the assumption that ‘right’ means AFSK on LSB, that the space must be the high tone AND the low(est) frequency.
What _is_ the consensus (if any)?

This from the author of MMTTY and in the Help :-

You should understand the concepts of mark and shift before operating RTTY.
 
Mark, Shift, and Space

Think of a RTTY signal as a continuous, constant -amplitude, suppressed-carrier
signal that shifts back and forth between two distinct frequencies. RTTY signals
can also be seen as a combination of two CW carriers at different RF frequencies,
only one of which is on at any given time. The pattern in which the two tones
are transmitted is what codes the letters.
 
One of these RF frequencies is known as the mark frequency; the other RF frequency
is known as the space frequency. The difference between Mark and space is known
as the shift. For amateur radio, the shift has been somewhat standardized at
170 Hz. It is customary to refer to the mark frequency as the frequency you are
operating on. For example, if you say you are transmitting on 14085.00 kHz, that
means your mark frequency is 14085.00 kHz and your space frequency is 170 Hz away.
 
This brings up a question: Is the space frequency shifted 170 Hz. below the mark
frequency, or 170 Hz. above the mark frequency? The answer to this question is
established by convention. The ham radio convention is to operate RTTY on lower
sideband, and to call one carrier the mark tone, and the other carrier, shifted
170 Hz. (0.170 KHz.) below the mark tone frequency, the space tone. A shift above
the mark frequency is called a reverse-shift.
 
Where Are the Tones?
 
The standard for posting information on the location of a RTTY station is to post
the mark frequency, but many posts are in error. When you operate AFSK using a
standard ham transceiver, and use LSB mode, things get even more complicated.
 
Here is an outline of what is happening:
 
You are operating AFSK, LSB (transmit and receive). The transceiver frequency
display shows the location of your suppressed carrier. That is the location where
a tone of 0 Hz. would be transmitted.
The transceiver shows a frequency of 14080 kHz.
You have MMTTY set to use the HAM defaults, with a mark frequency of 2125 Hz.
(2.125 KHz.) and a shift of 170 Hz. (0.170KHz.).
You want to tell someone where your mark frequency is located, or you tuned in a
station at this location, and you want to post its mark frequency.
The mark frequency you are receiving is actually 14080 KHz. - 2.125 KHz.
= 14077.875 KHz. This last number is what you should post if you want to post the
mark frequency on a DX Cluster.
Your space frequency is shifted down another 0.170 KHz. from your mark frequency,
so your space frequency is on 14077.875 KHz. - 0.170 KHz. = 14077.705 KHz.

73 Eddie

73’s Minto pa3bca

 
--------------------------------------------------------------------------
Ceterum censeo Carthaginem delendam esse
 
From: g3zjo
Sent: Wednesday, March 06, 2013 11:44
Subject: LF: RTTY Protocol
 
Graham

Just confirmed by test, MMTTY is backwards compared to all other Software. Others idle on the high tone MMTTY idles on the low, unless the REV is pressed.


From Wiki
the upper tone used for idle condition (MARK).


Another site says
You also have to switch the correct phase position (whether the lower or higher frequency carrier in frequency modulation corresponds to "Mark" or "space").

Mind you a Ham education site on the subject says.
Remember that bfo’s help filter out the carrier waves to allow for the original signals to come through


Eddie


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