[GreenKeys] 66 WWPM ITTY

[Bill Henry]

Guys, for those of us old enough to recall Lucky Strike TV Commercials - "LSMFT" = "Low Space Means Fine Teletype".  This was featured in every issue of the RTTY Journal for many years - back when Dusty Dunn (W8CQ) was the owner and editor.   That was the standard for HF RTTY.  We achieved FSK by switching an extra capacitor across the VFO tank (with a polar relay).  Therefore Mark became the "normal frequency" and Space the frequency when we switched in the capacitor - lower.  BUT, for whatever reason, the opposite tone polarity was standardized for VHF AFSK RTTY.  In those systems, Mark was 2125 Hz and Space was 2975 Hz.  And then we discovered that we could use our SSB stuff on RTTY with a VHF TU - "just feed the tones into the mike jack".  This gave us upside down HF RTTY so we used lower side band to reverse the polarity and it all "worked out in the wash".  When we switched to narrow shift, Mark stayed at 2125 Hz but Space moved down to 2295 Hz.  Dusty had another acronym for encouraging 170 shift - "Broad minds use narrow shift".  

One more bit of trivia: Collins S-Line equipment had filters so good and narrow they wouldn't accept high tones.  So some of us started using "low tones" - 1275 Hz Mark and 1445 or 2125 Hz Space (170/850 shift).  These tones were then adopted as the International CCIR standard - everywhere but in the U.S.!  

Just a few bits of old goat trivia!

Bill Henry

-----Original Message-----
From: greenkeys-bounces at mailman.qth.net [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Ralph Mowery
Sent: Monday, February 07, 2011 5:49 PM
To: GreenKeys at mailman.qth.net
Subject: Re: [GreenKeys] 66 WWPM ITTY

The mark/space tones have been a problem for years.  I think the problem is when the hams started feeding audio into the SSB transmitters.  Due to the way it works, the tones are actually reversed when trnsmitted.  The usage was that the mark tone was the higher frequency and shifted lower for space.  By putting the audio tone of 2125 into the ssb transmitter and then shifting up to the higher space frequency (2295 or 2975) and using lower sideband, the actual radio frequency would be shifted lower.
 
Difficult for me to explain in print.  However when just the audio is used on the FM or AM transceivers the tones will be mark of 2125 and the space will be a higher frequency.

Anyway, going from the way hams and much  comercial tones are sent, one is usually reversed from the other.  Using the most common demodulators require that you use lower sideband and normal for hams and upper sideband and normal for comercial , or you can reverse the sidebands and also the normal and reverse positions of the demodulator.  Slightly confusing isn't it ?





  ----- Original Message ----- 
  From: Martin 
  To: GreenKeys at mailman.qth.net 
  Sent: Monday, February 07, 2011 5:20 PM
  Subject: Re: [GreenKeys] 66 WWPM ITTY


  I sent this yesterday meaning it to go to the list but got it wrong again!


  PS Its not an issue, but am I wrong in thinking the mark and space 
  convention is supposed to be the other way around!

  Martin


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