The RF mark frequency had to be high (on HF) since they normally used a capacitor to pull the FSK VFO down. One of my ham peers who was licensed the same year as I was (1963) indicated that to his knowledge, all AFSK tones were set up as mark tone low on the one generators. It could be a chicken and egg situation, but that would only work with LSB to get the mark high on RF.
Some hams were transmitting AFSK on VHF equipment (I was very active with that starting in 1981 or so until packet radio eclipsed the RTTY VHF regenerative repeater in our area) and we just used the tones the way the AFSK generator produced them and injected them into VHF FM rigs. The 2975 Hz audio frequency would have no bearing on whether you needed to operate LSB or USB. The only thing that really mattered is that the tones had the correct shift. That is why low tones could be used as well as high tones and the stations could contact each other. My friend also advised me that the European tones, which were the low tones, also were reversed so that they injected the mark high as an audio signal so it came out as mark low on the RF side. Most tone generators could send the tones "normal" or "reversed" for decades so that was not something new. The first TU I built was the one ARRL promoted around 1980 as a "State of the Art TU." In fact, it was a very low end unit, but very simple to make using some of the XR chips of the time. Many of the list members will probably recall the XR-2206 and 2211. The XR-2211 tone decoder only decoded one of the two tones! Not uncommon for a budget design. And surprisingly, if the shift was different between stations, it really did not matter. Of course, its overall performance was terrible compared to a really quality TU:) 73, Rick, KV9U Mark Miller wrote: >Rick, > >I wasn't around back then but from what I have read, the standard for >RTTY was set that the Mark was the high RF frequency and the Space >the low RF frequency. To avoid problems with audio harmonics and the >fact that some rigs could not handle 2975, LSB had to be used. Most >demodulators would treat the absence of Mark as Space. I wish I >could find the article, but I remember reading how one amateur got >around the FCC regulations in the early days before FSK RTTY was >authorized by only decoding the presence and absence of Mark, so it >was like decoding CW. The LSB thing is just one of those things that >happened because of equipment limitations, and then got a life of its >own, like LSB for phone below 20 meters. MixW was the first program >with RTTY that I had used that send AFSK with the Mark high and the >Space low. I thought, gee about time. > >73, > >Mark N5RFX > > > > >>Would LSB be a requirement of 850 Hz shift if they could have chosen >>either sideband? Why couldn't they select USB with the audio tones >>"reversed" from LSB? This would have made the two RF frequencies normal >>to FSK RTTY with a mark of 2975 and a space of 2125. >> >> > > > >Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org > >Other areas of interest: > >The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ >DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) > > >Yahoo! Groups Links > > > > > > > > > > > > > > Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/digitalradio/join (Yahoo! ID required) <*> To change settings via email: mailto:[EMAIL PROTECTED] mailto:[EMAIL PROTECTED] <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
