On Wed, 2007-11-21 at 21:04 -0600, Duane Calvin wrote:
> Ah, yes, and the Orion is upside down from what I think it should be.
> Thanks - now I know what you're talking about.
>
> 73, Duane
> Duane Calvin, AC5AA
> Austin, Texas
> www.ac5aa.com
>
>
Right side up or inverted for RTTY is a lot like LSB vs USB on the
various ham bands. They both work on any frequency. They both come from
almost accidental choices in pioneering hardware.
For RTTY which started out only allowed on VHF, with AFSK on AM. Ma Bell
used tones of 2125 and 2975 for wire line signaling (I'm not sure it was
even for teletype) and those filters were available surplus. So a RTTY
pioneer in NYC (was it W2BDR?) selected the low tone as mark and the
upper tone as space thus picking 850 Hz shift. For many years the FCC
regulations that resulted from petitions to allow such digital modes
specified shift must be 850 Hz. No tolerance was given. I never heard of
any ham being cited for wrong shift but the precision of the shift was a
regular part of RTTY operator's contacts and off the air discussions.
Then HF operation came along and stability in most transmitters and
receivers of the era was better on 80 so using AFSK on LSB swapped the
high and the low on the air.
Eventually special experiments showed that 170 Hz shift worked better in
a fading media and after petitions the rules were changed to allow any
shift up to 850 Hz and ham practice settled on 170 Hz shift. Computer
users picked 200 Hz for packet from 300 baud modems that used 200 Hz
shift.
And LSB on 80 came from the first activity using a SSB generator at 9
MHz with a 5 MHz VFO (sound familiar? Analogue Tentecs continued to use
the scheme through the Omni VI). And many a SSB generator got the
sideband selection via phasing, which in theory all you had to do to
change sidebands was to invert one of the audio signals to the balanced
modulators. Trouble was that the 90 degree audio networks weren't exact
(some I analyzed to be closer to 80 degrees) so when you got the RF
phase shift to match and did get the unwanted side band down 40 dB,
flipping the side band switch left you with the new unwanted side band
down only 10 dB if that good. So once an exciter was tuned for USB on
20m, it came out automatically LSB on 75 meters.
With many years of operating USB on 75 meters, I've found that the
opposite sidebands overlap less destructively than the same sidebands
and if we used both, we could have at least 50% more QSOs in the same
spectrum. As it is now, QSOs on the "wrong" sideband tend to have
enhanced privacy with very few break ins of none family members.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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