[GreenKeys] Re: M20-RO

[David Ross]

RTTY folks -

  There is precious little teletype MUX stuff on HF lately, most all
data traffic has migrated up to satellites.  Lately is it just a fast
bitstream sent via satellite, no need for audio tones at all...

  A common teletype MUX format in the 1960s & 1970s had 16 tone pairs in
a 3 KC audio passband.  The scheme was intended to accommodate 16
simultaneous 75 baud bitstreams.  Each tone pair used 85 CPS shift,
center freqs were 170 CPS apart and started at 425 CPS -  tones are like
this:

 382.5 &  467.5
 552.5 &  637.5
 772.5 &  807.5
 892.5 &  977.5
1062.5 & 1147.5
1232.5 & 1317.5
1402.5 & 1487.5
1572.5 & 1657.5
1742.5 & 1827.5
1912.5 & 1997.5
2082.5 & 2167.5
2252.5 & 2337.5
2422.5 & 2507.5
2592.5 & 2677.5
2762.5 & 2847.5
2932.5 & 3017.5

  In the '60s, both Collins & TMC built HF radios with filters
compatible with this format -  the radios used special IF filters with
well-defined widths & passband flatness & group delay.
  Varying delays across the filter's passband were critical, since one
optional MUX configuration had the 16 separate channels set up as 8
dual-diversity channels.  Between two diversity channels analog voting
was used, and the scheme worked best if the incoming sigs were exactly
in sync.

  The CCITT spec that defines this old-timey MUX format also specifies
four sidebands called A2, A1, B1, & B2 -  if you see a radio with
sidebands named like this then it probably uses these fancy superflat
filters...

 -  A2 is the LSB of a suppressed carrier 6.29 KCs
    above the dial freq.
 -  A1 is plainjane USB
 -  B1 is plainjane LSB
 -  B2 is the USB of a suppressed carrier 6.29 KCs
    below the dial freq


Dave Ross    N7EPI    ross@hypertools.com