[RTTY] 75 baud RTTY with EXTFSK ...

[Joe Subich, W4TV]

 From time to time I see comments that some individuals would like to
see JE3HHT modify EXTFSK to support 75 baud RTTY.  I have recently
exchanged e-mail with Oba, JA7UDE about that possibility and Oba
advises that he attempted to add 75 baud support to EXTFSK some years
ago without success due to problems with bit period instability
(jitter).

EXTFSK uses the Windows Multimedia Timer that has a maximum resolution
of 1 millisecond.  However, the timer is not stable due to the nature
of Windows process swapping, etc.  I have measurements (scope photos)
made by OM7ZZ at microHAM which show the 22 millisecond bit period
from EXTFSK will vary +/- 2 ms (+/- 10%) on either a motherboard UART
or USB converter (measurements made on both devices to eliminate the
effects of USB latency).  The instability (jitter) is due to the MM
timer and thread switching in the Windows operating system.

If we apply a 1 millisecond resolution and 2 millisecond jitter to 75
baud, we start out at 77 baud (13 ms bit period).  With 2ms jitter,
the baud rate can vary from as low as 66.6 (15 ms bit duration) to as
high as 91 (11 ms bit duration).  A +/-15% instability in baud rate
will result in significant performance issues.

There are other solutions for 75 baud FSK operation with MMTTY.  If you
have a serial port (motherboard or add-in card) MMTTY will support it
directly.  If you have only USB ports, some of the better interfaces
(all of the microHAM "Keyers", RigExpert Standard and Plus, Navigator
by US Interface) support 45 and 75 baud operation using a UART (serial 
port) compatible interface, or you could use an AFSK to FSK converter
like the FSKit by K4DSP in the August 2011 QST (pp 40-42).

Finally, one can always use AFSK.

By the way, JE3HHT has released the source code of EXTFSK to the public
domain.  Anyone is free to add 75 baud support if they wish.  There are
additional timers with better resolution in Windows 2000 and later.  It
would not be at all inconceivable for a dedicated programmer who was
willing to do the research and work to use the same basic approach with
a different timer to add 75 and 110 baud support to EXTFSK if they
wanted to do so.  The source code is part of EXTFSK106.zip available
from www.hamsoft.ca ... have at it boys!.

73,

    ... Joe, W4TV