[RTTY] [MMTTY] 75 baud RTTY with EXTFSK ...

Tue Sep 20 19:57:03 PDT 2011

Hello Joe,

Thanks for the detailed technical background regarding the limitation of 
the implementation of the EXTFSK driver. It's no secret I have been 
vocal about the possibility of extending the capability to 75 baud. 
Unfortunately I don't have the programming background or skill to meddle 
with the source code.

And of course, as you pointed out, there are a variety of other methods 
to operate at 75 baud. My contesting computer is an older Dell laptop 
C640 with a docking station. It provides one serial and one parallel 
port. The serial port runs the CAT and FSK / PTT run from the LPT port. 
This has worked fine for years. (Yes, I am waiting for Ed McMahon to 
show up with the check, then I'll upgrade!)

This discussion made a light bulb come on... the laptop has a pair of 
PCMCIA ports. I see a variety of PCMCIA adapters that provide serial 
ports. Here's a couple examples:

*http://tinyurl.com/6ed3869*

*http://tinyurl.com/2z2c2w*

These manufacturers discuss thr top speed but there is no mention of the 
lowest speed supported. The second link refers to the MCS9835 LSI device 
which supports 5, 6, 7, and 8 bits, which sounds promising.

What is your opinion of finding a PCMCIA adapter that might be "slow 
enough" to direct key FSK from MMTTY?

73 de Bob - KØRC in MN
------------------------------------------------------------------------

On 9/20/2011 8:35 PM, Joe Subich, W4TV wrote:
>
>
> 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
>
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