[RTTY] FSK is bad?

[Kok Chen]

> On Oct 25, 2016, at 5:37 PM, Richard Ferch <ve3iay at gmail.com> wrote:
> 
> I hope someone like Chen or Andy
> who really knows (not someone just knee-jerk defending their rig because
> they have dropped a lot of money on it) will correct me if I am wrong.

As far as I know, you are not wrong, Rich.

Flex Radio has an opportunity to do much better, seeing their transmitter is a real DUC (digital up conversion).  But I wouldn't count on it ever happening.  

The Elecraft rigs are numerical DSP only up to the "last" I.F. (some tens of kHz sampling rate), where the I/Q signal it is converted to a scalar analog signal.  From there, it still goes through the analog superhet architecture to translate the RTTY signal up to R.F..  In the case of the K3, two more analog mixers and crystal filters.

True SDRs today, like the Flex Radio and the HPSDR are completely numerical (and thus no non-linearities) all the way to the actual R.F. sampling rate (above 120 MHz for HPSDR and above 200 MHz for the big Flexes).  It is like generating a virtually perfect RTTY signal right at the antenna itself :-).

BTW, I suspect Ed, W0YK has a lot to do with arm twisting the Elecraft guys to do the halfway correct thing (DSP filtering the RTTY transmit signal).  

The proper thing though, is for the "FSK" mode to generate two waveshaped Mark and Space signals like what is done in 2Tone and fldigi.  Don't hold your breath -- rig manufacturers are not known to cater to RTTY, even though David (2Tone) himself uses a K3.  Andy has both a K3 and an HPSDR radio.  I had a K3 and abandoned it some years ago for a couple of Hermes and an Angelia (all HPSDR radios).

In the meantime (unless you are crazy enough to build RTTY modems right into the SDR software like I do), a 2Tone signal injected into the Flex is probably the best RTTY signal that you can put on the air today (hear that, W7TI? :-)

73
Chen, W7AY