[RTTY] SPOT:SV2ASP/A 18.107.6 RTTY

[Kok Chen]

On Apr 28, 2011, at 3:19 PM, Don Hill AA5AU wrote:

> Someone needs to find out why he's using 200 Hz shift on transmit.  If he is receiving 200 Hz shift, it will
> affect his copy on weak signals.


He is probably using an old TNC that only allows him to use 200 Hz shift (the dreaded Amtor shift).  I will conjecture further that it likely means is that his demodulator is also pretty poor quality (none of the 200 Hz-only TNC had a good RTTY demodulator in them).

The trick I have learned a long time ago is to use precisely what the DX uses to get my wimpy signal copied by them.  If the other end uses 200 Hz shift, then also switch to using 200 Hz.  If he uses 183 Hz shift (the shift which RITTY uses), then switch over to 183 Hz shift.  

It is only part of a dB difference in SNR, but it might be just what is needed to get copied.  To paraphrase Sen. Dirksen, a fraction of a dB here and a fraction of a dB there and soon you are outside the decoding threshold :-).  You guys with amplifiers and big antennas can get your dBs from elsewhere :-).

You should be able to change the shift in an instant with most software modems.

However, if you are truly stuck at 170 Hz and cannot switch to 200 Hz, tune him in so that your center frequency matches his center frequency; that will at least result in an unbiased slicer.  Do not try to match his mark frequency (unless you know he is using Mark-Only demodulation; it almost never happens :-).  With crossed bananas, tune him in so that the horizontal and vertical ellipses each have about the same amount of tilt error -- don't try to zero beat one tone to the detriment of the other tone.  If he is working split, this will only help on the receiving end.

Oh yeah, if he is transmitting with 200 Hz shift, open up your receiving filter a little too.

73
Chen, W7AY