[RTTY] FSK vs AFSK

[Kok Chen]

On Dec 10, 2004, at 1:02 PM, Reid Hill wrote:
> But FSK has a BIG advantage over AFSK for one reason
> in my mind, FILTERS.

I have no problem at all with using narrow filters on my FT-1000MP in 
AFSK (what Yaesu calls "packet" mode).

Ditto the dial offset (someone else mentioned that as an advantage of 
FSK, but it is not true: it depends on your rig.  My 4 year old rig can 
do it.  I think the MP is much older than 4 years, but that was about 
how long I have had mine.).

With my FT-1000MP, I am in fact running a dial offset of 1615 Hz with 
one of the selectable Packet tone pairs.  The reason for such an odd 
frequency is because Packet tone pairs are 200 Hz apart, referenced to 
the center frequency whereas the "standard" RTTY is 170 Hz, referenced 
to the Mark (unmodulated, un-diddled) carrier.  By choosing this 
offset, I can use very narrow filters and still have the signal right 
smack in the middle of a narrow IF passband.

I had previously used the high tone pair, but W1ZT convinced me that 
the lower tones are more pleasing to the ears (I think as you age :-).

I have set my (software) modem to center the Mark tone on the crossed 
banana at 1615 Hz, and centered the DSP filters of the modem 
appropriately.  With both a KAM Plus and an ST-8000, you can also 
arbitrarily set their mark and space tones, so it is not just with 
software modems that you can do this to.

I use the exact same mode/offset setting when I operate PSK31.  In my 
case, my waterfall display's frequency scale is calibrated to show the 
offset of a line in the waterfall from the rig's dial (given the known 
1615 Hz offset on the rig).

I use AFSK and FSK interchangeably.  When I transmit through the KAM, I 
use FSK, for example.  When I finally get the software modem working 
with the microKEYER, it will be yet one more box that works with FSK.

73
Chen, W7AY