[RTTY] Code 10

[f6irf at free.fr]

>From:  Brian <bejo at clear.net.nz>
>is there any way in which
>I can improve decode under the influence of the pole, ie..Flutter, echo,
>multipath etc, I run MMTTY in FIR but beyond that I don't know, or is it that
>RTTY tends to get a bit messy under these conditions. Gotta try something to
>get past the 8.6 Q's an hour.


Hi Brian
- Just in case you only have the MMTTY engine, recommend you install the full
version - The full version has some presets programmed by Mako, which I found
extremely useful-  The DSP/FFT parameters are stored in the userpara.ini and
can also be used with the "engine". (an example is copied below) - you can use
the "save to file" and "load from file" profile utilities under profile menu.
- If not already done, recommend you read 2 chapters in MMTTY help - advanced
receiving and soundcard calibration (often overlooked, but really important
with a few soundcards...). You may not understand everything (my case !) about
advanced receiving, but it is not completly useless to read it...

Now to be concrete, from my experience (I am not a specialist - don't have
sufficient theoretical background !), I found that  in a large majority of
cases the IIR filter gives better results than the FIR...  (btw never found any
use for the PLL- seems to be used only for the 23hz shift MMTTY special mode).
According to Mako, the FIR maybe used for sharper filters, but needs more
processing power, and in practice found that having very sharp filters is not
often a good thing.
For example the "fluttered signals" preset copied below uses  the IIR filter and
an IIR bandwidth of 90hz - works very well on polar signals like KL7 to EU, but
also quite well on any sort of signal... (the standard RTTY which uses narrower
IIR filter is only useful when the S/N is limit... maybe the FIR could provide a
slight improvement in this case, but it requires the signal to be clean and
perfectly tuned, so in the contest reality never managed to see any
advantage...)
The "multipath" preset use very similar parameters to "fluttered"  but with the
ATC as echo canceller - also use this one a lot, especialy on 80m where middle
distance strong signals can't often be decoded due to multipath if you stick to
"standard RTTY" - However the ATC reduces the sensitivity, so it has to be used
only when needed...

In general it is extremely useful to have at least 2 instances of MMTTY running
(with different parameters) - Then you can really see the differences and in
which case one will take advantage over the other... You can also play a bit
with the second instance when the rate is low...
My conclusion would be that there is no definitive answer, only a good
compromise for given conditions, band and stations worked...
Patrick



[Define1]
Name=Fluttered signals
Enabled=1
AFC=1
AFCFixShift=1
AFCSQ=32
AFCTime=8.000000e+00
AFCSweep=1.000000e+00
UOS=1
TxNet=1
TxDisRev=0
LimitDxGain=2.000000e+02
LimitOverSampling=0
ATC=0
ATCTime=4
Majority=1
IgnoreFreamError=0
SQ=1
SQLevel=6.000000e+02
DefMarkFreq=2.125000e+03
DefShift=1.700000e+02
DefStopLen=0
OutputGain=2.457600e+04
TXECHO=0
TXBPFTAP=48
TXLPF=0
TXLPFFreq=1.000000e+02
TXWaitType=0
TXCharWait=0
TXDiddleWait=0
TXCharWaitDiddle=0
TXRandomDiddle=0
TXWaitTimerDiddle=0
Rev=0
ShiftFreq=1.700000e+02
RXBPF=1
RXBPFTAP=56
RXBPFAFC=1
RXBPFFW=1.000000e+02
RXlms=0
RXlmsDelay=0
RXlmsMU2=3.000000e-03
RXlmsGM=9.999000e-01
RXlmsAGC=0
RXlmsInv=0
RXlmsTAP=56
RXNotchTAP=72
RXlmsBPF=1
RXlmsType=1
RXlmsNotch=2136
RXlmsNotch2=0
RXlmsTwoNotch=0
DEMTYPE=0
BaudRate=4.545000e+01
TTYBitLen=5
TTYStopLen=4
TTYParity=0
SmoozType=0
SmoozOrder=5
SmoozIIR=4.000000e+01
Smooz=7.000000e+01
Tap=72
IIRBW=90
pllVcoGain=3.000000e+00
pllLoopOrder=2
pllLoopFC=2.500000e+02
pllOutOrder=4
pllOutFC=2.000000e+02
Diddle=2
TxFixShift=0
InvPTT=0
TXUOS=1