Don't know if it has already mentionned here, but this is a simple but extremely
useful tool to compair decoders, fine-tune MMTTY filters and settings, etc...
http://www.qsl.net/ae4jy/pathsim.htm
Works from WAV file - so everyhting can be done on a single PC:
- First record a RTTY signal, for example using AFSK output of MMTTY as
generator and pathsim as simple WAV recorder (disable noise generator and other
pathes); anything like "THE QUICK BROWN FOX " or a list of callsigns... (record
at relatively low level i.e max -6dB or 25% as adding paths later may overdrive
the soundcard)
- Open the WAV-file created with PathSim and set the output to soundcard - add
white noise, multipath, flutter either manually or using predefined presets,
and try to decode...
Have fun !
Patrick
Additional comments:
- Don't miss the "HighLatitudeDisturbed" preset which gives a realistic polar
path sounding...
- It is interesting to compair decoders, but do not forget that you won't have
any pre-filtering (what your transceiver is normaly providing -especialy if you
have a built-in DSP)- so do not make conclusions too fast: for example with
MMTTY the major improvement is obtained by using/narrowing the BPF filter... In
the real world the BPF is almost useless if you have a transceiver with
DSP-filter or if you are using an external DSP).
- It can also be used to test external devices like DSP filters, or hardware
terminals...
- It can also be used to compair modes (once you have reached the limits of
RTTY, you may give a try at PSK, MSFK16 and CW...)
- Don't forget that the S/N ratio is expressed for a bandwidth of 3kHz - S/N
expressed in dBs follows the usual log base10 progression I.E. S/N -10dB in
3kHz bandwidth is equivalent to 0dB in 300hz BW -3dB in 600Hz or -6dB in
1200Hz or +10dB in 30Hz.
- To give a reference point Joe, K1JT, Nobel Prize of Physics and author of
WSJT, considers that the limit for CW "human decoding" at 12WPM is -12dB(*) in
2500Hz BW - (roughly -13dB in 3kHz BW - this is a for signal in white noise or
AWGN channel) - ref appendix D of WSJT version 3 manual page 50
http://pulsar.princeton.edu/~joe/K1JT/WSJT300.PDF
(*) the graph page 51 rather shows -14dB, anyway, if you are a CW-op you can
try... Interesting to see what it gives at "equal speed" ie 60WPM (45bd RTTY
speed) - the theory says 10log (60/12) or roughly 7dB more signal needed due
to speed increase...
Taking the most favourable (the graph) option, that's make a minimum S/N level
of -8dB in 3kHz bandwidth for CW at 60WPM...(-6dB taking the text option). Now
you can compair CW with RTTY ! (interesting isn't it?)
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