> On May 26, 2017, at 1:20 AM, John Barber <john@xxxxxxxxxxxxxxxx> wrote:
>
> With low power and poor
> propagation, PSK can work a lot better than RTTY.
Perhaps you mean PSK31 is better than RTTY when conditions are _good_? PSK31
fails pretty badly when there is multipath, for example.
PSK31 also cannot take advantage that RTTY can under selective fading -- when
there is selective fading, the two tones of an RTTY signal do not fade
simultaneously. You can imagine that when Mark has faded completely but Space
has not faded, a good decoder can behave like a Space-only decoder and still
decode the character, albeit with a greater error rate than having both tones
available. With PSK31, there is no complementary signal to decode from;
selective fading impacts PSK31 as much as flat fading. This is the "frequency
diversity" that is associated with RTTY.
However, when conditions are good, a perfect PSK decoder will beat out a
non-coherent FSK decoder by about 3 dB in SNR.
A 50 watt PSK transmitter will produce similar error rates as a 100 watt
non-coherent FSK transmitter when all else is equal. However, not everything
is equal, and PSK31 has other advantages.
PSK31 further beats out steam RTTY because under optimal filtering for both
cases, PSK31 gives another 1.6 dB advantage over RTTY due to the slower baud
rate. The efficiency of Varicode vs Baudot helps some more. Start and stop
bits are very wasteful when it comes to synchronizing characters.
(A _coherent_ FSK signal can make up for much of the loss when compared to PSK;
many will remember how nice the error rate of Amtor (SITOR-B) was when decoded
by a coherent FSK demodulator. But unless we stop [sic] using stop bits of
nondeterministic length ("stop bit jitter"), we are stuck with non-coherent FSK
decoders -- although a couple of us are playing with "pseudo-coherent" decoder
ideas for RTTY. )
An extreme case to illustrate PSK31 under poor propagation is polar- and
transequatorial- flutter. PSK31 will never get through however much power you
use, but RTTY will eventually get through with enough power.
How PSK31 fares under the CCIR Flutter condition is shown in the blue line here:
http://www.w7ay.net/site/Technical/DominoEX/Measurements/ITU/flutter.html
Notice that no matter how much SNR improves (towards the right hand side of the
chart), the PSK31 error rate stays pretty much at about 50%. I.e., you might
as well toss a coin and guess what was being sent :-). Basically, the flutter
has caused so much phase error that no matter how clean the signal is compared
to background noise, the error rate does not improve when you are depending on
phase information to decode your data.
On the other hand, when conditions are good and all you have is Gaussian noise,
PSK31 does quite well, see the blue line here
http://www.w7ay.net/site/Technical/DominoEX/Measurements/AWGN/dominoex11.html
You can compare the AWGN error rate of this blue curve to the RTTY AWGN error
rate at Alex VE3NEA's web site.
Other ITU conditions than flutter is linked from this page:
http://www.w7ay.net/site/Technical/DominoEX/Measurements/itu.html
I had made up those charts some years back for DominoEX. But each plot has a
PSK31 curve too.
There is one hidden advantage RTTY has over PSK31: the power from RTTY is
constant (well, almost constant in the case when you waveshape an AFSK signal).
To keep the bandwidth tight, the PSK31 signal is envelope modulated. The
result is a 3 dB peak-to-average power envelope ratio ("crest factor"). Since
many PA are peak power limited, you are forced to run lower average (RMS)
power. If the PA clips at 100 watts, you simply cannot run PSK31 at over 50
watts without severely harming others who are sharing the spectrum. With RTTY,
if the PA can handle power without melting, you can run pretty much the power
where the amplifier begins to clips.
What mode is better is really a pretty complex question. As you can see, there
is no black-and-white answer.
As an aside, some of the worst flutter I have experienced is the path from
Tahiti to California, and at one time, from Phoenix to Fernando de Noranha.
Neither are polar paths. In the Noronha case many years ago, I heard a JA who
was visiting PY0F issue a CQ on 20m PSK31. Lots of errors, but I decided to
try anyway. However, all I got back are "AGN?" and I gave up. A few minutes
later, the op switched to RTTY and I got through in one call.
73
Chen, W7AY
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