Apologies for the provocative title, but the lamenting of PSK31's perceived
slowness during the FD festivities prompted me to crunch some actual numbers.
The question I thought to investigate was "how much slower PSK31 is vs. RTTY in
various contest-like exchanges?" I'm giving my results for discussion. For
the record, I have never participated in in a PSK31 contest, so this is all
back-of-the-envelope math and not based on any kind of actual contest
experience. I have other thoughts regarding PSK31 in contesting, but this
message is strictly about information transmission speed.
I took several messages that are typical of RTTY contests. For all messages I
assumed a CR/LF at the beginning of the message. RTTY is sent with USOS on,
1.5 stop bits at 45.45b. PSK31 is sent in lowercase (more on this below) at
31.25b. (Given the recent thread on character encoding in email you may want
to cut and paste the table below into a fixed width editor to get columns to
line up right.)
Message, Bits PSK31, Bits RTTY, Time PSK31, Time RTTY, PSK31 advantage
<CR><LF>cq k0sm k0sm cq <EOT> 98 172.5 3.14
3.80 17.37%
<CR><LF>cq aa5au aa5au cq <EOT> 98 187.5 3.14
4.13 23.98%
<CR><LF>aa5au andy ny ny <EOT> 104 172.5 3.33
3.80 12.31%
<CR><LF>aa5au 4a wny <EOT> 72 150 2.30 3.30
30.19%
<CR><LF>aa5au 599 678 <EOT> 122 180 3.90
3.96 1.42%
It turns out its really quite hard to engineer a message where 31.25b PSK31 is
slower than 45.45b RTTY! Even numeric exchanges, which are quite long in
varicode end up being at parity or better. Replacing the CR/LF with a single
leading space makes PSK31 even faster.
Because of the variable-length encoding in PSK31 your own callsign could
potentially take longer to send. This is a function of how many "unusual
letters" you have in your callsign. Here are some examples, although it should
be pointed out that "wa5zup" breaks nearly even with RTTY if you include
leading and trailing spaces (which you would in a message), which are only one
bit in varicode:
n6ee 17 45 0.54 0.99 45.06%
aa5au 27 52.5 0.86 1.16 25.20%
k0sm 27 45 0.86 0.99 12.74%
4x6z 35 60 1.12 1.32 15.16%
wa5zup 41 60 1.31 0.99 -32.51%
This begs the question as to why PSK31 is slow. How many people are using
their RTTY macros in all caps for PSK31? PSK31 averages about 10 bits for
capital letters, and exactly 6 bits for lowercase. One way to change this
behavior would be to have contesting software always send lowercase, and if one
wanted, always print uppercase on the screen for legibility. I can't imagine a
contest where case would matter, and you get instant speed improvement and
better copy without having to change your beloved macros. Like USOS for RTTY,
forcing lowercase transmission would a kind of hack to make things go a little
faster and improve copy, and people who don't know any better still reap the
advantage of lower-case transmission by default. This seems to this armchair
observer to be a much better solution to increasing the baudrate (and thus
losing SNR) by switching to PSK63 or whatever I to overcome poor utilization of
the character table.
I hope this is helpful!
Cheers,
Andy K0SM/2
_______________________________________________
RTTY mailing list
RTTY@contesting.com
http://lists.contesting.com/mailman/listinfo/rtty
|