[RTTY] WPX suggestion

[Andreas Rehberg]

only true if the number is sent once only;
with 599 123 123 the probability that both USOS 
characters are hit is just marginal..

I vote for "no dashes"!

Andy, DF4WC


-------- Original-Nachricht --------
> Datum: Fri, 08 Feb 2013 08:45:26 -0800
> Von: Kok Chen <chen at mac.com>
> An: RTTY Reflector <rtty at contesting.com>
> CC: Bill Turner <dezrat1242 at yahoo.com>
> Betreff: Re: [RTTY] WPX suggestion

> On Feb 8, 2013, at 4:19 AM, Bill Turner wrote:
> 
> > Please, please, please, NO dashes. While it does speed up sending, it
> > interferes with the USOS (Un Shift On Space) function which most folks
> > agree is a much better thing to have. USOS greatly reduces requests for
> > repeats, especially when there is QRM and QRN. 
> 
> 
> It is another myth that an extra character helps when you are transmitting
> all numerals.
> 
> The extra FIGS that follows the Space actually hurt.  The reason is this:
> 
> if the probability of the error of a single character is Q, then the
> probability that you do *not* have to repeat an exchange of N characters (all N
> characters survives) is (**N is the power of N)
> 
> 1 - (1-Q)**N
> 
> For example, if Q is 0.1 and N is 8 (e.g., 599-123, i.e. transmitting
> <figs>599<dash>123), then the above probability is 57%.
> 
> By transmitting that extra character, the probability that all characters
> are received correctly is now
> 
> 1 - (1-Q)**(N+1)
> 
> Or, using the same probabilities, the probability of not having to repeat
> is now 61%.
> 
> The above with random errors.
> 
> Lets see what happens when a dash is specifically replaced by Space that
> is followed by a FIGS, and we allow partial reception to count (i.e., the
> other person can still see the 3 digit that what was sent correctly -- it not
> safe, but I present this in case it is further argued that a FIGS somehow
> will magically transform a 599-123 into something readable again).
> 
> Recall that the dash can be received wrongly, but unless it turns into
> LTRS, it will just be printed as some other FIGS character, e.g., into a dot. 
> Recall that a dash is Baudot 0x03 and a LTRS is a Baudot 0x1F.  The
> Hamming distance is 3.  So the probability of a dash turning into a LTRS is very
> low.  I.e., if we use the same Q above of 0.1 (10% of the characters are
> wrong), the probability of a dash turning into a LTRS is 0.3%.
> 
> Lets see what happens when you send the Space followed by a FIGS.  The
> Space can still get mangled and turn into something other FIGS character.  But
> lets look at the FIGS that has to also be sent.  Given the same Q as
> above, that probability that the FIGS can be turned into a non-FIGS character is
>  9.7%!  When that happens, you will receive "599 xQWE"  where x is
> anything but a FIGS character.
> 
> What if the space is mangled? well, it is the same as if the dash is
> mangled... and guess what, you print exactly the same thing as what dash
> message.  I.e., If the space is mangled into a dot, you copy 599.123, which you
> also copy 599.123 if the dash turned into a dot.  Remember that "123" only
> prints incorrectly for the dash guy when the dash turns into a LTRS, and the
> receiver prints 599QWE.
> 
> The only other possibility is if the preamble FIGS is received as
> something else (say, as a dot).
> 
> The Space guy is received as ".TOO 123" and the Dash guy is received as
> ".TOO AWE"  Most of us (and some software) will know what either means, and
> not need a repeated exchange, but a newcomer to RTTY will be scratching his
> head in both cases, and ask for a repeat.
> 
> So, do yourself a favor.  If the exchange consists of only numerals, use a
> dash in place of a space.  If the exchanged is mixed, e.g., "599 NH" then
> don't use a dash.  But, don't do it blindly.   Dashes are not always
> better, but in the case that the original posted presented it (all numerals) it
> is better. 
> 
> 73
> Chen, W7AY
> 
> 
> 
> 
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