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Re: [RTTY] Dashes in Contest Reports

To: RTTY Reflector <rtty@contesting.com>
Subject: Re: [RTTY] Dashes in Contest Reports
From: Kok Chen <chen@mac.com>
Date: Tue, 26 Jan 2010 14:09:18 -0800
List-post: <rtty@contesting.com">mailto:rtty@contesting.com>
On Jan 26, 2010, at 1:25 PM, Jim Reisert AD1C wrote:

> MTBF, or mean time between/before failure, is a fixed quantity.  When
> you INCREASE the number of transmitted characters, the MTBF stays the
> same, but the likelihood of experiencing a failure INCREASES.

The only time I can think of, when what Jim mentioned above is not true is when 
the extra characters are part of an error correction scheme.

The simplest code is repeating.  Repetition is even used in hardware, for 
example in early JPL spacecrafts in the form of "triple modular redundancy" 
(Avizienis' papers come to mind).

If you send an exchange twice, such repetition forms an error *detection* 
scheme.  I.e., if you receive two responses that are different, you know there 
is an error and can ask for a repeat.  If you send an exchange three times, it 
forms an error *correction* scheme for many kind of errors (it does not provide 
correction in an exchange where all three numbers are different :-).

Notice that repeating an exchange also has the (good) property of temporal 
diversity.  Frequency-, polarization- and spatial-diversity are also powerful 
tools to counter channel problems on HF -- all RTTY users already take 
advantage of frequency-diversity because two separate carriers are used in FSK.

Better ways to detect and correct errors exist (and used by all modern modes 
such as QPSK31, MFSK16, etc).  All of them introduce structured redundancies 
into the data stream.  Repetition codes are actually extremely inefficient.  
You don't find computer memory systems using repetition for the ECC mechanism 
:-).

It is also known that you can introduce bad redundancies -- the MTBF case that 
Jim mentioned is a prime example.

73
Chen, W7AY

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