If the error correction circuitry only trys for a best guess this will
always be the result - random noise in, random characters out. If the
error checking routine measures the amount of error, for example with
ET1 in Coherent how many of the bits are in error, then outputs an
error symbol if more than a particular threshold is reached, there is
less likelihood of noise generating random messages.
The error detection method in Amtor Broadcast mode (no handshaking) does
this. An incorrectly received character prints out as an error symbol
rather than a, probably more serious, wrong character.
Another analogy to the light description, is to listen to noise than
has been bandpass filtered to just a few Hz wide - it sounds remarkably
like a tone modulated at the rate of the bandwidth. The WJ8711 HF
receiver has a 56Hz wide filter included - noise sounds so remarkably
like a weak RTTY signal that we've tried to decode it before now !!
Andy G4JNT
I'm sure there are better mathematical terms for this, but
the essence is:
if the input to COHERENT is truly random, the output is a "weighted
randomness." It's just as truly random as the input. All
legal symbols will
eventually appear, and there will be no real order to them.
However, the chance of any one character appearing at a given
instant is no
longer a simple 1-chance-in-x-to-the-power-of-y probability.
It's more like
the randomness of energy from an incandescent lamp. Energy levels of
individual photons in the flux are definitely random, yet the
spectral curve
peaks at some color temperature around which most of the
energy will be found.
A suitable analogy for the Grab feature might be an optical
filter that
happens to have the same response curve as the incandescent
light source
being shone through it. The more such filters one stacks in
front of the
light (that is, the greater Grab depth), the narrower the range of
wavelengths which will be visible to the eye. Ultimately,
the light would
appear nearly monochromatic, just as the decoder would
clearly begin to
output one character more often than all others; but it would
never truly
become--pardon the expression--coherent.
73,
John KD4IDY
--
The Information contained in this E-Mail and any subsequent correspondence
is private and is intended solely for the intended recipient(s).
For those other than the recipient any disclosure, copying, distribution,
or any action taken or omitted to be taken in reliance on such information is
prohibited and may be unlawful.
|
