On Jun 11, 2010, at 10:26 AM, Robert Chudek - K0RC wrote:
> I do have a question about the MMTTY software and the AEA PK232,
> Kantronics KAM, and other hardware decoders. Do they effectively "fill in"
> the missing mark or space tone when selective fading completely takes one
> or the other tones out of the audio stream?
I don't know about the PK232, but the KAM Plus does not appear to do anything
other tha usingn an LM358 comparator with a fixed threshold.
ATC (adaptive threshold correction) does not really "fill in" a missing tone.
What it does is to bias the threshold level when one of the tones is weaker
than the other.
Imagine that you have the demodulated output from two matched filters, one
centered on the Mark tone and one centered on the Space tone. These two
outputs represents the amplitude at each of the two tones. When both tones are
received with equal strengths, an unbiased slicer will simply use look at
whether M-S is positive or negative. If it is positive, a Mark is decoded, if
it is negative, a space is decoded.
Now, consider when the Space signal has selectively fade by 6 dB (i.e., the
amplitude S is now 0.5 of M). You should no longer use the same threshold (0
volts) to determine whether Mark or Space was received. The threshold should
be moved to halfway of M-S (which is now M-0.5*S). A simple way of doing this
is to use a filtered versions of M and S amplitudes, call them m and s. The
decision of whether M or S was received is then the process of comparing M-S
against m/2-s/2 instead of to 0. I.e., the slider voltage of an ATC is m/2-s/2
instead of 0.
Notice that this words with Mark only copy (when S = 0) since the threshold for
deciding if Mark was received is simply m/2.
In the analog days, the "filter" to get the slower changing 'm' from 'M' is
similar to how we used to build AGC circuits (fast attack, slow decay). You
can do much better today with digital modems since it is easy to delay the
incoming signal so you can get more accurate estimates of m and s.
One of the earliest patents, titled "Variable Decision Threshold Computer" (US
Patent 2,999,925, issued September 1961) for ATC is issued to Mr. Elmer Thomas
and assigned to Page Communications Engineers, Inc -- Page was very big in
military communications.
The patent's Summary says:
"This invention relates generally to frequency shift keying (FSK) receivers and
to multiple level AM digital systems and more particularly to a new device
usable with such receivers to modify the criteria for determining which of
binary signals is being received by shifting the decision threshold, or the
signal with respect to a fixed threshold, under changing conditions and upon
fading of said signals."
The patent show very understandable waveforms and circuits (tubes, of course
:-), and I highly recommend that anyone with interest in RTTY take a look.
The easiest way to get a copy of the patent in PDF form (the USPTO site seems
to have a preference for Internet Explorer and old scanned patents like '925
don't work well with some browsers), go to the following site and enter 2999925
as the patent number, click the Fetch button, and it will build a PDF file for
you to download:
http://www.pat2pdf.org/
73
Chen, W7AY
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