[RTTY] BARTG 75 RTTY Sprint?

Kok Chen chen at mac.com
Fri Jun 11 11:49:20 PDT 2010


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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