On Feb 16, 2013, at 1:37 PM, Bill Turner wrote:
> Can someone explain why slower is better for 2Tone? Is this true only for
> 2Tone or for other decoders as well?
It is true under selective fading, for all demodulators that implements a good
ATC circuit.
For an overview of ATC, see these two articles:
http://w7ay.net/site/Technical/RTTY%20Demodulators/Contents/am.html
http://www.w7ay.net/site/Technical/ATC/index.html
I don't know what MMTTY uses for ATC (or even if it has such a circuit), so I
can't speak for it, but both fldigi and 2Tone use pretty much state of the art
ATC.
Both 2Tone and fldigi have some form of squaring circuits before the slicer.
As the second of the two above articles show, the squaring can either be
applied to the signal itself (what Leonard Kahn had used for diversity
reception) or to the ATC control voltage, as I had done. I think fldigi lets
you select between both methods, and 2Tone squares the Mark and Space signal.
The reason Kahn's work (for diversity reception) applies to RTTY is because
when you use an AM type detector, the Mark and Space tones can be considered as
independent (but complementary) tones which operates under frequency diversity.
Both 2Tone and fldigi use AM detectors instead of FM detectors. For a
distinction between AM and FM RTTY detectors, see here:
http://w7ay.net/site/Technical/RTTY%20Demodulators/Contents/am.html
The Allnatt, Jones and Laws (researchers with the British Post Office Research
Station) paper that I mentioned in the web article mentions that they should
have used a squarer, but did not because of the complexity (the original 1957
paper actually references Kahn's 1954 paper). Today, in software, that is
half a line of code :-).
I mention all this because none of this squaring stuff to do ATC is really new
-- merely forgotten by hams, or they could not justify the additional hardware
cost.
A good ATC depends on precise knowledge of the signal envelope, and this is
where I believe 2Tone will prove superior to the current fldigi.
You definitely don't want the receivers' AGC to interfere with the modem's ATC
when a good ATC is better at adjusting the threshold than some arbitrary AGC
circuit that was not designed for RTTY reception. Just like you don't want a
receiver's filter to interfere with the modem's filter, if the modem's filter
is already optimal.
I usually run with no AGC, and just use the RF gain control manually to back
off the receiver gain when an extra large signal arrives. Beyond that, I just
use a high dynamic range sound card to send a very high dynamic range audio
signal to the modem.
The only time you will want some receiver AGC is when the signal undergoes
periodic, deep "flat fading" where both Mark and Space fades at the same time,
and the fading depth exceeds the dynamic range of your receiver. But even
there, you would still want to use very slow AGC (as G3YYD recommends) so that
the AGC activity does not affect the ATC circuit. If the AGC is slow enough,
the ATC will adapt to it. If the AGC is a typical fast-charge-slow-discharge
AGC, or worse still, a hang AGC that suddenly drops, the ATC envelope tracker
will have more problems.
Not to be flippant, but if fast AGC helps copy, the modem needs a better ATC
circuit.
Oh, you do not, do not, do not, want to use fast AGC with a fluttered signal.
Turn off AGC (and if there is a software button, even turn off ATC) when you
hear flutter and set the RF gain control to keep the receiver and sound card
from overloading. And if you really must use AGC, choose the slowest AGC and
without hang, for flutter conditions.
AGC is also useful before a limiter that has insufficient dynamic range when
you use an FM type detector. But that is a different story since 2Tone does
not use FM. But even there, you really don't want to use AGC that is much
faster than a hundred milliseconds.
73
Chen, W7AY
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