Update text in User Guide related to *NA VHF Contest Mode*.

git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@8046 ab8295b8-cf94-4d9e-aec4-7959e3be5d79

doc/user_guide/en/protocols.adoc

@@ -40,11 +40,22 @@ additional information is sent in place of the grid locator or by
 encoding additional information into some of the 6 million available
 slots mentioned above.
 
-Finally, the message compression algorithm supports messages starting
-with `CQ AA` through `CQ ZZ`.  Such messages are encoded by
-sending `E9AA` through `E9ZZ` in place of the first callsign of a
-standard message.  Upon reception these calls are converted back to
-the form `CQ AA` through `CQ ZZ`.
+As a convenience for sending directed CQ messages, the compression
+algorithm supports messages starting with `CQ AA` through `CQ ZZ`.
+These message fragments are encoded internally as if they were the
+callsigns `E9AA` through `E9ZZ`.  Upon reception they are converted
+back to the form `CQ AA` through `CQ ZZ`, for display to the user.
+
+The FT8 and MSK144 modes support a special feature allowing convenient
+transmission and acknowledgment of four-character grid locators, the
+required exchanges in most North American VHF contests.  With this
+Contest Mode enabled, _WSJT-X_ supports messages of the form `W9XYZ
+K1ABC R FN42` by converting the grid locator to that of its
+diametrically opposite point on Earth.  The receiving program
+recognizes a locator implying a distance greater than 10,000 km, does
+the reverse transformation, and inserts the implied "`R`".  Obviously,
+this mode should not be used on the HF bands or under other
+circumstances where world-wide propagation is possible.
 
 To be useful on channels with low signal-to-noise ratio, this kind of
 lossless message compression requires use of a strong forward error
@@ -289,15 +300,6 @@ with constant envelope, equivalent to a Minimum Shift Keying (MSK)
 waveform.  Frame duration is 72 ms, so the effective character
 transmission rate for standard messages is up to 250 cps.
 
-Contest Mode in MSK144 conveys an additional acknowledgment bit (the
-"`R`" in a message of the form `W9XYZ K1ABC R FN42`) by using the fact
-that meteor scatter and other propagation modes usable with MSK144 are
-generally effective only out to distances of order 2500 km.  To convey
-the message fragment `R FN42`, WSJT-X encodes the locator as that of
-its antipodes.  The receiving program recognizes a locator with
-distance greater than 10,000 km, does the reverse transformation, and
-inserts the implied "`R`".
-
 MSK144 also supports short-form messages that can be used after QSO
 partners have exchanged both callsigns.  Short messages consist of 4
 bits encoding R+report, RRR, or 73, together with a 12-bit hash code

doc/user_guide/en/settings-advanced.adoc

@@ -31,9 +31,10 @@ IMPORTANT: For the health of your T/R relays and external
 preamplifier, we strongly recommend using a hardware sequencer and
 testing to make sure that sequencing is correct.
 
-- Check *FT8 and MSK144 Contest Mode* to enable generation and
-auto-sequencing of MSK144 messages with four-character grid locators
-in place of signal reports.
+- Check *FT8 and MSK144: NA VHF Contest Mode* to enable generation and
+auto-sequencing of messages using four-character grid locators in
+place of signal reports, as required for most North American VHF
+contests.
 
 - Check *x 2 Tone spacing* to generate Tx audio with twice the normal
 tone spacing.  This feature is intended for use with specialized LF/MF

doc/user_guide/en/vhf-features.adoc

@@ -279,11 +279,11 @@ messages at 50 or 70 MHz.  At these frequencies, most pings are long
 enough to support standard messages -- which have the advantage of
 being readable by anyone listening in.
 
-- A special *Contest Mode* for MSK144 can be activated by checking a
-box on the *Settings | Advanced* tab.  This mode is configured
-especially for VHF contests in which four-character grid locators are
-the required exchange.  When *Contest Mode* is active, the standard QSO
-sequence looks like this:
+- A special *VHF Contest Mode* for FT8 and MSK144 can be activated by
+checking a box on the *Settings | Advanced* tab.  This mode is
+configured especially for VHF contests in which four-character grid
+locators are the required exchange.  When *Contest Mode* is active,
+the standard QSO sequence looks like this:
 
  CQ K1ABC FN42
                       K1ABC W9XYZ EN37