                 NOTES ON IAMBIC MODE-A AND MODE-B TIMINGS
                 -----------------------------------------

  There seems to be a lot of confusion concerning the differences between
Mode-A and Mode-B timings in iambic morse keyers. This document tries to detail
the Mode-A and Mode-B timings as CWirc implements them, and as I believe they
should be implemented after having researched the matter quite a bit.



  1- Common timings for Curtis Mode-A or Mode-B keying

  Iambic keying is used to generate a clean morse signal faster with less hand
movements. It uses 2 paddles, one to generate a stream of dits and the other to
generate a stream of dahs. Usually, for right-handed operators, the left paddle
makes dits and the right paddle makes dahs, but it's perfectly okay to reverse
them if you prefer.

Here are the timing diagrams for each paddle :

CW signal    _____-----_____-----_____-----_____-----_____-----_____-----_
Left paddle  _____--------------------------------------------------------
             |    |    |    |    |    |    |    |    |    |    |    |    |
             <1dit>

CW signal    _____---------------_____---------------_____---------------_
Right paddle _____--------------------------------------------------------
             |    |    |    |    |    |    |    |    |    |    |    |    |
             <1dit>

When a paddle is released, the keyer finishes the element being currently sent
(dit or dah) then goes silent. If a paddle is pushed again after the last
element is sent, the keyer makes sure the next element, or stream of elements
corresponding to that paddle is sent after at least a silence that's one dit
long.

Example :

CW signal    _____-----_____-----_____-----_____---------------___________
Left paddle  _____-----------------------_________________________________
Right paddle _________________________________----------------------------
             |    |    |    |    |    |    |    |    |    |    |    |    |

When one paddle is held, tapping on the other paddle, however shortly, will
insert that element into the stream from the first paddle at the next available
"slot" in the morse stream.

Examples :

CW signal    _____-----_____-----_____---------------_____-----_____-----_
Left paddle  _____--------------------------------------------------------
Right paddle ________________------_______________________________________
             |    |    |    |    |    |    |    |    |    |    |    |    |

CW signal    _____-----_____-----_____---------------_____-----_____-----_
Left paddle  _____--------------------------------------------------------
Right paddle ______________________-______________________________________
             |    |    |    |    |    |    |    |    |    |    |    |    |

CW signal    _____---------------_____-----_____---------------_____------
Left paddle  ________---__________________________________________________
Right paddle -------------------------------------------------------------
             |    |    |    |    |    |    |    |    |    |    |    |    |

When the 2 paddles are squeezed together, they generate a stream of alternating
dits and dahs. It's the first paddle that makes contact that determines if the
stream starts with a dit or a dah. In the incredibly unlikely event that the 2
paddles make contact exactly at the same time, the first element being sent is
determined by chance.

Examples :

CW signal    _____-----_____---------------_____-----_____---------------_
Left paddle  _____--------------------------------------------------------
Right paddle ______-------------------------------------------------------
             |    |    |    |    |    |    |    |    |    |    |    |    |

CW signal    _____---------------_____-----_____---------------_____-----_
Left paddle  ______-------------------------------------------------------
Right paddle _____--------------------------------------------------------
             |    |    |    |    |    |    |    |    |    |    |    |    |

CW signal    _____---------------_____-----_____---------------_____-----_
Left paddle  ______________-----------------------------------------------
Right paddle _____--------------------------------------------------------
             |    |    |    |    |    |    |    |    |    |    |    |    |



  2 - Differences between Mode-A and Mode-B timings

  The only time when A and B modes of operations are different is when 2
squeezed paddles are released. In Mode-A, the last element being sent when the
last paddle is released is finished, then the keyer goes silent.

Examples :

CW signal    --------------_____-----_____----------------________________
Left paddle  ----------------------------------___________________________
Right paddle -----------------------------------__________________________
             |    |    |    |    |    |    |    |    |    |    |    |    |

CW signal    --------------_____-----_____________________________________
Left paddle  -----------------------______________________________________
Right paddle ----------------------_______________________________________
             |    |    |    |    |    |    |    |    |    |    |    |    |

In Mode-B however, and that's were most of the confusion comes from, if the
last paddle released is released after the midpoint of the element currently
being sent, an opposite element is automatically inserted after that element
finishes. It is not inserted if both paddles are released before the midpoint
of the element currently being sent.

Examples :

CW signal    __________------------------------------__________----------_
Left paddle  --------------------------------_____________________________
Right paddle -------------------------------______________________________
             |         |         |    ^    |         |         |         |

CW signal    __________------------------------------_____________________
Left paddle  ----------------------_______________________________________
Right paddle -----------------------______________________________________
             |         |         |    ^    |         |         |         |

CW signal    __________------------------------------__________----------_
Left paddle  ----------------------_______________________________________
Right paddle ------------------------------_______________________________
             |         |         |    ^    |         |         |         |

CW signal    __________----------___________-----------------------------_
Left paddle  ------------------------_____________________________________
Right paddle -----------------------______________________________________
             |         |    ^    |         |         |         |         |

CW signal    __________----------___________-----------------------------_
Left paddle  ------------_________________________________________________
Right paddle -----------------____________________________________________
             |         |    ^    |         |         |         |         |

CW signal    __________----------_________________________________________
Left paddle  -------------________________________________________________
Right paddle ------------_________________________________________________
             |         |    ^    |         |         |         |         |

It is believed that Mode-B wasn't orinigally an intended "feature" in keyers,
but a bug in the keyer's design that added extraneous elements, and that many
OMs found it nicer to use than standard Mode-A, so Mode-B became a standard as
well.

Note that if you key iambic by "slapping" keys, you will be able to walk from
a Mode-A station to a Mode-B one and key without any problem. If you're used
to one mode and use a station that's set up for another however, you'll find
that you'll have trouble. In particular, if you're used to Mode-B and its
element-insertion feature, and try to key a "C" on a Mode-A keyer, chances are
you'll send a "K" instead for example.
