|CCW (Coherent CW)
The coherent CW is a pure digital mode because it is only done for computers (although it is
readable by a human being). The keying produced by the computer is perfect so as to allow synchronization and correct
evaluation of the bit (dot or dot abscence).
The description done hereafter corresponds to the CCW of Multipsk, which has introduced some
modifications (F6CTE/DK5KE) and new modifications in the Multipsk version 3.10.
Created by: Raymond Petit (W7GHM) in 1975
Speed : 12 wpm in standard. Here 12, 24 or 48 wpm respectively corresponding to a dot
length of 100, 50 and 25 ms
Modulation : On-Off keying of an audio tone,
Receive mode: indifferent (LSB or USB)
Character set : A..Z 0..9 ? = / + and some others signs more or less used
(“ Varicode ” characters type). The waiting character (-_---) is used as idling character. It is
specific to the CCW (of Multipsk) to keep the synchronization in case of absence of characters to send. The line feed
character is transformed in a "message start" signal (_-_-_). The removal of a character is done with the
transmission of an error character (--------), correction which is taken into account by the receiver.
Shape of pulse : rectangular
Bandwidth : up to about 100 Hz according to speed,
Demodulation : coherent,
Synchronization: automatic using the signal
Correction code: no
Convolution code: no
Interleaving : no
Drift tolerance : 15 to 25 Hz/min (depending on the CCW speed and on the signal-to-noise
Pmean/Ppeak: 0.5 but very variable
Lowest S/N : the slower is the CCW speed, the better is the decoding.
The minimum S/N ratio depends on the speed and the letters sent. For the standard speed (12
wpm) the minimum S/N is about -12 dB (-8 dB at 24 wpm and -5 dB at 48 wpm).
Note 1: at the beginning of transmission, "CCW", a space and a waiting
character (-_---) are transmitted. The user, to win time, can supress the prefix CCW and its space but the opposite operator
will ignore that it is CCW.
Note 2: in order not to lose the synchronization, it is not possible to type more than
one space between two words.