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
CCW (Coherent CW) has been
created, for amateur radio, by Ramond Petit (W6GHM) in 1975. The CCW presented
here derives from the "traditional" CCW with some modifications
(F6CTE/DK5KE). For example, the standard speed is 12 wpm . Here the user can
choose between 12, 24 and 48 wpm.
minimum S/N ratio can be very low, down to -12 dB for the standard speed of 12
wpm, depending on the speed and the characters sent.
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.
Click on the copy of the screen in order to
listen to the corresponding audio
Created by: Raymond Petit (W6GHM) 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 ratio)
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.
frequencies for CCW(-OOK)
Il est proposé de choisir, pour la CCW qui est un mode proche de
la CW, les fréquences suivantes:
1844, 3561, 7031, 10107, 14061, 21061,
24907, 28061 Khz (fréquences QRP + 1KHz).