Marine VHF Audio and Signalling Processor
CMX885
The coded message and voice message is transmitted over the NWR transmitter network using standard
pre-emphasis for narrow band VHF Frequency Modulation (FM) of 6dB per octave increasing slope from
300Hz to 3,000Hz applied to the modulator.
Preamble. The preamble and header code are transmitted three times with a one second pause (± 5%)
between each coded message burst prior to the broadcast of the actual voice message. Then the
preamble and End Of Message (EOM) code are transmitted three times with a one second pause (± 5%)
between each EOM burst.
Preamble Byte. The first 16 bytes (prior to the header code and EOM) of the data transmission constitute
a preamble with each byte having the value $AB (8 bit byte [10101011]). For all bytes, the least significant
bit (LSB) is sent first. The bytes following the preamble constitute the actual message data transmission.
NOTE: For NWR system maintenance, NWS will occasionally send a continuous string of preamble code,
$AB or a continuous tone through its communications links to the NWR transmitters, for several seconds
up to around one minute.
Bit Definition. The following definitions of a bit are based on a bit period equaling 1920 microseconds (±
one microsecond).
o
o
o
o
The data rate is 520.83 bits per second
Logic zero is 1562.5 Hz.
Logic one is 2083.3 Hz
Mark and space bit periods are equal at 1.92 milliseconds.
Header. Bit and byte synchronization is attained by the preamble code at the beginning of each header
code or EOM data transmission. The message data (header) code is transmitted using American
Standard Code for Information Interchange (ASCII) characters as defined in ANSI INCITS 4 (rev 86,
2002), with the eighth bit always set to zero. Each separate header code data transmission should not
exceed a total of 268 bytes if the maximum allowable geographic locations (31) are included.
Warning Alarm Tone. The Warning Alarm Tone (WAT), if transmitted, is sent within one to three seconds
following the third header code burst. The frequency of the WAT is 1050Hz (±0.3%) for 8 to 10 seconds at
no less than 80% modulation (±4.0 kHz deviation minimum, ±5.0 kHz deviation maximum).
Voice Message. If transmitted, the actual voiced message begins within three to five seconds following
the last NWR SAME code burst or WAT, whichever is last. The voice audio ranges between 20%
modulation (± 1 kHz deviation) and 90% modulation (± 4.5 kHz deviation) with occasional lulls near zero
and peaks as high as, but not exceeding, 100% modulation (± 5.0 kHz deviation). Total length of the voice
message should not exceed two minutes.
Preamble. A repeat of above.
End Of Message. EOM is identified by the use of “NNNN.”
7.8.1 Message Code Format
(Preamble) ZCZC-ORG-EEE-PSSCCC-PSSCCC+TTTT-JJJHHMM-LLLLLLLL- (1 second pause)
(Preamble) ZCZC-ORG-EEE-PSSCCC-PSSCCC+TTTT-JJJHHMM-LLLLLLLL- (1 second pause)
(Preamble) ZCZC-ORG-EEE-PSSCCC-PSSCCC+TTTT-JJJHHMM-LLLLLLLL- (1 to 3 second pause)
1050 Hz Warning Alarm Tone for 8 to 10 seconds – (if transmitted) (3 to 5 second pause)
Voice /spoken oral text of message – (if transmitted) (1 to 3 second pause)
(Preamble) NNNN (1 second pause)
(Preamble) NNNN (1 second pause)
(Preamble) NNNN
© 2010 CML Microsystems Plc
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D/885/3
CMLMICRO [ CML MICROCIRCUITS ]
