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Decoding the DCF 77 Radio Time Signal (DOC)

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									Decoding the DCF-77 Radio Time Signal
The DCF-77 radio time transmission is utilised by many NTP server and PC
computer systems to provide accurate synchronisation of time critical
applications. This article describes how the DCF-77 time signal is
decoded by NTP server and computer systems to provide an accurate timing
reference.
The DCF-77 Time Transmitter.
The DCF-77 time signal is a long-wave radio time signal broadcast from
Mainflingen, near Frankfurt, Germany. The radio signal is maintained by
T-Systems, a sub-division of Deuche Telecom, and has been in operation
since 1959. The DCF-77 signal is generated from extremely accurate atomic
clocks located at the German National Physics Laboratory. When decoded,
it provides a highly accurate timing reference for clocks and computer
timing equipment.
DCF-77 Time Code.
Time and date information is transmitted continuously, repeated each
minute. The data is transmitted as an amplitude modulated, pulse-width
coded data signal. Each data bit is transmitted as one pulse per second.
The data transmitted consists of the current time and date, leap second
indicator, daylight saving time indicator, transmitter identifier and
parity bits.
The duration of each transmitted pulse is decoded as follows: a second
mark with a duration of 100 milliseconds is decoded as a binary zero; a
second mark with a duration of 200 milliseconds is decoded as a binary
one.
Transmitted Data.
Time and date information is presented in BCD (Binary Coded Decimal)
format and is encoded as follows: bits 0-14 are unused, but may provide
future status information; bit 15 indicates use of backup transmitter;
bit 16 (A1), indicates the announcement of daylight saving change; bit 17
(Z1), indicates daylight saving is in use (CEST); bit 18 (Z2), indicates
standard time (CET); bit 19 (A2), announces a leap second; bit 20,
indicates start of time information; bits 21-27, BCD encoded minutes; bit
28 (P1), parity bit covers bits 21-27; bits 29-34, BDC encoded hours; bit
35 (P2), parity bit covers bits 29-34; bits 36-41, BCD encoded day of
month; bits 42-44, BCD encoded day of week; bits 45-49, BCD encoded month
of the year; bits 50-57, BCD encoded year; bit 58 (P3), parity bit covers
bits 36-57.
The DCF-77 data bits 1-14 are generally unused by any decoding algorithm
and may provide status information in future broadcasts. The time zone
bits Z1 and Z2 indicate current daylight saving time status. When CET
time is being broadcast, Z1 is zero and Z2 is one, for CEST time, Z1 is
one and Z2 is zero. The daylight saving change announcement bit, A1,
indicates an imminent change to or from CET. The leap second announcement
bit indicates the imminent insertion of a leap second. The three parity
bits P1-P3 compliment the preceding information to an even number of ones
(even parity).
Examples:
Received DCF-77 bit stream:

00000000000000000010100000000011010110000001001001000110011

Decoded time and date: Tuesday 01/12/1998 16:00
Received DCF-77 bit stream:
00000000000000000010110000001011010110000001001001000110011

Decoded time and date: Tuesday 01/12/1998 16:01
Dave Evans develops atomic clock time synchronisation systems and NTP
server systems to ensure accurate time on PC networks. Click here to find
out more about DCF-77 NTP Server solutions.


								
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