The DCF-77-time transmission is considered by many NTP server and PC computer systems are used for precise synchronization of the time made available to applications. This article describes how the DCF-77 signal is decoded by NTP server and computer systems to present an accurate timing reference.
The DCF-77 Time Transmitter.
The DCF-77 signal is a long-wave radio clock signal transmission from Mainflingen near Frankfurt, Germany. The radio signal is maintained by T-Systems, asub-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 from the current time and date, leap second indicator, daylight savings indicator, transmitter ID, and parity bits.
The duration of each transmitted pulse is decoded as follows: a second brand with a duration of 100 milliseconds is decoded as a binary zero, a second brand comes with a duration of 200 milliseconds decoded as a binary.
Transmitted Data.
Date and time information 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, 35 bits (P2), includes parity bit Bit 29-34; bits 36-41, BCD encoded day of the month, bits 42-44, BCD encoded day of the week, bits 45 -- 49, BCD coded months of the year, bits 50-57, BCD encoded year, bit-58 () P3, covers Parity Bit Bit 36-57.
The DCF-77 data bits 1-14 is usually done by an unused decoding algorithm, and can provide status information in future mailings. The timezone bits Z1 and Z2 indicate current DST status. If time is CETBroadcast, Z1 and Z2 is zero, as is CEST time is a Z1 and Z2 is equal to zero. Daylight saving time change announcement bit, A1, has an impending change or CET. The leap second announcement bit indicates the imminent introduction of a leap second. The three parity bits P1-P3 compliment the latest information to an even number of ones (even parity).
Examples:
Received DCF-77-bit stream:
00000000000000000010100000000011010110000001001001000110011
DecodedTime and Date: Tuesday 01/12/1998 16:00
Received DCF-77-bit stream:
00000000000000000010110000001011010110000001001001000110011
Decoded Date and Time: Tuesday 01/12/1998 16:01
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