Decoding the DCF 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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