The DCF-77 radio time and frequency transmission is broadcast from Mainflingen, Germany. The DCF-77 signal can be used by timing equipment to provide an accurate time reference. Computer systems, such as NTP time server systems, which provide a precise time reference for networks can utilise the broadcast as an accurate external source of time. This article describes how the DCF-77 signal can be decoded to provide an accurate time reference for NTP time servers and other computer applications.
Before describing the format of the transmission, it's worth mentioning a little about the history of the transmitter. The German National Standard time and frequency radio broadcast, call sign DCF-77, has been in operation since 1959. The broadcast is currently operated by T-Systems, a division of the Deuche Telecom group. Precise timing information for the broadcast is generated from caesium atomic clocks maintained by the German National Physics Laboratory. The transmission has a relatively high power of 50W, which can be received up to 2000 km from Frankfurt. Reception can easily be obtained throughout much of central and northwestern Europe. The transmission is broadcast as an amplitude-modulated, pulse-width encoded data signal. Precise timing information is continuously broadcast, repeated each minute. Data is transmitted as a series of 59 pulses, one pulse each second. Each pulse represents a data bit, either a binary one or binary zero. A one hundred millisecond pulse represents a binary zero, while a two hundred millisecond pulse represents a binary one. The pulses can be decoded at the end of the minute into date and time information.
Time and date information is encoded in the DCF-77 transmission as Binary Coded Decimal format. Data bits are encoded into the transmission as follows. Data bits in the range 1 to 14 are reserved for future developments. Bit 15 indicates the transmitter in current operation - primary transmitter or backup transmitter. Bit 16 represents the announcement of daylight saving change. Bit 17 indicates whether daylight saving is currently in use. Bit 18 indicates standard time and is the inverse of bit 17. Bit 19 indicates that an imminent leap second is to be inserted. Bit 20 indicates the start of time and date information. Bits 21-27 represent BCD encoded minutes. Bits 29-34 represent BCD encoded hours. Bits 36-41 represent BCD encoded day of month Bits 42-44 represent BCD encoded day of week Bits 45-49 represent BCD encoded month of the year. Bits 50-57 represent BCD encoded year. Finally, parity bits are inserted into the encoded data as follows. Parity bit 28 covers bits 21-27. Parity bit 35 covers bits 29-34. Parity bit 58 covers bits 36-57.
To summarise, the DCF-77 time transmission is a reliable and accurate source of time from NTP server systems and other computer timing applications. The signal can be generally be received indoors close to the NTP time server itself, thus reducing installations costs. Additionally, the simple encoding of time and date information within the transmission enables straightforward implementation into NTP server software. Indeed, the standard NTP server software distribution for Linux includes a reference clock driver for the DCF-77 signal.
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