There are a number of national time and frequency transmissions broadcast from a number of countries around the world. In the US, the WWVB signal is transmitted from Colorado. In Europe, the MSF and DCF-77 signals are broadcast from Cumbria in the UK and Frankfurt in Germany. All provide encoded time and date information that is accurate to within a few microseconds of the correct time.
This article provides information on the DCF-77 radio time signal, broadcast from Mainflingen in Germany. The article describes the following aspects of the broadcast:
- The history of the DCF-77 time signal.
- Typical uses of the time signal.
- Signal characteristics.
- DCF-77 signal encoding.
The History of the DCF-77 Time Signal
The German national time and frequency transmission is broadcast from Mainflingen near Frankfurt, Germany. The radio signal it is broadcast at 77.5 KHz, hence it’s radio call sign is known as DCF-77. The DCF-77 transmission is one of the oldest in the world, it has been in continuous operation since 1959. The transmitters are currently operated and maintained by the German telecommunications company T-Systems, a sub-division of the German Telecoms group Deutsche Telecom. There are two transmitters located at Mainflingen to provide a redundancy. The main transmitter is used in general operation, however, when maintenance work is scheduled, the backup transmitter can be switched on to provided an uninterrupted signal.
Caesium atomic clocks maintained by the German National Physics Laboratory provide a highly accurate source of time for the transmission. The broadcast has a relatively high power output of 50W, which can be received up to 2000 km from Frankfurt. This range covers most of central and north-western Europe. The signal can be successfully received as far away as the United Kingdom.
Typical Uses of the Time Signal.
The signal can be used by timing equipment to provide precise time and date information. There are many manufacturers of clocks and watches that receive the signal to provide accurate time. Additionally, network time servers utilise low-cost radio receivers to pick up accurate time from the DCF-77 signal to synchronize the internet and local networks.
The transmission is broadcast as an amplitude-modulated, pulse-width encoded data signal. Accurate time and date information is continuously broadcast, repeated each minute. Timing data is transmitted as a series of 59 pulses, one pulse each second. Each pulse represents a data bit. A 100 millisecond pulse represents a binary zero, while a 200 millisecond pulse represents a binary one. At the end of each minute the pulses can be decoded into date and time information.
DCF-77 Signal Encoding
Each minute, the transmitter broadcasts the current time in hours and minutes, the current date as day, month and year, a leap second indicator and a daylight saving time indicator. A number of parity bits is also contained within the transmission that can be used for error checking purposes.
The leap second indicator is enabled if a leap-second is to be imminently inserted. While the daylight saving time indicator is enabled if daylight saving time is currently in operation.
Data is broadcast as a series of bits in a binary coded decimal (BCD) format.
Bits 0-14 are reserved for future use.
Bit 15, when reset, indicates use of normal transmitter, when set, indicates use of backup transmitter.
Bit 16 (A1), indicates the announcement of daylight saving change.
Bit 17 (Z1), indicates Central European Summer Time (CEST) is in operation.
Bit 18 (Z2), indicates standard Central European Time (CET) is in operation.
Bits Z1 and Z2 indicate if daylight saving time is currently in operation. When bit Z1 is zero and Z2 is one, Central European Summer Time (CET) is being transmitted. When Z1 is one and Z2 is zero, then Central European Summer Time (CEST) is in operation.
Bit A1 indicates an imminent change to or from CET to or from CEST.
Bit 19 (A2), when set, announces the imminent insertion of a leap second.
Bit 20, indicates the start of time information in the data sequence.
Bits 21-27, BCD encoded minutes – 0 to 59.
Bit 28 (P1), parity bit, compliments bits 21-27 to an even number of bits (even parity).
Bits 29-34, BDC encoded hours – 0 to 23.
Bit 35 (P2), parity bit, compliments bits 29-34 to an even number of bits (even parity).
Bits 36-41, BCD encoded day of month, 1 to 12.
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, compliments bits 36-57 to an even number of bits (even parity).
To conclude, the DCF-77 time and frequency broadcast is a particularly reliable source of time. It can be used for many applications that require a hardware time reference. However, the broadcast does have a finite range and can only be reliably received in Central and North-Western Europe. Decoding the time signal is relatively straight-forward and can be easily achieved with a low-cost, low-power microprocessor.