DCF77

Coordinates: 50°0′56″N 9°00′39″E / 50.01556°N 9.01083°E / 50.01556; 9.01083

DCF77 in Mainflingen

Low cost DCF77 receiver

DCF77 range from Physikalisch-Technische Bundesanstalt

DCF77 is a longwave time signal and standard-frequency radio station. Its primary and backup transmitter are located in Mainflingen, about 25 km south-east of Frankfurt am Main, Germany. It is operated by Media Broadcast GmbH (previously a subsidiary of Deutsche Telekom AG), on behalf of the Physikalisch-Technische Bundesanstalt, Germany's national physics laboratory. DCF77 has been in service as a standard-frequency station since 1959; date and time information was added in 1973. The timestamp sent is either in UTC+1 or UTC+2 depending on daylight saving time.^[1]

The 77.5 kHz carrier signal is generated from local atomic clocks that are linked with the German master clocks in Braunschweig. With a relatively high power of 50 kW, the station can be received in large parts of Europe, as far as 2000 km from Frankfurt (and further away depending on signal propagation and local interference. As an example, reception with high-grade consumer clocks is possible in Portugal and Gibraltar (during night hours). Its signal carries an amplitude-modulated, pulse-width coded 1 bit/s data signal. The same data signal is also phase modulated onto the carrier using a 512-bit long pseudorandom sequence (direct-sequence spread spectrum modulation). The transmitted data repeats each minute.

• Current date and time;
• Leap second warning bit;
• Summer time bit;
• Primary/backup transmitter identification bit;
• Several parity bits.

Since 2003, 14 previously unused bits of the time code have been used for civil defence emergency signals. This is still an experimental service, aimed to one day replace the German network of civil defense sirens.

The call sign DCF77 stands for D=Deutschland (Germany), C=long wave signal, F=Frankfurt, 77=frequency: 77.5 kHz.

Radio clocks and watches have been very popular in Europe since the late 1980s and most of them use the DCF77 signal to set their time automatically.

Time code details

Like most longwave time transmitters, DCF77 marks seconds by reducing carrier power for an interval beginning on the second. The duration of the reduction is varied to convey one bit of time code per second, repeating every minute. The carrier is synchronized so the rising zero-crossing occurs on the second. All modulation changes also occur at rising zero-crossings.

Amplitude modulation

The DCF77 signal is reduced to 15% power (−8¼ dBFS) for 0.1 or 0.2 seconds at the beginning of each second. A 0.1 second reduction (7750 cycles of the 77500 Hz carrier) denotes a binary 0; a 0.2 second reduction denotes a binary 1. As a special case, the last second of every minute is marked with no carrier power reduction.

There was also a morse code station identification, sent during minutes 19, 39 and 59 of each hour, however this was discontinued as the station is easily identifiable by the characteristic signal.^[2] A 250 Hz tone was generated by square wave modulating the carrier between 100% and 85% power, and that tone was used to send one letter per second, between the second marks. During seconds 20–32, the call sign "DCF77" was transmitted twice.

Phase modulation

In addition, for 793 ms beginning at 200 ms, each time code bit is transmitted using direct-sequence spread spectrum. The bit is mixed with a 512-bit pseudo-random chip sequence and encoded on the carrier using ±13° phase-shift keying.^[3] The chip sequence contains equal amounts of each phase, so the average phase remains unchanged. Each bit spans 120 cycles of the carrier, so the exact duration is cycles 15500 through 76940 out of 77500. The last 560 cycles (7.22 ms) of each second are not phase-modulated.^[4]

The chip sequence is generated by a 9-bit LFSR, repeats every second, and begins with 00000100011000010011100101010110000….

A software implementation of a Galois LFSR can generate the full chip sequence:

  unsigned int i, lfsr;
 
  lfsr = 0;
  for (i = 0; i < 512; i++) {
 
    unsigned int chip;
 
    chip = lfsr & 1;
    output_chip(chip);
 
    lfsr >>= 1;
    if (chip || !lfsr)
      lfsr ^= 0x110;
  }

Each time code bit to be transmitted is exclusive-ored with the LFSR output. The final chipped sequence is used to modulate the transmitter phase. During 0 chips the carrier is transmitted with a +13° phase advance, while during 1 chips it is transmitted with a −13° phase lag.

In lieu of the special minute marker used in the amplitude code, bit 59 is transmitted as an ordinary 0 bit, and the first 10 bits (seconds 0–9) are transmitted as binary 1.

Time code interpretation

The time is represented in binary-coded decimal. It represents civil time, including summer time adjustments. The time transmitted is the time of the following minute; e.g. during Dec 31 23:59, the transmitted time encodes Jan 1 00:00.^[5]

The first 20 seconds are special flags. The minutes are encoded in seconds 21–28, hours during seconds 29–34, and the date during seconds 36–58.

Two flags warn of changes to occur at the end of the current hour: a change of time zones, and a leap second insertion. These flags are set during the hour up to the event. This includes the last minute before the event, during which the other time code bits (including the time zone indicator bits) encode the time of the first minute after the event.

DCF77 time code

Bit	Weight	Meaning		Bit	Weight	Meaning		Bit	Weight	Meaning
:00	M	Start of minute, always 0.		:20	S	Start of encoded time, always 1.		:40	10	Day of month (continued)
:01	Civil warning bits,[6] provided by the Bundesamt für Bevölkerungsschutz und Katastrophenwarnung (Federal Office of Civil Protection and Disaster Relief). Also contain weather broadcasts.[5][7]			:21	1	Minutes 00–59		:41	20
:02				:22	2			:42	1	Day of week Monday=1, Sunday=7
:03				:23	4			:43	2
:04				:24	8			:44	4
:05				:25	10			:45	1	Month number 01–12
:06				:26	20			:46	2
:07				:27	40			:47	4
:08				:28	P1	Even parity over minute bits 21–28.		:48	8
:09				:29	1	Hours 0–23		:49	10
:10				:30	2			:50	1	Year within century 00–99
:11				:31	4			:51	2
:12				:32	8			:52	4
:13				:33	10			:53	8
:14				:34	20			:54	10
:15	R	Call bit, set during abnormal transmitter operation.[5] Previously indicated use of the backup antenna.		:35	P2	Even parity over hour bits 29–35.		:55	20
:16	A1	Summer time announcement. Set during hour before change.		:36	1	Day of month. 01–31		:56	40
:17	Z1	Set to 1 when CEST is in effect.		:37	2			:57	80
:18	Z2	Set to 1 when CET is in effect.		:38	4			:58	P3	Even parity over date bits 36–58.
:19	A2	Leap second announcement. Set during hour before leap second.		:39	8			:59	No bit transmitted during last second of each minute.

In the event of an added leap second, a 0 bit is inserted during second 59, and the special missing bit is transmitted during the leap second itself, second 60.^[5]

Although the time code only includes two digits of year, it is possible to deduce two bits of century using the day of week. There is still a 400-year ambiguity, as the Gregorian calendar repeats weeks every 400 years, but this is sufficient to determine which years ending in 00 are leap years.^[8]

The time zone bits can be considered a binary-coded representation of the GMT offset. Z1 set indicates UTC+2, while Z2 indicates UTC+1.

See also

• Coordinated Universal Time
• Radio clock
• Time from NPL (MSF)

References

1. ^ "Time and Standard Frequency Station DCF77 (Germany)". http://www.eecis.udel.edu/~mills/ntp/dcf77.html.  100503 eecis.udel.edu
2. ^ (in german) Zeit- und Normalfrequenzverbreitung mit DCF77, Physikalisch-Technische Bundesanstalt, p. 6, http://www.ptb.de/de/org/4/44/pdf/dcf77.pdf, retrieved 2009-08-12 
3. ^ DCF77 phase modulation, Physikalisch-Technische Bundesanstalt, http://www.ptb.de/en/org/4/44/442/dcf77_pm_e.htm 
4. ^ Hetzel, P. (16 March 1988). "Time dissemination via the LF transmitter DCF77 using a pseudo-random phase-shift keying of the carrier". 2nd European Frequency and Time Forum. Neuchâtel. pp. 351–364. https://www.ptb.de/cms/fileadmin/internet/fachabteilungen/abteilung_4/4.4_zeit_und_frequenz/pdf/5_1988_Hetzel_-_Proc_EFTF_88.pdf. Retrieved 11 October 2011. 
5. ^ ^{a b c d} DCF77 time code, Physikalisch-Technische Bundesanstalt, 2007-05-09, http://www.ptb.de/en/org/4/44/442/dcf77_kode_e.htm 
6. ^ Warnings to the general public by means of CF77?, Physikalisch-Technische Bundesanstalt, 2007-05-09, http://www.ptb.de/en/org/4/44/442/dcf77_bbk_e.htm 
7. ^ Piester, D.; Bauch, A.; Becker, J.; Polewka, T.; Rost, M.; Sibold, D.; Staliuniene, E. (2006-12-05), "PTB's Time and Frequency Activities in 2006", Proc. 38th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting, pp. 37–47, http://www.piester.de/dp200701.pdf, retrieved 2009-03-24 
8. ^ The date XX00-02-28 must fall on a Monday, Sunday, Friday, or Wednesday. Only the first case is a leap year, followed by Tuesday the 29th. In the other three cases, the next day is March 1.

External links

• Official DCF77 web page at the PTB
• (German) The atomic clock in Frankfurt/Main Germany
• DCF77 on the Global Frequency Database
• Time code description
• Observation of DCF77 during a leap second
• Bau einer Uhr
• (German) Zeit- und Frequenzverbreitung mit DCF77: 1959–2009 und darüber hinaus ("Time and frequency dissemination with DCF77: 1959–2009 and beyond") – a detailed description of the history of DCF77