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DCF77

DCF77

Overview


DCF77 is a longwave
Mainflingen longwave transmitter
Mainflingen longwave transmitter is a large facility for commercial longwave transmissions at Mainflingen, Hesse, Germany, which was built in 1956. It uses several T- and triangle antennas, which are mounted on guyed masts of lattice steel, insulated against ground...

 time signal
Time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...

 and standard-frequency radio station. Its primary and backup transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

 are located in Mainflingen
Mainflingen
Mainflingen is a small settlement of about 3800 inhabitants, a constituent community of the municipality Mainhausen in Hesse, Germany....

, about 25 km south-east of Frankfurt am Main, Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

. It is operated by Media Broadcast GmbH (previously a subsidiary of Deutsche Telekom AG), on behalf of the Physikalisch-Technische Bundesanstalt
Physikalisch-Technische Bundesanstalt
The Physikalisch-Technische Bundesanstalt is based in Braunschweig and Berlin. It is the national institute for natural and engineering sciences and the highest technical authority for metrology and physical safety engineering in Germany....

, Germany's national physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 laboratory. DCF77 has been in service as a standard-frequency station since 1959; date and time information was added in 1973.
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Encyclopedia


DCF77 is a longwave
Mainflingen longwave transmitter
Mainflingen longwave transmitter is a large facility for commercial longwave transmissions at Mainflingen, Hesse, Germany, which was built in 1956. It uses several T- and triangle antennas, which are mounted on guyed masts of lattice steel, insulated against ground...

 time signal
Time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...

 and standard-frequency radio station. Its primary and backup transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

 are located in Mainflingen
Mainflingen
Mainflingen is a small settlement of about 3800 inhabitants, a constituent community of the municipality Mainhausen in Hesse, Germany....

, about 25 km south-east of Frankfurt am Main, Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

. It is operated by Media Broadcast GmbH (previously a subsidiary of Deutsche Telekom AG), on behalf of the Physikalisch-Technische Bundesanstalt
Physikalisch-Technische Bundesanstalt
The Physikalisch-Technische Bundesanstalt is based in Braunschweig and Berlin. It is the national institute for natural and engineering sciences and the highest technical authority for metrology and physical safety engineering in Germany....

, Germany's national physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 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
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

+1 or UTC
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

+2 depending on daylight saving time
Daylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

.

The 77.5 kHz carrier signal is generated from local atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

s that are linked with the German master clocks in Braunschweig
Braunschweig
Braunschweig , is a city of 247,400 people, located in the federal-state of Lower Saxony, Germany. It is located north of the Harz mountains at the farthest navigable point of the Oker river, which connects to the North Sea via the rivers Aller and Weser....

. With a relatively high power of , the station can be received in large parts of Europe, as far as from Frankfurt (and further away depending on signal propagation
Radio propagation
Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere...

 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
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...

 using a 512-bit long pseudorandom sequence (direct-sequence spread spectrum
Direct-sequence spread spectrum
In telecommunications, direct-sequence spread spectrum is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals...

 modulation). The transmitted data repeats each minute.
  • Current date and time;
  • Leap second
    Leap second
    A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

     warning bit;
  • Summer time
    European Summer Time
    European Summer Time is the arrangement in Europe by which clocks are advanced by one hour in spring to make the most of seasonal daylight. This is done in all of the countries of Europe except Iceland which observes Greenwich Mean Time all year round, and Russia, which has two time zones in...

     bit;
  • Primary/backup transmitter identification bit;
  • Several parity bit
    Parity bit
    A parity bit is a bit that is added to ensure that the number of bits with the value one in a set of bits is even or odd. Parity bits are used as the simplest form of error detecting code....

    s.


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 siren
Civil defense siren
A civil defense siren is a mechanical or electronic device for generating sound to...

s.

The call sign
Call sign
In broadcasting and radio communications, a call sign is a unique designation for a transmitting station. In North America they are used as names for broadcasting stations...

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

Radio clock
Radio clock
A radio clock or radio-controlled clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock...

s 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
DBFS
Decibels relative to full scale, commonly abbreviated dBFS, measures decibel amplitude levels in digital systems such as pulse-code modulation which have a defined maximum available peak level....

) 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
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...

 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. A 250 Hz tone was generated by square wave
Square wave
A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels...

 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
Direct-sequence spread spectrum
In telecommunications, direct-sequence spread spectrum is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals...

. The bit is mixed with a 512-bit pseudo-random chip
Chip (CDMA)
In digital communications, a chip is a pulse of a direct-sequence spread spectrum code, such as a pseudo-noise code sequence used in direct-sequence code division multiple access channel access techniques....

 sequence and encoded on the carrier using ±13° phase-shift keying
Phase-shift keying
Phase-shift keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal ....

. 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.

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
Binary-coded decimal
In computing and electronic systems, binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by its own binary sequence. In BCD, a numeral is usually represented by four bits which, in general, represent the decimal range 0 through 9...

. 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.

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, provided by the Bundesamt
für Bevölkerungsschutz und Katastrophenwarnung
(Federal Office of Civil Protection and Disaster Relief).
Also contain weather broadcasts.
: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.
Previously indicated use of the backup antenna.
:35 P2 Even parity over hour bits 29–35. :55 20
:16 A1 Summer time
Daylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

 announcement.
Set during hour before change.
:36 1 Day of month.
01–31
:56 40
:17 Z1 Set to 1 when CEST
Central European Summer Time
Central European Summer Time is one of the names of the Daylight saving time offset using the UTC offset of UTC+02:00, 2 hours ahead of Coordinated Universal Time. It is used as a summer daylight saving time in most European countries. During the winter, Central European Time is used...

 is in effect.
:37 2 :57 80
:18 Z2 Set to 1 when CET
Central European Time
Central European Time , used in most parts of the European Union, is a standard time that is 1 hour ahead of Coordinated Universal Time . The time offset from UTC can be written as +01:00...

 is in effect.
:38 4 :58 P3 Even parity over date bits 36–58.
:19 A2 Leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

 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.

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
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

 repeats weeks every 400 years, but this is sufficient to determine which years ending in 00 are leap years.

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
    Coordinated Universal Time
    Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

  • Radio clock
    Radio clock
    A radio clock or radio-controlled clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock...

  • Time from NPL (MSF)

External links