DCF77

DCF77 is a German system of transmission of the standard time by radio waves, on a broad coverage area. It was created by the governmental Office Physico-Technique (PTB), on an initiative of the German government. Its transmitter is located at Mainflingen (50,02°N 9,00°E), close to Francfort-sur-le-Main. It has a Atomic clock with the Césium and thus gives the absolute hour (with one second a theoretical variation of error for a million years, which is however rather remarkable). The two antennas are supported by horizontal cables, themselves maintained in height by several posts (to approximately 200 m height).

Information is emitted in long waves by a transmitter of 25 kw whose range is of 1500 km (and thus largely admissible on several Ouest-européens countries, of which the Metropolitan France, Germany or Italy for example).

General principle

The carrier frequency is of 77,5 K Hz. The only defect of this frequency is to be very sensitive to the parasites. It is thus necessary to check the coherence of the received data (from where the use of bits of parity). One notices it in particular on the increase in the range to 2000 km the night (considering there are much less parasites the night than the day).

The protocol also makes it possible to really control the receivers by radio (from where name radio-controlled). The transmitter can inform time change be/winter or addition of a intercalated Seconde at the end of the current hour to mitigate the irregularities of rotation of the ground.

It is not possible to make entirely function a clock with this radiopiloté signal. Because reception of the signal perhaps stopped (storm, distance, buildings, displacements…).

The time of encoding of the atomic hour and its transmission radio inevitably prevents from reaching the hour " atomique" receiving side. On the other hand, to be fixed on the received signal very strongly makes it possible to limit the deviations which a quartz watch would have for example on the long run. The total error thus is very limited and the precision obtained excellent.

Each user can very simply know the standard time and also pass automatically per hour of été/d' winter. (Under acceptance requirement of the signal, obviously.)

Its reception is much easier and economic to use the hour GPS.

Details on the protocol

Information is coded in BCD (decimal coded into binary). It is about carrying to 77,5 Khz simply modulated in amplitude (lowering the amplitude with 25% the maximum at the time of the impulses; in phase with the carrying one).

An impulse is emitted every second. It lasts 100ms for one “0” logic and 200ms for one “1”. Only the 59e second is not modulated and makes it possible to announce the beginning of a new screen (see below). It is noticed that it is the duration of the signal which transports information , and not the amplitude (it would be if not with these low frequencies quasi-impossible to hope for a certain reliability).

The synchronization of the clocks is done on the first bit (bit N 0). The appearance of the first modulation, then marks the beginning of a new minute. The coded signal corresponds per hour that it will be at the time of next “the signal departure” (" … with the next signal it will be… " ).

A screen is made up of 59 values (over 59 seconds):

values 0 to 14 are reserved but not used. They were planned initially for launching possible alarms to the population (in the place of the sirens).
values 16 to 19 bring precise details like the passage to the summer-time/winter, the addition of a intercalated Seconde, the current time zone or the " bit of appel" ,
value 20 strictly speaking indicates the beginning of the transmission of the hour,
values 21 to 28 relate to the minutes (+ bit of even parity of the minutes),
values 29 to 35 relate to the hours (+ bit of even parity of the hours),
values 36 to 41 relate to the day of the month,
values 42 to 44 relate to the day in the week (of 1=lundi with 7=dimanche),
values 45 to 49 relate to the month,
values 50 to 57 relate to the year,
value 58 relates to the bit of parity pair of values 36 to 57,
value 59 is not modulated.

Let us recall that the diffused screen contains the data of the following minute, which will start with next “the signal” (beginning of the following transmission).

In accordance with the legal texts an international transmitter must indicate its identity. The identifier " DCF77" was retained and is emitted in Morse three times each hour (19th, 39e and 59e minutes; seconds 20 to 32; between two signal; by lowering of amplitude of -25% at the rate of 250 Hz; without stopping the normal signal).

Moreover, since there exists an inaccuracy of a few milliseconds over the duration separating the emission and the reception, light a Phase modulation (pseudo-random) makes it possible to be synchronized if necessary with more precision (that is completely useless for the general public).

Table of correspondence

Let us recall that the bit n59 (the 60e) is not modulated in normal weather.

Example

for 22:29:

21 is to 1: Being weight 1, one adds 1 to the number of minutes
22 is with 0
23 is with 0
24 is to 1: Being weight 8, one adds 8 to the number of minutes (what makes 1+8=9)
25 is with 0
26 is to 1: Being weight 20, one adds 20 to the number of minutes (what makes 1+8+20=29)
27 is with 0
28 is to 1: That wants to say that the bits from 21 to 27 are odd, having 3 bits to 1 (21, 24 and 26), we received good the valeur

29 is with 0
30 is to 1: Being weight 2, one adds 2 to the number of the hours
31 is with 0
32 is with 0
33 is with 0
34 is to 1: Being weight 20, one adds 20 to the number of the hours (what makes 2+20=22)
35 is to 0: That wants to say that the bits from 29 to 34 are even, having 2 bits to 1 (30 and 34), we received the good value

The parity is very useful to check if there no were error during the reception and not to post an incorrect value. It is also recommended to check bit 20 to 1 (if the synchro arrives at the bad place).

The year is sent on two digits, it is thus convenient to check that the date does not exceed 99, in the event of problem of reception, it is frequent to be found with dates higher than 99 (example: to find the 25/25/2165 with 45h85, which arrives if one receives all the bits with 1).

Applications

One of course finds this system on the alarm clocks and watches in any kind, avoiding the adjustment of the hour, and the adjustment be/winter. The applications can be very diverse.

There exist electronic modules making it possible to receive and to demodulate the signal (compounds of an antenna and receiver), the signal leaves directly on one the pins and it does not remain any more which has to decode it (see near the retailers of electronics components). The module is not either very hard to realize.

Lastly, there exist also some modules peripheral in kit, to connect to a computer to synchronize it. Nevertheless, if it is possible to be connected to Internet, one can avoid this kind of expenses via software of setting per hour: to see the protocol NTP.

See too

Time signals emitted in other countries

France: Allouis (" France-Inter")
the United States: ,
Canada:
Venezuela:
Australia:
Russia:
Japan: ;
the United Kingdom:
Swiss: HBG Prangins
China: ;

Put at the data-processing hour via NTP (Network Time Protocol)

External references

Official site of the Governmental Office Physico-Technique: http://www.ptb.de/en/org/4/44/442/dcf77_1_e.htm
Information on the protocol: http://www.aurel32.net/elec/dcf77.php

Modules of reception are rather easily near the retailers of electronics components.

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