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View of the Mainflingen long-wave radio station for transmission of the DCF77 signal: transmitter building (in the back), antenna building (yellow bricks) and antenna masts.

DCF77 time code

Encoding scheme of the time information transmitted with DCF77; details in the following text.

The different duration of the modulated second marks serves for binary coding of time and date: a second mark with a duration of 0.1 s correspond to a binary zero, and a second mark with a duration of 0.2 s to a binary one. Once a minute, the numbers of minute, hour, day, day of the week, month and year are transmitted on the basis of a BCD code (BCD: Binary Coded Decimal, every digit of a number is encoded separately). From the calendar year, only the unit place and the decimal place are transmitted, i.e. the year 2005 only as 05. Each code emitted contains the information for the following minute.

Details of the information transmitted

For many years, operational information has been transmitted via the DCF77 control facility with second marks Nos. 1 to 14. Although prolonged second marks in this range generally indicated an irregularity in the control or transmitting facilities, this did not mean that the time information emitted was erroneous. Such an indication of a malfunction was transmitted only very rarely, so that some developers of DCF77 decoding software carelessly assumed that these bits would never carry information. Since the middle of 2003 only the second mark #15 (“call bit“) is used to signalize irregularities in the control facilities and to alarm PTB.

The DCF77 control electronics were adapted such that the data input to bits 1 to 14 which are broadcast via amplitude modulation is now provided by a third party. The contractual relation between PTB and T-Systems was changed accordingly. Primarily, these bits shall be used for the purpose of public warning. In addition weather data are transmitted, which are supplied by the Swiss firm Meteo Time GmbH. The same data are transmitted also by the Swiss standard frequency and time signal service HBG (75 kHz). The PTB explicitly denies any responsibility for the data content transmitted during bits 1 to14. Radio-controlled clocks manufactured and operated up to now are not disturbed by the new service, but cannot make use of it.

Zone time bits Z1 and Z2 (second marks Nos. 17 and 18) show to which time system the time information transmitted after second mark 20 refers. For the emission of CET, Z1 has the state zero and Z2 the state one. When CEST is being emitted, this is reversed.

The announcement bit A1 (No. 16) indicates an imminent change-over of the time system. Before the transition from CET to CEST or back takes place, A1 is emitted for one hour in state one: before change-over from CET to CEST (CEST to CET) from 01:00:16 h CET (02:00:16 h CEST) until 01:59:16 h CET (02:59:16 h CEST).

Announcement bit A2 (No. 19) adverts the imminent introduction of a leap second. A2 is also emitted for one hour in state one before a leap second is inserted. Before a leap second is inserted on the 1st of January (1st of July), A2 is therefore emitted sixty times from 00:00:19 h CET (01:00:19 h CEST) until 00:59:19 h CET (01:59:19 h CEST) in state one.

The day of the week is encoded in accordance with standard ISO 8601 or DIN EN 28601, Monday being day one of the week. The three test bits P1, P2 and P3 complement the preceding information words (7 bits for the minute, 6 bits for the hour and 22 bits for the date including the number of the weekday) to an even number of ones.

In the case of the AM second marks, a leap second is inserted as follows: The 59th second mark preceding mark 01:00:00 h CET or 02:00:00 h CEST is emitted – different than before – with a duration of 0.1 s. After that, the inserted 60th second mark is emitted without carrier reduction. The probability that leap seconds must be omitted is negligible. However, the technical facilities on the transmitter allow it.


Further information