DCF77: the long-wave time signal from Mainflingen on 77.5 kHz

What you hear

In CW mode DCF77 is a steady note, and at first nothing seems to happen. Listen for the dip: at the start of every second the loudness drops sharply for a tenth of a second or for two tenths. A short dip is a binary zero, a long dip is a one, and fifty-nine of them make a minute. In the fifty-ninth second there is no dip at all; that gap tells every clock in range that the next dip is the top of a new minute. With a little patience you can read the code by ear: the minute comes in bits 21 to 27, the hour in 29 to 34, then the day, the weekday, the month and the year, each with a parity bit, plus flags for summer time and a warning that a leap second is coming. Bits 1 to 14 at the start of each minute carry encrypted weather forecasts for consumer weather stations, which is why that stretch looks busier than the rest.

Underneath, invisible on an ordinary waterfall, the carrier phase is also modulated with a pseudo-random sequence that gives precise receivers a much sharper time mark than the dips can. To the ear it changes nothing.

Who transmits and why

The time comes from the atomic clocks of PTB, the German national metrology institute in Braunschweig, and goes on the air from Mainflingen, a village about 25 km south-east of Frankfurt am Main, from tall masts on flat ground that have been carrying the signal since 1959. The transmitter is run by a commercial network operator under contract to PTB, and the 50 kW it delivers reaches roughly 2000 km in every direction, which covers nearly all of Europe. The digital time code was added in 1973, and since then DCF77 has been the clock everyone sets by without knowing it: radio-controlled wristwatches and bedside clocks, station clocks, church bells, street lighting, parking meters, building controllers, and plenty of laboratory equipment that needs a frequency reference more than it needs the date.

The name is usually explained as D for Germany, C for the long-wave block of callsigns, F for the Frankfurt region and 77 for the frequency; that is the conventional reading, not an official statement.

When and where to listen

Around the clock, from anywhere in Europe, as long as the receiver you borrow covers long wave: not every public receiver does, but many do. Long-wave propagation is steady by day, when the ground wave does the work, and livelier at night, when a sky wave arrives too and the two can add or cancel, so at a distance the note swells and fades over minutes. Receivers in Germany, the Netherlands, Austria and Switzerland hear it at full strength; from Britain, Scandinavia, Poland and Italy it is still comfortable; from Portugal or Finland it is weaker but present. The station goes off briefly for maintenance now and then, and a second transmitter on the same site takes over for longer work.

The same stretch of dial holds the other European time signals: MSF on 60 kHz, RBU on 66.66 kHz, ALS162 on 162 kHz, so an afternoon can be spent comparing their habits.

In ShackDeck

Station catalogue → Time signals → DCF77 (77.5 kHz). The program picks a public receiver that covers 77.5 kHz, tunes it in CW with a narrow passband so that the carrier is an audible note, and shows the waterfall. No receiver, antenna or account of your own is needed. The program does not decode the time code; the pleasure here is hearing the dips and reading the bits yourself.