The DCF77 signal

The protocol used to transmit time data is quite simple: every minute the full time and date are transmitted in 59 bit (from 0 to 58). At the beginning of every second a bit is transmitted by reducing to 25% (-6dB) the 77.5kHz carrier for a while (a sort of digital amplitude modulation). If the power stays low for 100 ms the transmitted bit is 0 and if the power stays low for 200 ms it's 1. The 60th (number 59) bit is not transmitted and this is used as a marker to detect the beginning of each minute (and each group of 59 bit).

Bit	Name	Description	Comment
0..14	-	Reserved
15	R	Antenna bit	1 when secondary antenna is used
16	A1	Delta DST bit	1 when DST change occurs in the next hour
17	Z1	Time zone bit 1	1 when DST is set
18	Z2	Time zone bit 2	Usually the opposite of Z1
19	A2	Commutation bit	1 when a 61 seconds minutes is inserted (leap second)
20	S	Start bit	Always 1
21...27	M	Minutes	7 bit (BCD, LSB first, 00...59)
28	P1	First parity bit	Even parity for all transmitted bits
29...34	H	Hours	6 bit (BCD, LSB first, 00...23)
35	P2	Second parity bit	Even parity for all transmitted bits
36...41	DM	Day of month	6 bit (BCD, LSB first, 01...31)
42...44	DW	Day of week	3 bit (BCD, LSB first, 1 = Monday ... 7 = Sunday)
45...49	MN	Month	5 bit (BCD, LSB first, 01...12)
50...57	Y	Year	8 bit (BCD, LSB first, 00...99)
58	P3	Third parity bit	Even parity for all transmitted bits

Three times per hour (during the 19th, the 39th and the 59th minute) the call ID (DCF77) is transmitted in Morse code by a 250 Hz 25 % amplitude modulation of the carrier between second marks (without suspending time signals).