Télé Distribution Française

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Télé Diffusion de France, or TDF, is a time signal service, broadcast on shortwave radio by the French "Laboratoire primaire du temps et des frequences" (LPTF). This time signal is generated by an extremely accurate cesium fountain atomic clock. The time signal is broadcast from Allouis, France at the "Centre National d'Etudes des Télécommunications" with a power of 2 MW at 162 kHz.

1 Signal format
2 Phase modulation pattern
3 Note
4 Reference

[edit] Signal format

TDF is actually an amplitude modulated longwave broadcasting station, transmitting the programs of the France Inter Network of Télédiffusion de France (TDF). Time signals are transmitted by phase-modulating the carrier by ±1 radian in 0.1 s every second except the 59th second of each minute. This modulation pattern is doubled to indicate a binary one. The numbers of the minute, hour, day of the month, day of the week, month and year are transmitted each minute from the 21st to the 58th second, in accordance with the French legal time scale. In addition, a binary one at the 17th second past the minute indicates that the local time is 2 hours ahead of UTC (i.e., summer time), a binary one at the 18th second past the minute indicates when the local time is 1 hour ahead of UTC (i.e., winter time). A binary one at the 14th second indicates that the current day is a public holiday (14 July, Christmas, etc.) and a binary one at the 13th second indicates that the current day is the day before a public holiday. The relative uncertainty of the carrier frequency is 2 parts in 10¹².

[edit] Phase modulation pattern

One signal element consists of the phase of the carrier shifted linearly by +1 rad in 25 ms (known as "ramp A"), then shifted linearly by -2 rad in 50 ms ("ramp B"), then shifted linearly again by +1 rad after another 25 ms ("ramp C"), returning the phase to zero. One signal element is always sent at each second between 0 and 58. Two signal elements are sent in sequence to represent a binary one; otherwise it is interpreted as binary zero. During ramp B of the initial signal element, the exact point the signal phase is at zero represents the top of the UTC second. Since the phase is the integral of the frequency, this triangular phase modulation corresponds to a square frequency modulation with an amplitude of about ±6 Hz.

Both the average phase and the average frequency deviation are thus zero. More data is sent by phase modulation during the rest of each second. But the second marker (and data bit) is always preceded by 100 ms without any phase modulation. The signal is not phase-modulated at all during the 59th second past the minute.

The binary encoding of date and time data during seconds 20 through 58 is "similar to DCF77" (Betke 9-10).