CHU: Canada's time signal from Ottawa on 7850 kHz

What you hear

On 7850 kHz the seconds are marked not by clicks but by short tone pulses, each a burst of 1000 Hz about three tenths of a second long, just enough to have a pitch of its own. The pulse at the start of the minute is longer, and the pulse of the 29th second is left out. Then come the voices. In the closing seconds of each minute a recorded announcer gives the time, in French in one minute and in English in the next: "CHU Canada, quatorze heures vingt-six minutes, temps universel coordonné", and a minute later "CHU Canada, fourteen hours twenty-seven minutes, Coordinated Universal Time". Shortly after the half-minute there is a short burst of warbling, like an old dial-up modem: that is a digital time code in the Bell 103 format at 300 bits per second, which a computer with a modem could decode to set its clock long before network time existed.

Who transmits and why

CHU is run by the National Research Council of Canada, the keeper of the country's official time, from a transmitter site in the countryside south-west of Ottawa. Time signals have been going out from Ottawa since the 1920s, and the letters CHU have been on the air since the 1930s, which makes it one of the oldest time stations anywhere. It broadcasts around the clock on 3330, 7850 and 14670 kHz, with the most power on 7850. The middle frequency is the newcomer: until the beginning of 2009 it was 7335 kHz, inside a range that was handed over to shortwave broadcasters, and the station moved up to 7850 to get clear of them.

The signal is upper sideband with a full carrier, a form of AM compatible with ordinary receivers. That is why the catalogue sets USB: it gives the cleanest audio and the narrowest filter. In AM the voice is just as clear with a little more noise; in LSB you hear nothing but the carrier.

When and where to listen

Across eastern North America 7850 kHz is loud day and night, and receivers in the United States and Canada hear it like a local station. From Europe CHU is one of the easiest North American time signals to catch: in the evening and through the night the 7 MHz path across the Atlantic opens regularly, and receivers in Ireland, Britain, the Netherlands and Scandinavia hear the pulses and both languages clearly from around midnight UTC until European sunrise, with winter nights the best. The frequency sits just above the 40 m amateur band, so there is little competition on it. 14670 kHz is the one for daytime long-distance reception, 3330 kHz for the hours of darkness near the station.

A detail worth watching on the waterfall: the pulses show up as a regular row of dashes to the right of the carrier, and the modem burst as a short wider smudge once a minute.

In ShackDeck

Station catalogue → Time signals → CHU 7850 kHz. The program finds a public receiver that covers 7850 kHz, tunes it in USB with a passband wide enough for the voice and shows the waterfall. No receiver, antenna or account of your own is needed; if the voice sounds thin, switch the receiver to AM and it will sound the same with a touch more noise.