CHU is the callsign of a shortwave radio station in Ottawa, Ontario, Canada (exact location - 45°17'47"N, 75°45'22"W), operated by the Institute for National Measurement Standards of the National Research Council of Canada. Like its American counterparts, WWV and WWVH, CHU's signal is used for dissemination of official Canadian government time signals. The CHU time signal, as well as the station's transmitted frequencies, are derived from atomic clocks.
CHU transmits 3 kW signals on 3330 and 14670 kHz, and a 10 kW signal on 7335 kHz. The signal used is amplitude modulated, with the lower sideband suppressed (emission type H3E). The same information is carried on all three frequencies simultaneously. The actual time signal is a series of 1000 Hz tones, transmitted once per second, on the second. The top of the minute is marked by a half-second-long beep, and the top of the hour is marked with a one second-long beep, followed by nine seconds of silence. Thereafter, every second except for the 29th second past the minute, CHU transmits a 300-millisecond tone. Between one and sixteen seconds past the minute, CHU transmits the difference between UT1 and Coordinated Universal Time (UTC), in much the same way that the value is transmitted on WWV, but with split tones instead of doubled "ticks". Between 31 and 39 seconds past the minute inclusive, the once-per-second tones are reduced to 10-millisecond "ticks" while a digital time code is transmitted. The digital time code used by CHU is unique, in that it is decodeable by a Bell 103-compatible modem. At ten seconds before each minute, the once-per-second tones are again cut to 10 milliseconds each, this time while voice announcements of the next minute in UTC, alternating between French and English, are transmitted. French announcements are transmitted first on the odd minutes, while English announcements come first on the even minutes.