A numbers station is a category of shortwave radio station characterized by broadcasts of formatted numbers, addressed to intelligence officers operating in foreign countries. Most identified stations use speech synthesis to vocalize numbers, although digital modes, such as Phase-shift keying and Frequency-shift keying as well as Morse code transmissions are not uncommon. Most stations have set time schedules, or schedule patterns, however other stations appear to be broadcast at random times. Stations may or may not have set frequencies in the HF band.