Numbers stations are dedicated shortwave radio stations that mostly broadcast. . .numbers.
When I first got a short wave radio, the instruction book listed several number station frequencies. I have listened to several of them on the air. They broadcast a few spoken words often, but mainly numbers. There are both artificial voices and real voices--usually female. What makes them fascinating to listen to is that no one knows what these hundreds of stations actually do. But, naturally, there is plenty of speculation.
In the 1950s, Time magazine reported that the numbers stations first appeared shortly after World War II using a format that had been used to send weather data during that war. But, what mostly people believe is that the transmissions are used to send messages to spies. They do send out QSL cards (a tradition among ham radio people) if you send them a reception report. Who knows, maybe that's just part of their cover?
According to The Irdial Discs website, "Shortwave Numbers Stations are a perfect method of anonymous, one way communication. Spies located anywhere in the world can be communicated to by their masters via small, locally available, and unmodified Shortwave receivers. The encryption system used by Numbers Stations, known as a “one time pad” is unbreakable. Combine this with the fact that it is almost impossible to track down the message recipients once they are inserted into the enemy country, it becomes clear just how powerful the Numbers Station system is."
The U.S. has prosecuted several spies from Cuba (and some people living in the U.S.) for espionage, and charged they used Cuban numbers stations to communicate.
According to The Wikipedia:
"The one-way voice link (OWVL) described a covert communications system that transmitted messages to an agent's unmodified shortwave radio using the high-frequency shortwave bands between 3 and 30 MHz at a predetermined time, date, and frequency contained in their communications plan. The transmissions were contained in a series of repeated random number sequences and could only be deciphered using the agent's one-time pad. If proper tradecraft was practiced and instructions were precisely followed, an OWVL transmission was considered unbreakable. [...] As long as the agent's cover could justify possessing a shortwave radio and he was not under technical surveillance, high-frequency OWVL was a secure and preferred system for the CIA during the Cold War."
Also from The Wikipedia:
"The Conet Project has since become somewhat of a cult sensation and counts many musicians and filmmakers among its fans, including Wilco frontman Jeff Tweedy, Melvins collaborator David Scott Stone, Boards of Canada, Manu Chao The Besnard Lakes, Devendra Banhart, former Faith No More vocalist Mike Patton, and director Cameron Crowe. Samples from the collection have been used in numerous films and albums, including Crowe's film Vanilla Sky, Porcupine Tree's Stupid Dream album, We Were Promised Jetpacks' These Four Walls album, and Wilco's Yankee Hotel Foxtrot album, the last of which was an issue of legal dispute; Jeff Tweedy did not seek permission to use the Conet sample and Irdial sued forcopyright infringement. The incident sparked debate about who exactly owns copyright concerning recordings of numbers station transmissions, but Tweedy ultimately decided to avoid taking the matter to court, agreeing to pay Irdial royalties and reimburse its legal fees. The Besnard Lakeshave also used recordings from numbers stations throughout their album, The Besnard Lakes Are the Dark Horse and frontman Jace Lasek is said to be a fan of The Conet Project. Kronos Quartet incorporated live reception of the Conet numbers into "4Cast Unpredictable", a performed sound sculpture in collaboration with Trimpin. Ten years in the making, the piece was performed once only, at Montclair State University Performing Arts Center, New Jersey, in 2007."
"In keeping with its 'Free Music Philosophy', the Irdial-Discs label has made the entire collection available for download in MP3 form (along with a PDF version of the included booklet) on its website completely free of charge and encourages fans to freely distribute it on file sharing networks."
The Conet project collected four CDs worth of numbers broadcasts. You can download them for free from the internet archive here: http://archive.org/details/ird059
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