These QRM-generators were used mostly by Eastern Bloc countries to jam shortwave transmissions from the West. They used all manner of continuous tones and raucous audio, anything to make the desired signal unintelligible.
The Radio Jamming website has some excellent material backgrounding their existence along with sound clips so you can hear just how pervasive their cacophony was. The website is the work of Rimantas Pleikys, not only a shortwave listener but also the former minister of communications and informatics and past member of the Lithuanian parliament!
Thanks to Paul Rawdon for letting us know that the DVD Empire of Noise mentioned on the home page is on YouTube
[youtube=http://www.youtube.com/watch?v=nXV4nTfGHuI]
