As far back as the 1970s there was a mainstream variant of this myth involving a `Trunk Line Monitor', which supposedly used speech recognition to extract words from telephone trunks. This is much harder than noticing keywords in email, and most of the people who originally propagated it had no idea of then-current technology or the storage, signal-processing, or speech recognition needs of such a project. On the basis of mass-storage costs alone it would have been cheaper to hire 50 high-school students and just let them listen in. Twenty years and several orders of technological magnitude later, however, there are clear indications that the NSA has actually deployed such filtering (again, very much against U.S. law).
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