These are pictures of a new art installation in Derby Square Liverpool.
The techniques for extracting it from recordings have been known since the early 19th century when Joseph Fourier invented Fourier analysis it been used for forensic analysis for more than 10 years but to describe it as "surveillance" is simply tin hat conspiracy theory click baiting. The only thing that HMG actually watches for this technique is the mains itself and it isn't just them people who specialise in forensic analysis for court use make their own recordings.
The signal in the UK has always been tightly regulated in part to allow synchronous clocks to work, there was a requirement for the number of cycles in a day to be a fixed number, so the national grid had to make adjustments to counter the natural variation. In conjunction with Radio Clocks and the time signal, this created a nationwide accurate time, which replaced railway time, which had replaced solar time.
It is just a pity that the entire thing has been hijacked by the word surveillance.
Oh and one other thing if you are an electronic engineer or a HiFi enthusiast who has spent years trying to get rid of the 50hz buzz or perhaps someone who likes to sit quietly in Derby Square and watch the world go by, it is too noisy.
If anyone fancies playing with FFTW I have used this C library on several occasions FFTW there is even a play one in Java for the amateur in JTransforms. It is not limited to sounds but can also be applied to images see here.