In the UK Guardian news I’d read an article about all the Snowden/GCHQ kerfuffle, which though correct about the issues regarding surveillance and free speech, was riddled through with all manner of technical errors (thse also being how their journos got in bother anyway!).
One bit also said “government interception has moved on from attaching crocodile clips to telephone lines” – which made me laugh as it was only private investigators locally monitoring a target who would do this, the govt lot put their intercept in via a jumper wire at the Telephone Exchange distribution frame, and even if another engineer knew why it was there everyone kept quiet.
Last week I recovered two “Block Terminals” from the era of the General Post Office in Britain, and the date code of the better looking one was 1972 (year of my birth), so I thought “that will go on my wall for the telephone extensions”.
Then I read the Grauniad article, which inspired me to build an old style audio intercept point as well. Now I can of course do the whole lot digitally via softphones, but I’d rather use my PC’s for better quality audio (the output of the tap currently goes to a squawk box made from a cheap amplifer PCB and the speakers out of a knackered clock radio) or an analogue cassette recorder (remember those?)
This is the block terminal and clips to the line being monitored
Below the intercept circuit and the inside of the block terminal cover (the internal extension dial plan of my system is no secret, and 399 (where the circuit is connected) or any number ending in 99 is generally used in Britain as a test extension by telephone engineers (which ends up often connected to all manner of specialist equipment))
close up of intercept circuit, made up of parts pulled from circuit boards of scrap equipment (things like winmodems for which drivers are no longer produced but often have a useful transformer, and kaput PSUs from PCs which yield many elcos and also high voltage capacitors) This is very simple analogue circuit, using a capacitor and resistor (to present high impedance to the telephone line and not seize it (the capacitor blocks the DC voltage), then to the isolation trafo from the old modem, the secondary of this then connected to 2 x rectifier diodes back to back as quick and dirty limiter to protect audio equipment (it limits strong ringing voltage to 0.5V RMS). the 10k pot is audio output level control….
