Black box (phreaking)

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Black boxes were devices which, when attached to home phones, allowed all incoming calls to be received without charge to the caller.^[1]

The black box (as distinguished from

in series with the line. It relied on (now-obsolete) telephone exchanges controlled by mechanical relays.

These exchanges used a relay to detect a drop in line voltage (usually to less than -10V off-hook, compared to -48V when on-hook) to begin billing for a call; a separate relay controlled ringing on the line. The black box placed a resistor in series with the line, so that the off-hook voltage was closer to -36V: just enough to stop the ringing, but not enough to trigger billing. A bypass capacitor was often added to prevent the device from attenuating AC signals such as transmitted voice.^[2]

A call originating from a telephone fitted with a black box would still be charged for by the telephone company unless some method to circumvent the call charging was deployed. Black boxes were commonly built by phone

that were popular back in the 1980s and early 90s.

Electronic switching systems rendered black boxes obsolete, as no audio path was established until the call was answered. The infinity transmitter, an eavesdropping device which in its original design relied on an audio path to the target line remaining open before a call was answered or after it was hung up by the recipient, was similarly affected by the demise of mechanical switching.

See also

• Phreaking

References