Black box

In science, computing, and engineering, a black box is a device, system or object which can be viewed in terms of its inputs and outputs (or transfer characteristics), without any knowledge of its internal workings. Its implementation is "opaque" (black). Almost anything might be referred to as a black box: a transistor, algorithm, or the human brain.

To analyse something, as an open system, with a typical "black box approach", only the behavior of the stimulus/response will be accounted for, to infer the (unknown) box. The usual representation of this black box system is a data flow diagram centered in the box.

The opposite of a black box is a system where the inner components or logic are available for inspection, which is most commonly referred to as a white box (sometimes also known as a "clear box" or a "glass box").

History

The modern term "black box" seems to have entered the English language around 1945. In electronic circuit theory the process of network synthesis from transfer functions, which led to electronic circuits being regarded as "black boxes" characterized by their response to signals applied to their ports, can be traced to Wilhelm Cauer who published his ideas in their most developed form in 1941. Although Cauer did not himself use the term, others who followed him certainly did describe the method as black-box analysis.Vitold Belevitch puts the concept of black-boxes even earlier, attributing the explicit use of two-port networks as black boxes to Franz Breisig in 1921 and argues that 2-terminal components were implicitly treated as black-boxes before that.

Black box (phreaking)

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

The black box (as distinguished from blue boxes and red boxes) was a small electronic circuit, usually a resistor or zener diode 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.

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 phreaks during the 1960s to 1980s (and in some places like Eastern Europe, well into the 2000s) in order to provide callers with free telephone calls. Sometimes several friends would incorporate a black box into each of their telephones to enable them to hold long conversations with each other without having to pay for them. Another use of black boxes were in the incoming modems of computers running bulletin board systems that were popular back in the 1980s and early 90s.

Flight recorder

A flight recorder, colloquially known as a black box, although it is now orange-coloured, is an electronic recording device placed in an aircraft for the purpose of facilitating the investigation of aviation accidents and incidents.

Any type of aircraft in any condition of flight can be viewed in terms of its input parameters (e.g. control instructions) and output parameters (e.g. flight sensors), without any knowledge of its internal workings, as a black box model. The flight data recorder (FDR) is a device that preserves the recent history of the flight through the recording of dozens of parameters collected several times per second. The cockpit voice recorder (CVR) preserves the recent history of the sounds in the cockpit including the conversation of the pilots. The two recorders give an accurate testimony, narrating the aircraft's flight history, to assist in any later investigation.

The FDR and CVR may be combined in a single unit. The two recorders are required by international regulation, overseen by the International Civil Aviation Organization, to be capable of surviving the conditions likely to be encountered in a severe aircraft accident. For this reason, they are typically specified to withstand an impact of 3400 g and temperatures of over 1,000 °C (1,830 °F) as required by EUROCAE ED-112. They have been required in commercial aircraft in the US since 1967.