A black box is a unit of which the transmission characteristics are known, but not its internal functioning.
A black box is a device or system component with known properties, which are expressed by the fact that the output signal has a functional relationship to the input signal
. Theinner function may be a logical function, but it may also be
function expressed in termsof gain, frequency response
or impulse response, or other parameters. Knowledge of the inner function, of its structure or mode of operation, is not of interest for the use of black boxes, since they are regarded as analogue or digital functional units.