Different criteria for "broad" have been applied in different contexts and at different times. Its origin is in physics, acoustics, and radio systems engineering, where it had been used with a meaning similar to "wideband", or in the context of audio noise reduction systems, where it indicated a single-band rather than a multiple-audio-band system design of the compander. Later, with the advent of digital telecommunications, the term was mainly used for transmission over multiple channels. Whereas a passband signal is also modulated so that it occupies higher frequencies (compared to a "baseband" signal which is bound to the lowest end of the spectrum, see line coding), it is still occupying a single channel. The key difference is that what is typically considered a "broadband signal" in this sense is a signal that occupies multiple (non-masking, orthogonal) passbands, thus allowing for much higher throughput over a single medium but with additional complexity in the transmitter/receiver circuitry.