background noise. The engineer may mix programming that will sound pleasing on the widest range of playback systems used by regular listeners (i.e. high-end audio, low-quality radios in clock radios and "boom boxes", in club PA systems, in a car stereo or a home stereo). While some broadcasters like the BBC generally believe in using monitors of "the highest practicable standard of performance", some audio engineers argue that monitoring should be carried out with loudspeakers of mediocre technical quality to be representative of the regular systems end-users are likely to be listening with; or that some technical defects are apparent only with high-grade reproducing equipment and therefore can be ignored. However, as a public broadcaster dealing with a lot of live material, the BBC holds the view that studio monitors should be "as free as possible from avoidable defects". It is argued that real life low-grade sound systems are so different that it would be impossible to compensate for the characteristics of every type of system available; technical faults must not be apparent to even a minority of listeners while remaining undetected by the operating staff. It is further argued that, because of technical progress in the science of sound transmission, equipment in the studio originating the programme should have a higher standard of performance than the equipment employed in reproducing it, since the former has a longer life. In fact, most professional audio production studios have several sets of monitors spanning the range of playback systems in the market. This may include a sampling of large, expensive speakers as may be used in movie theatres, hi-fi style speakers, car speakers, portable music systems, PC speakers and consumer-grade headphones. Amplification: Studio monitors may be "active" (including one or more internal power amplifier(s)), or passive (requiring an external power amplifier). Active models are usually bi-amplified, which means that the input sound signal is divided into two parts by an active crossover for low and high frequency components. Both parts are amplified using separate low- and high-frequency amplifiers, and then the low-frequency part is routed to a woofer and the high-frequency part is routed to a tweeter or horn. Bi-amplification is done so that a cleaner overall sound reproduction can be obtained, since signals are easier to process before power amplification. Consumer loudspeakers may or may not have these various design goals. History 1920s and 1930s In the early years of the