Real analog-to-digital converters lack the discrimination to pick up sub-microvolt, nanowatt radio signals. Therefore a low-noise amplifier must precede the conversion step and this device introduces its own problems. For example if spurious signals are present (which is typical), these compete with the desired signals within the amplifier's dynamic range. They may introduce distortion in the desired signals, or may block them completely. The standard solution is to put band-pass filters between the antenna and the amplifier, but these reduce the radio's flexibility - which some see as the whole point of a software radio. Real software radios often have two or three analog "channels" that are switched in and out. These contain matched filters, amplifiers and sometimes a mixer.