It was the invention of an effective noise reduction circuit by Ray Dolby in the 1960s that got Dolby Laboratories onto the road to where it is today. This circuit was developed into Dolby Type A noise reduction, which became very widely used for professional analog recording onto tape, prior to the advent of digital recording. Most analog recording media produce background noise, typically like white noise. The simple solution would be to boost the middle and upper frequencies during recording, then cut them on playback, thus also cutting the hiss. But this causes overloading problems. Dolby noise reduction systems rely on the masking effect of sounds. If the signal was loud, the hiss would be largely inaudible anyway. So Dolby's system tracked the level of the signal, leaving it unaltered when it was strong, but introducing the boost/cut system when the signal was low. Dolby noise reduction later made it onto consumer equipment in the form of Dolby B, C and S noise reduction systems which all work along similar lines. It was Dolby B, in particular, that allowed the compact cassette to become an established media. Dolby HX, which also appears on recent cassette decks, is not to be confused with the noise reduction systems.