This simple explanation doesn't tell the full story of how the Dolby systems work. To keep from making a mess of the recording whenever the high-pitched musical sounds were loud to begin with, Ray Dolby made sure his circuit left all loud sounds alone. And to make sure that extra quiet musical tones had a fighting chance to beat the hiss, he made sure that they were boosted all the way. Here's where the hard part comes in. When the tape is played back, the Dolby systems have to treat the music in the exact opposite way that they treated it in recording.
Every boost has to be matched by an equal but opposite cut.
If one thing is off if, say, the tape is dull-sounding to start with the Dolby systems will exaggerate that dullness and make the music sound terrible. The way to avoid problems is to make sure that the tape you use gives a flat sound, with no brightness or dullness when it is played back. Professionals make sure their tapes meet this requirement through the use of a few thousand dollars' worth of test equipment. But there's a much easier and cheaper way of making sure your tape is matched to your cassette desk for proper Dolby operation. All it takes is your FM tuner or receiver.