improved DNL

Dynamic Noise Limiting (DNL) is a noise reduction system patented by Philips, which is particularly useful for the reproduction of (cassette) tape recordings. As the name suggests, the system is dynamic, i.e. the noise is only suppressed at the moments when it is most intrusive which, in the case of a music signal, is during the quieter passages. The system also exploits an interesting psychoacous-tic effect, namely that during quiet passages the high frequency signal components are less important than is the case during louder sections of the music. A DNL circuit utilises this fact by attenuating the high frequency components, and hence the noise, during low amplitude portions of the input signal.