Dynamic Noise Limiter

The Dynamic Noise Limiter, or DNL for short, is a noise reduction system introduced by Philips in the late 1960s to improve the quality of audio recordings on cassette tape. It is used on playback only, and does not require encoding the original audio signal. It was not widely used, as Dolby noise reduction became very popular. Third-party implementations called it Dynamic Noise Reduction, or DNR. As Dolby may be initialized DNR, there is some confusion between the two.

The small physical size of cassette tapes, compared to reel-to-reel tapes, means they must use a slow tape speed to allow practical recording times. Their smaller width also limits the dynamic range of the signal that can be recorded to it. This results in much higher amounts of audible tape hiss compared to reel-to-reel systems. Tape hiss occurs on all magnetic tape systems, as an ever-present low-volume, high-frequency signal. On reel-to-reel systems the higher dynamic range and faster tape speeds allowed the signal to be recorded at volume levels that overwhelm the hiss. On cassettes, there is simply not enough dynamic range to do this, especially in quiet passages, and the hiss becomes very audible.