Hiss, and other mid- to high-frequency noise, can be reduced using the DeNoise application.
Hiss reduction – how it works
Suppose you had a 1024-band graphic equalizer, and that you could adjust all 1024 gain controls 80 times a second. Impossible? Quite possible! DeNoise operates as an intelligent graphic equalizer, dividing the incoming audio into 80 overlapping frames per second, automatically and continuously adjusting the gain of 1024* frequency bands. It uses algorithms that judge what will be perceived as noise, and what will be perceived as desirable audio, and applies a balancing act to suppress the noise without doing perceptible damage to the sound.
* 2048 bands if the sample rate exceeds 48kHz.
If it's not broken ...
Have you ever wondered why mp3 and mp4 can sound as good as it does? The amazing thing is that 90% of the original data has been thrown away! It's all done using the science of psycho-acoustics – the study of how we actually perceive sound. DeNoise uses the same scientific principles as these audio codecs. Algorithms from mathematical statistics first judge what might be fixed after which methods from psycho-acoustics decide between what should to be fixed and what is best left alone. Some of the noise will be masked by the actual sound – such noise is best not touched. Moreover, the DeNoise interface allows you see precisely what it is doing, and why, to help in choosing appropriate settings for each particular audio file.
DeNoise is designed to be automatic, but the results may be monitored using headphones or speakers. Sound output can be selected to play the incoming untreated audio, the outgoing repaired audio, or the difference, which is what has been discarded as audible noise. Processing can be stopped at any time, so that the effect of different settings can be auditioned using a Preview function.