Tutorials > How to perform noise removal

Noise removal, or noise reduction, is the process of removing noise from a signal. Active Sound Editor comes with two different filters for removing two different types of noise:

•	DeHiss Filter

Hiss degradation is a form of random additive noise composed of electrical circuit noise, irregularities in the storage medium etc. Analog magnetic tape recordings contain hiss due to the size of the magnetic particles used to make the tape. It can be reduced by using finer magnetic particles or by increasing the amount of tape used per second for recording. Due to its random nature, Hiss noise generally has significant components at all audio frequencies, and thus simple filtering and equalization procedures are inadequate for restoration purposes.

The filter implemented by Active Sound Editor is a multi-band digital noise gate and filter is automatically shaped from noise profile. This filter works in two steps:

1.	Generate Noise Profile

In this step, a small portion of input audio is selected (which you think only contains noise and no other audio). The selected portion is supplied to filter to generate noise profile through the Effects.DeNoiseFilterProfileSet method. In this process, filter checks each noise sample and determines its threshold in each frequency band. Threshold from all input samples is accumulated to noise gate.

2.	Remove Noise

In this step, started through the Effects.DeNoiseFilterApply method, all input audio (from which noise is to be removed) is processed through the noise gate generated above and the result is noise free audio.

•	DeClick Filter

Clicks are finite duration defects in audio occurring at random positions. There are many mechanisms by which clicks can occur. Typical examples are specks of dirt and dust adhering to the grooves of a gramophone disc or granularity in the material used for pressing such a disc. Further click-type degradation may be caused through damage to the disc in the form of small scratches on the surface.

Similar artifacts are encountered in other analogue media, including optical film sound tracks and early wax cylinder recordings, although magnetic tape recordings are generally free of clicks. Ticks can occur in digital recordings as a result of poorly concealed digital errors and timing problems.

Pops are low frequency noise pulses due to scratches or even breakages in the surface of gramophone disc. The form of pulses also depends upon the characteristics of playback system.

The filter is started through the Effects.DeClickFilterApply method and works in two phases. In the first phase it detects clicks and pops in the source audio and the second phase it removes them. For detection filter searches for short pieces of audio that are much louder than the region before or after it. In clicks/pops removal phase, filter removes original audio from suspected regions and fills it by interpolating audio from surroundings.