A click is a short, needle-shaped, wide band disturbance peak which in the case of Figure 1 is followed by a low frequency thump. The system searches through the frequency spectrum to discover the characteristics of this peak, which we hear as a click, then encloses this impulse and deletes the signal. The signal before and after the click is used as a guide to reconstruct the content at the deleted spot. Since hard disk systems do not normally write over an original but add new data to the original sound information, this enables the user to reconstruct the original signal at any time should the substitute signal not be satisfactory. The advantage of such hard disk management is that the user can correct each disturbance directly until such time as a satisfactory result has been attained.
Crackles are comparable to the surface noise on old records.
Unlike the above-mentioned clicks, the crackle is a constant accumulation of tiny clicks. Like clicking, the computer recognises crackling by means of its frequency spectrum, although tiny crackles are much shorter and numerous than the rougher clicks previously removed in declicking. The decrackling gives excellent results, especially for old films. The types of disturbance which do not fit into the limited scheme of automatic fault finding must be removed manually. Whenever a click-like disturbance appears this can be done with the aid of interpolation.