A software solution
ClickRepair is an application for finding and repairing clicks and crackle. Most of the time it will do its task automatically. Displayed left is a click – caused by a small scratch in the record – which has been automatically detected and repaired. It would be heard quite clearly without the repair, afterwards it may be imperceptible, or at worst replaced by a far less objectionable artefact. Of couse there will be some who prefer to listen to the original and let the human auditory system separate music from noise. The more intrusive the noise, the more likely it is that some restoration will be preferred.
Detection and repair
Declicking is a two-stage process – detection followed by repair. The samples which are not deemed to be damaged are left unchanged, whilst the damage itself is replaced by an interpolation based on analysis of the surrounding sound. Some audio editors offer an interpolation option, but it is unlikely that you will have enough time and/or patience to find and make the necessary repairs without a high degree of automation. Because 99.5% of a good LP will not be affected by scratches and similar problems, filtering the file is not a good solution. The typical artefact introduced by the single restoration shown on the left will be imperceptible. On a good LP they should be sufficiently separated (in time) that they remain imperceptible. The choices involved in restoring older 78rpm shellac records, and badly damaged LPs, are much more complex and subjective. On a 78rpm shellac record defects may occur so regularly that they are replaced by some kind of lower frequency "fuzziness".
• It is impossible to distinguish, with absolute certainty, music from damage.
• Conventional filtering will introduce degradation that is worse than the noise.
• ClickRepair is not a filter – it only changes samples judged to be in need of repair.
Listen to the samples to see what can be achieved in reducing clicks and crackle using ClickRepair. You might even change your opinion about the utility of statistics! And remember that, the higher the "repair rate", the more likely it is that the indivdual artefacts will be replaced by some kind of vague "fuzz" or "distortion".
ClickRepair is designed to be automatic, but it is possible to examine each and every repair. With a relatively fast computer, automatic processing is better than real time (it can be better than 20x), or the results may be monitored using headphones or speakers (which slows it down to real time). 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 noise. Processing can be stopped at any time, or at repairs longer than a chosen length, and repairs can be monitored or even changed, although this makes for very slow going!