Wave Corrector is less effective against crackle than against other types of noise. The program is designed to restore vinyl recordings which generally contain discrete clicks rather than constant crackle. Crackle is more typical of old shellac records (78's). Unfortunately, crackle is a type of noise which is too continuous for click corrections to be totally effective; but it is too impulsive for a hiss filter to be effective. In Wave Corrector, clicks are detected by a statistical algorithm which analyses the music and looks for statistically significant departures from the norm. A constant background crackle makes it very difficult for this algorithm to differentiate between the music and the noise. On the other hand, the hiss filter assumes a smooth constant level of background noise (rather like when you de-tune an FM radio). Because crackle contains high amplitude instantaneous peaks, it cannot be completely removed by the hiss filer either. In fact, if you apply the hiss filter to crackly recording, the crackles may produce unpleasant artefacts.
The best approach for removing crackle is to Wave Corrector's SuperScan command to make multiple passes of the click corrector. This superimposes new corrections on top of those already present and will significantly reduce the crackle. It's probable however that some crackle will remain even after this process.
Note, crackle can be a by-product of inappropriate cleaning. This is because the cleaning process can leave behind microscopic residues. If you think this is the cause of your crackle, then you should have these residues removed by a professional record cleaning service.
After applying the Hiss Filter, I hear warbling noises in quiet sections of the music. What can I do to prevent this?
These noises are typical of the artefacts you get when the hiss filter is working too hard. This can happen if the hiss filter has been incorrectly calibrated or if the noise present is not suitable for removal by a hiss filter.
The hiss filter is calibrated using a 'noise profile'. It is important to understand how this works if you are to avoid these unwanted artefacts. The 'noise profile' consists of a section of the recording where the noise is present but no music. The noise profile acts as a 'mask' which is subtracted from the rest of the recording. For the process to work properly, the original recording must include a second or two of lead-in or lead-out where there is no music playing. The program uses a portion of the lead-in or lead-out as the noise profile.
However, even when a lead-in is present, the program can still sometimes mis-measure the noise. For example, some live recordings can start with applause. In this case, the program may mistake the applause for noise and cause the program to assume the noise level is much higher than it really is. This in turn causes the filter to over-correct the hiss and the artefacts will then be much worse than they should be.
To avoid this, Wave Corrector provides the 'View Profile' and Capture New Profile' commands. Select the command View Profile on the Waveform menu and then use the Play Block command to verify that the profile is hiss only and that there is no musical content or applause or other sounds are present. If it is not genuine hiss, then you should manually capture a new noise profile as described in the Help file.
Another possible reason for filtering artefacts is if the noise is too spiky. The hiss filter is designed to remove tape hiss which is relatively constant broadband noise (like when you detune an FM radio) If you try to use the hiss filter, for example, on vinyl records, the characteristic of the noise is such that there are instantaneous peaks left behind after the filtering process. These also result in the warbling artefacts.