Unfortunately, the algorithms used in Wave Corrector are proprietary, so we cannot reveal exactly how they work. However, the program implements two distinct functions: click detection and waveform regeneration.
For click detection, the program calculates the instantaneous rate of change for each sample point and performs a statistical analysis of these rate of change values. Outlier values are marked as potential clicks, and then further analysis is performed to differentiate actual clicks from musical transients.
Waveform regeneration is performed using an iterative technique. The program performs a frequency analysis of the waveform in the vicinity of the click being corrected. The most significant frequency components are used to generate an initial replacement waveform, and then the process is iterated using a technique similar to successive approximations.
A newly inserted manual correction is given minimum width and hence has no effect until it is accurately positioned over the click being corrected and the width adjusted to an appropriate value.
If, having done this, a click is still audible then there is probably a second click very nearby which you will need to correct as well.
What this demonstrates is that Wave Corrector's click detector can mis-fire due to musical content. Although this might seem a problem, in practice it usually has a negligible effect on the quality of the end result. This is because the replacement waveform that Wave Corrector generates (the 'correction') is calculated to accurately match the surrounding wave. Hence you are unlikely to hear any aberrations as a result of these 'false positives'. The only exceptions are with certain instruments (eg low frequency brass) which can cause clusters of corrections very close together. These can alter the quality of the sound making it more 'rasping'. The signature command on the view menu enables you to determine if this is happening and you can remove these spurious corrections using the block commands.
Click discrimination is a difficult process because the program has to find clicks in the presence of such a wide variety of possible musical content. Wave Corrector, in common with most similar software, uses a statistical method which looks for sudden changes in the statistics describing the wave.
All click eliminator programs suffer from this problem to some extent. In Wave Corrector, we've concentrated on making the 'corrections' as accurate as possible and also on providing the tools to enable you to audition and correct any mistakes the program makes.
There is a section in the Help File on 'false negatives' and 'false positives' which goes into this in more detail.
If Wave Corrector doesn't completely remove a click, there are two possible procedures for manually removing it:
1. It may be possible to adjust the correction for the particular click. Depending on the characteristics of the click, Wave Corrector may underestimate its severity and hence under-correct it. To adjust the correction, first use the horizontal scale controls and the lower scroll bar to move to the point in the waveform where the click is occurring. Zoom in to about 50 samples per division. if you then double click with the mouse over the click, its correction will be selected in the correction list. You can now drag the edge of the correction with the mouse to widen it; and also use the left/right arrow keys to move the correction from side to side. Doing a combination of these things, you may be able to eliminate the pop. You can audition the effect of your adjustment using the ?Audition Corrected? toolbar button.
See the next question for a more detailed description.
2. It may be, if the click is very severe, that you cannot completely eliminate it with the above procedure. In this case, you can use 'Cut and Splice' to remove the click completely. To do this drag, with the mouse from left to right over the click. This will select the click as a block. Then use Block - Cut & Splice, to remove the click from the waveform. Note, before using Cut & Splice you can use the Audition Corrected toolbar button to audition the effect of the operation.
The easiest way to understand the Adjust/Insert procedure is to use it to adjust an existing correction (rather than inserting a new one).
Select a correction from the correction list, preferably a large yellow or red one. If the horizontal scale is sufficiently zoomed in, then you can adjust the correction as follows: move the mouse to the 'edge' of the correction; the mouse pointer will change to East-West arrows. Hold down the left mouse button and drag with the mouse. As you drag, the correction will become wider or narrower and you can see how the corrected wave changes versus the underlying uncorrected wave. Note, if the display is zoomed out too much, then you will not see the East-West arrows and you will have to invoke the command using the Adjust/Insert toolbar button. As well as adjusting the width of the correction, you can also move it to the left or right by using the cursor left/right keys. You will see how by adjusting the width and position of a correction, you can visually adjust the shape of the wave to get the best looking result; you can use the audition before and after toolbar buttons to hear the effect of your changes.
Having familiarised yourself with adjusting corrections, you can then move on to the slightly more complicated task of inserting new ones.
To insert a new correction, you need to visually identify the point in the waveform where the click is occurring. Sometimes this is obvious but other times it can be difficult and you need to use trial and error. Having identified the point where you think the click is, centre the display on that point and zoom in to say, 50 samples per division. (note, to centre the display you can simply double click over the point you want to centre on). Then, use the same procedure as described above to 'drag' a correction over the click. Again, you can use the audition commands to see if your correction has eliminated the click.
If I have a very noisy recording, can I keep re-processing it in Wave Corrector to achieve optimum noise reduction?
As a general rule, it is best not to perform multiple passes of the click corrector. This is because of the statistical method that Wave Corrector uses to discriminate between clicks and music. When it scans a waveform that has already been corrected, it is much more likely to trigger falsely. And if you continue to re-scan several times, eventually audible distortion will result.
However, for exceptionally noisy recordings, you can use Wave corrector's Super-Scan command to make multiple passes.
When you insert or adjust a correction, the program needs to have access to the waveform data in the vicinity of the correction. However, when you apply a filter, the waveform data is altered; and therefore it is not available for the insert/adjust commands. Hence, Insert/Adjust is disabled.
If you find you need to insert or adjust a correction after you've applied the rumble (or other) filter, then you need to use the 'Remove Filtering' command on the Waveform Menu. This will remove the filtering and re-enable Insert/Adjust. After you've made your insertions/adjustments you can then re-apply the filter.
I've corrected a recording at sensitivity 4 (or 5) and now some sections are noticeably distorted. What can I do?
Unfortunately, sensitivity levels 4 and 5 are quite aggressive and can cause distortion with certain types of music. Human voice can be affected as well as some low frequency brass and woodwind instruments. The most sensitive settings should therefore be used as sparingly as possible.
If you find that you definitely need to use setting 4 or 5 for most of the file, then you can use Wave Corrector Block commands to re-scan the distorted sections at a lower threshold. You simply need to drag a block over the section where distortion is occurring, and then select the command 'Re-Scan Block'. Select a lower setting (say 3) and the distortion should be removed. Of course, with the less aggressive setting, you may get the return of some of the noise. However, this will almost certainly be preferable to the distortion. Note, it is possible to manually insert corrections over clicks. So if re-scanning at level 3 causes some clicks to re-appear, you can try manually removing them. However, this is quite time consuming and requires a degree of skill/practice.