As already mentioned, the Hiss and Hum reduction filters use a technique known as 'Noise Subtraction'. This technique takes a 'snapshot' of the unwanted noise that is to be removed. This snapshot is known as a 'noise profile' and it is used to generate a noise mask that is 'subtracted' from the entire file.
For the technique to be successful, it is very important that the noise profile consists of typical noise from the file. It must be taken from a quiet passage where there is noise present but no music. If there is any music at all present, then program will over-correct the noise and unpleasant artefacts are likely to be the result.
It is also important to remember that the noise must be constant throughout the file. If the noise varies throughout the file then it won't be possible to locate a typical noise profile. This will result in some sections being over-corrected and others under-corrected.
The technique is really only successful for removing tape hiss, hum or interfering tones. However, in the case of interfering tones, these might be more effectively removed with a notch filter. See the section on the Graphic Equaliser/Filter.
Wave Corrector automatically generates a noise profile when you load a wave file. To do this it searches the first and last 30 seconds of the file, and if that is unsuccessful, it looks for a quiet section within the file (for example at a track change). To aid the program in its task, you should always ensure that your recordings include a few seconds of lead-in or lead-out. In this way you will help the program to locate a suitable profile. There are certain types of recording however, that it is not possible to auto-profile. An example is a live concert that starts with applause. Applause has very noise-like characteristics and it is likely to be mistaken by the program for actual noise. As, in the example above, this will result in the program over-correcting the noise and causing artefacts.
If the program cannot find a suitable profile, it issues a warning that the profile may be unreliable. If you see this warning, or if you are at all suspicious that your recording may not be suitable for auto-profiling, then you should check the validity of the auto-profile and manually create a new one if necessary.
To check the auto-profile, use the Waveform - Edit Noise Profile command. This will centre the main display con the current profile and enable you to audition it.
If the auto-profile is unsatisfactory, you can move its position to somewhere more suitable.