Corrective Linear-Phase Noise Reduction (CLNR): A Comprehensive Solution for Removing Noise and Artifacts
Denoising software performs the complex task of reducing or removing undesirable noise and artifacts from audio, while affecting the program material as little as possible. CLNR is a patented set of algorithms that provide a highly effective denoising toolkit, while requiring greater efficiency and lower CPU overhead than other solutions. CLNR is the basis for BIAS' SoundSoap, SoundSoap Pro, and SoundSaver products, which provide consumers and professionals with high-performance, cost-effective denoising solutions.
Denoising is usually broken down into three tasks: click and crackle removal, hum and rumble removal, and broadband noise reduction. Each of these tasks is achieved through entirely different processes. The methods used and the order in which processes are executed are both determinants of the level of success a denoising tool can achieve.
Typically, removal of clicks and crackle is accomplished using a model that splits a signal into two parts: the tonal component and everything else that is not tonal. The non-tonal component of the signal is then examined, and crackle identified in it and removed. Many denoising systems use a process that fails to eliminate all of the crackle.
CLNR uses a more rigorous method of expunging crackle, ensuring maximum success in its removal. This type of processing can be very compute-intensive, but BIAS has developed a particularly efficient model for the high degree of effectiveness it exhibits.
Click removal looks for signals whose amplitude is out of proportion to the rest of the signal. CLNR has an intelligent envelope detector that yields excellent accuracy in click detection.
Rumble removal, the simplest processing in the package, consists simply of a high-pass filter, but the filter applied by CLNR is of a careful, linear-phase design. The hum reduction is also accomplished with linear-phase filtering, but it attenuates at the frequency set by the user using a slider, and then also attenuates at integer multiples of that frequency. A bandwidth parameter allows the user to adjust for the level of "dirtiness" in the hum, and an attenuation parameter lets the user determine how heavily the process is applied.
Broadband noise reduction is achieved through a well-known spectral subtraction technique: first, the profile of the noise floor in the signal is learned, then an expander applied to each FFT bin attenuates portions of the spectrum that are below the noise floor, thus pushing down the noise level. The thresholds are set initially as a result of the analysis, but CLNR provides 12 sliders that allow the thresholds to be modified by hand. The expansion ratios are also available for modification, in order to allow the processing to be adjusted for each application.
The level of complexity in the user interface is scalable to a level appropriate to the user and application. SoundSoap Pro is aimed primarily at audio professionals, so it offers users access to a considerable number of processing parameters, including the gate thresholds in the broadband noise reduction section. SoundSoap 2 is targeted at a broader audience, so it presents a very streamlined interface, with processing parameters like the gate thresholds being handled intelligently by the program "under the hood." The underlying technology is the same in both products. SoundSaver, the latest product from BIAS to include CLNR, is a low cost "all-in-one" solution optimized for archiving tapes and LPs.