Dynamic Noise Filter and Signal Processor

[Craig Maier]

Dynamic Noise Filter


(Analog Noise Filter)
This is a digital simulation of a dynamic analog filter. It is useful for dynamically attenuating "Hiss" from old record recordings or from old magnetic tape recordings. It performs better than a fixed Low-pass filter because it only attenuates high frequencies when there is no high frequency information present above the setting of its "Noise Threshold" slider adjustment. Sometimes this technique is referred to as "single-ended noise reduction". The Dynamic Noise filter's Low-pass corner frequency is frequency modulated by a rectified envelope signal that represents the amplitude of the signal content above a particular low-pass corner. So, normally, the bandwidth of this filter is limited until some high frequency content is measured by its high frequency detector. When this occurs, the bandwidth of the filter is opened up to allow the frequency of interest to pass through. When the high frequency signal diminishes again below a threshold value, the filter closes back down to a smaller bandwidth. The user has the ability to adjust a number of parameters with this filter, including noise threshold, filter frequency, attack time (the time constant associated with the signal whose job it is to increase the Low-pass filter frequency corner), release time (the time constant associated with the signal whose job it is to decrease the Low-pass filter frequency corner after a high frequency event has ceased) and HF Gain (high frequency gain). This filter should only be used on recordings that contain little or no impulse noise, or on recordings that have already been processed through the Impulse Noise filter first to minimize unnecessary filter “breathing” effects. This Filter can be used either as a dynamic noise reduction tool or as a dynamic high frequency enhancer, or both if two are placed in cascade within the Diamond Cut Multi-Filter.


The Dynamic Noise Filter
The Dynamic Noise Filter provides the following slider controls:

• Noise Threshold (0 to 10,000)

The Noise Threshold is the value of rectified and averaged High-pass signal level above which the output of the Dynamic Noise Filter starts to raise its corner frequency. Moving the noise threshold slider control vertically raises its value. This control must be adjusted so that when there are no highs present in the source material, background "Hiss" is attenuated, but when "highs" are present (such as cymbal crashes or the pronunciation of the letter "S"), the filter "opens up" and passes those “highs” to the output of the system.

• Filter Frequency (200 – 19,999 Hz)

This is the 1st order High-pass corner filter frequency which drives the Dynamic Noise Filter Detector / Rectifier / Attack & Release Time Constant circuitry. For modern reel-to-reel tapes, this parameter will be operated generally up in the 4 to 6 kHz range. For early 78s, it will be operated in the 1 to 3 kHz range. The Filter Frequency parameter range is from 200 Hz to 19.999 kHz. Operation of this setting above 10,000 Hz is rarely of any value, but it is allowable. Experimentation will be required to determine the best setting and will vary greatly depending on your source material.

Important Note:The entire frequency range of adjustment up to 19,999 Hz is only possible when utilizing a 44.1 kHz sampling rate or higher. At a 22.05 kHz sampling rate, the maximum effective frequency setting will be 10 kHz, and at an 11.025 kHz sampling rate, this value will drop to 5 kHz.

• Attack Time (1 to 300 mSec)

The Attack Time slider adjusts the time constant (in milliseconds) associated with the rising edge of a High-pass signal envelope. Fast music will require smaller values of attack time compared to slow music. The range of adjustment for the Attack Time parameter is 1 to 300 milliseconds.

• Release Time (1 to 500 mSec)

The Release Time slider adjusts the time constant (in milliseconds) associated with the falling edge of a High-pass signal envelope. This parameter will also require smaller values for fast music compared to the requirements of slow music. Also, the release time will almost always be set to a period of time greater than the Attack time for the algorithm to sound natural. The range of adjustment for the Release Time parameter is 1 to 500 milliseconds.

• HF Gain

Gain controls the amount of dynamic High-pass filter signal that is summed back into the output of the filter. This allows you to obtain upward or downward expansion of the high frequency portion of the audio spectrum. The "neutral" setting for this would be 0 dB that represents no expansion or compression. Setting this value greater than 0 dB will be produce a "Spectral Enhancer" function. Values of 0 dB and lower produce a Single Ended Noise Reduction function used to de-Hiss a sound source. This control is calibrated in dB. Its range of adjustment in normal noise reduction mode is from 0 to -90 dB. Its range of control in Spectral Enhancer mode is from 0 to + 10 dB.

• Enhancer Mode

Checking this box switches the Dynamic Noise Filter from Noise Reduction mode to Spectral Enhancer Mode. Enhancer mode provides upwards expansion of signals above Filter Frequency setting and its threshold setting. The degree of Enhancement is determined by the HF Gain control setting. Enhancer mode can add some life to dull recordings without introducing a great deal of noise into the signal (as would an ordinary Equalizer).

Important Note:
The controls can be adjusted "live" when the preview mode button is clicked.
Dynamic Noise Filter Operating Procedure (Tutorial)


Most of the parameter settings for this filter will vary considerably depending on the content of your particular .wav file. You will have to experiment to determine the values most to your liking. The values used below in the Procedure Example will get you started.

1. Highlight the portion of your .wav file on which you desire to apply the Dynamic Noise Filter. (You may choose to highlight the entire file or any portion thereof.)
2. Click on the "Filter Menu" with the left mouse button.
3. Click on "Dynamic Noise Filter."
4. Set the Noise Threshold slider all the way down; this is the minimum threshold position of the slider control.
5. Set the Filter Frequency to around 1.5 kHz.
6. Set the Attack Time to about 5 mSec. (Unless you are attempting to obtain some sort of special effect, the Release Time should always be set to a value greater than or equal to the Attack Time.)
7. Set the Release Time to around 50 mSec.
8. Set the Gain Control to - 6 dB as a starting point for noise reduction. (This control should only be set to higher positive value numbers if a "Spectral Enhancement" effect is desired and Enhancer Mode is checked.) This Gain Control will modify the "effective bandwidth" of your recording by incrementally amplifying the high end of the spectrum when there are enough “highs” present to trip the detector. This can be used for noise reduction or to increase the "presence" of a recording, or to enhance the sound of a vocalist. If greater noise reduction is desired set the gain control to higher negative values. If greater enhancement is required, set the gain control to higher positive values (when in Enhancer Mode).
9. Click on "Preview".

When the filter is operating properly, "Hiss" will be reduced, but when there is high frequency content on the recording, the filter should "open up" and pass through more "highs" to its output. If the Threshold is set too high, the filter will never open up, and the .wav file will sound "dull" although "Hiss" may be reduced. If the Threshold is set to low, the filter will always be opened up to full bandwidth, and there will be no noise reduction action. If the Attack time is set too long, there will be a delay heard before the filter changes bandwidth on musical high frequency transients such as cymbal crashes (loss of the leading-edge). If the Release time is set too long, there will be a residual "Hiss" left behind after a high frequency musical event, which will decay out, but too slowly and therefore, unnaturally.
When you determine the best setting of the controls for your particular .wav file, click Run filter. When the filter has completed its operation, the results will appear in the Destination Workspace.
Low Pass, Band Pass and High Pass IIR Filter Sub-Menu


The IIR based Low Pass, Band Pass and High Pass Filters are all in a sub-menu called “LP, BP, HP Filters”.