The NF575 Noise Filter is a specially designed set of algorithms used to remove noise (but not music!) from audio signals. This issue covers basic operations of the NF575 Noise Filter, provides a more in depth description of some of its features, and outlines some useful applications.
Filters can be broken down into four general categories – high pass, low pass, band stop (notch), and band pass.
High Pass Filter (HPF)
High pass filters remove signals below the selected frequency, and pass the signals above the selected frequency unaffected (hence the term high pass). High pass filters have a slope measured in the amount of signal reduction (in dB) per frequency octave. Typically these values are in multiples of 6 dB/Oct. At the exact selected frequency of the high pass filter the signal reduction is usually -3 dB, however this can change based on filter design.
A high pass filter is useful for removing low frequency rumble, unwanted vocal plosives, and dc offsets.
Low Pass Filter (LPF)
Low pass filters remove signals above the selected frequency, and pass the signals below the selected frequency unaffected (hence the term low pass). Low pass filters have a slope measured in the amount of signal reduction (in dB) per frequency octave. Typically these values are in multiples of 6 dB/Oct. At the exact selected frequency of the low pass filter the signal reduction is usually -3 dB, however this can change based on filter design.
A low pass filter is useful for background noise such as tape hiss, waterfalls and general ambient background noise.
Band Stop Filter
Band stop filters, or notch filters, are designed to remove a specified amount of signal at exactly the selected frequency. Band stop (notch) filters have a slope, or Q control, that determines the width of the actual notch shape. A Q of 1.0 is about one frequency octave. A Q of 12.0 is about 1/12 a frequency octave, or one semi-tone. At the exact selected frequency of the band stop (notch) filter the signal reduction is maximum. Some band stop (notch) filters, like those found on the NF575, allow user adjustment of the amount of signal reduction. This signal reduction adjustment is commonly referred to as range on noise filters.
A band stop (notch) is useful for removing cyclical noise like 60 Hz hum and camera shutter noise.
A Closer Look at the NF575 Plug-in
Now that we’ve covered some filter type basics, some features of the NF575 Noise Filter could use some more discussion.
Although there are several filtering options available for the Pro Tools™ platform, few, if any, have the precision of the NF575 Noise Filter. Not only designed for the ultimate in filtering accuracy, the high pass and low pass filters included in the NF575 have adjustable slopes with settings from 6 dB/Oct up to 36 dB/Oct. Because of the steepness of the NF575 Noise Filter high pass and low pass filters, even greater amounts of noise reduction are possible. The steep transition (slope) between unaffected and filtered frequencies allows even tighter control over audio signals and the noise removal process.
Notch Filter Soloing
Great notch filtering is nearly useless if the removed noise cannot be monitored accurately. The NF575 notch filter Solo buttons insert band pass filters in place of the notch filters, so the user can audition cyclical noise sources over a selectable frequency bandwidth.
When a notch filter band is soloed, the user is effectively listening to the removed signal. Note the Solo control only solos the filter band by as much as the selected Range value. When the Range control is at its maximum reduction, the band will be soloed the most. When the Range control is at its minimum reduction (no reduction) the Solo control has no effect.
For each notch filter band, soloing that band and sweeping it across its available frequency control range will reveal at what frequency the noise source is the loudest. Upon finding this loudest frequency location, un-solo the notch filter. Now that cyclical noise is removed from the audio signal. Adjust the Q control as tightly as possible to remove only the noise and not the desired audio data. Repeat this process for all notches, or until all audible cyclical noise is removed.
Notch Filter Linking
Sometimes the cyclical noise has a fundamental frequency, but the harmonics of the periodic noise also contaminate the audio. By linking all the notch filter frequencies to the first notch filter band in the NF575, cyclical noise can be quickly removed. Default settings put the notch filter frequencies at harmonic intervals automatically (ex: 60, 120, 180, 240, 300 Hz). If the noise does not contain harmonics, but rather seemingly unrelated cyclical noises at various frequencies, the NF575 notch filters can be linked at any frequency interval (ex: 120, 168, 240, 328, 400 Hz).
But what would you use the NF575 for? Why, noise removal of course! Here are some common (and not so common) examples of how to apply the NF575 Noise Filter in your next session.
60 Hz Buzz Removal
Cyclical noise like AC power hum is all too common in audio production. While a simple notch filter may remove some of this kind of noise, there are often several harmonics of noise that require multiple notch filters, often with varying amounts of reduction. The NF575 notch filters are extremely narrow and linkable, so the removal of cyclical (and harmonic) noise is possible, without affecting the desired audio.
Camera Shutter Noise
Camera noise can sometimes generate cyclical (and harmonic) noise problems as well. The use of the NF575 notch filters and their linking capability can make quick work of this kind of noise problem.
Low Frequency Rumble
Vocal or dialog plosives (ex: a really loud ‘buh’ sound when screaming ‘baby’ at the top of your lungs), the rumble from a mic stand, wind, or other low frequency signals can spoil the otherwise good recording. Using the NF575 high pass filters, these types of sounds can be easily removed. The steeper slopes can remove more of these unwanted sounds, while preserving the real content in the track.
While the NF575 is really designed for noise reduction, the NF575 notch filters can create some pretty drastic phaser like effects. Link all 5 notch filters to the first notch using the link control, select a wide Q setting, and then vary the frequency control in the first notch filter (band 1). The effects are interesting, and by automating the frequency control of the first notch filter, the phaser sweep can be made into a consistent effect during playback (instead of locking to the rate of an LFO, which could produce different phasor effects each playback pass).