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Harmonic Reject
The Harmonic Reject filter, which is sometimes referred to as a "comb" or "multiple notch" filter is used to attenuate periodic noises that contain harmonics. It is capable of attenuating either the odd or even harmonics of the selected fundamental frequency. This filter is very effective for attenuating a form of hum (line frequency related) noise that is rich in harmonics. This type of noise is sometimes referred to as "Buzz." Noise, such as this can be introduced into an audio recording from sources such as light dimmers or switching mode computer power supplies. "Buzz" can contain odd harmonics of the power line frequency, all the way through the entire audio spectrum. A 60-Hertz square wave is, by definition, the fundamental component of the waveform plus all the odd harmonics of that frequency up to infinity. If the waveform is perfectly symmetrical, it will include a fundamental component and only its odd harmonic multiples. The various bands (tines) of the Harmonic Reject filter will attenuate the fundamental as well as all of the harmonic by- products within the audio spectrum. Should you encounter wide-band line frequency related noise that is asymmetrical, it can produce some even harmonics.
An example of noise containing even harmonics would be that produced by an unsymmetrical sine, trapezoidal or square waveform (e.g.- a square wave having non-symmetrical duty cycle values). Non-linear devices in a signal pathway can introduce this effect. The Harmonic Reject filter can be placed in a mode in which it will attenuate the "evens" rather than the "odds" if you should encounter such noise. Before using the Harmonic Reject Filter, it is often useful to identify the fundamental frequency of the noise signal by using the Spectrum Analyzer in a high frequency resolution mode of operation (found under the "View" menu).
The Harmonic Reject Filter
The following is a summary of the control parameters and range of adjustment provided for the Harmonic Reject Filter:
This filter also incorporates a "Keep Residue" feature. This allows you to hear or keep only the noise component of the original signal. This feature is useful for "tuning" the filter to the maximum level of noise, so that when you actually run the filter with the "Keep Residue" feature turned off, the noise left behind will be minimized. Lastly, the keep residue function also allows you to produce "slot" filters, provided the slots that you need are harmonically related. See Appendix 1 for more details about slot filters.
Fine Tuning the Harmonic Reject Filter
Let’s assume that you have a .wav file having a buzz which contains roughly a 60 Hz fundamental frequency. This can be determined by applying the spectrum analyzer to the file and making a measurement of the first spike in the series. Set the Range (checkbox) to 60 Hz. Adjust the frequency of the Harmonic Reject filter in the range field to correspond to the reading viewed on the Spectrum Analyzer (direct numerical entry is a good way to do this). Start with a width of 0.2. Set the filter harmonics to 5 or 6 and check the “Keep Residue” mode. Next, click on the preview button and then point your mouse to the frequency slider. You should be hearing mostly buzz and not much signal in this mode. Now, use the up and down keys on your keyboard to move the frequency first above 60 Hz and then below 60 Hz one small increment at a time. Keep on adjusting the frequency in small increments until you hear the loudest buzz coming from your sound system. When you have found the maximum loudness value, switch back to non-keep residue mode. You will hear the signal with the buzz substantially attenuated. The next thing to do is to adjust the Maximum Harmonic number to the proper value required to attenuate the buzz without degrading the quality of your audio signal. Try increasing it until no further improvement is observed. If necessary, try decreasing it too. Do not use maximum harmonic numbers greater than necessary to accomplish noise reduction. The use of too many harmonics will damage your audio signal. Lastly, adjust the Width control to the minimum value that will do the job for you so as to minimize damage to the primary signal of interest.
Sometimes, a better result is achieved using the IIR (resonant notch filter) technique. You can enter that mode via the IIR checkbox. This allows a much higher frequency resolution adjustment capability and so you may be able to more closely hone in on the target signal of interest. However, when using the IIR method, try to minimize the number of harmonics to no more than what is needed to get the job done (Maximum Harmonic). The IIR method is much more math intensive and thus, slower, especially with high values of the Maximum Harmonic settings.
Important Notes:
For severe 50 or 60 Hz buzz situations, run at least two passes of the filter. The first pass should be run with a setting of 50 or 60 Hz, odd, and the desired Maximum Harmonics. On the second pass, run with a setting of 25 Hz (for 50 Hz situations) or 30 Hz (for 60 Hz situations), odd, and a lower Maximum Harmonics number. This sequence can be repeated more than once for further buzz reduction. You should also try running the system in “even only” mode after processing the odds first. Don’t forget that you can stack up (cascade) 2 or more of these filters in the Multi-Filter and apply them all at once as a convenience.
Note: Sometimes, when line-frequency related “buzz” is encountered containing only very high frequency components (spikes) the EZ Impulse or Expert Impulse Filter may perform a better job removing buzz and doing less damage to the main audio signal. Experimentation is the only way to know what filter will produce the best result on any given file.
The Harmonic Reject filter, which is sometimes referred to as a "comb" or "multiple notch" filter is used to attenuate periodic noises that contain harmonics. It is capable of attenuating either the odd or even harmonics of the selected fundamental frequency. This filter is very effective for attenuating a form of hum (line frequency related) noise that is rich in harmonics. This type of noise is sometimes referred to as "Buzz." Noise, such as this can be introduced into an audio recording from sources such as light dimmers or switching mode computer power supplies. "Buzz" can contain odd harmonics of the power line frequency, all the way through the entire audio spectrum. A 60-Hertz square wave is, by definition, the fundamental component of the waveform plus all the odd harmonics of that frequency up to infinity. If the waveform is perfectly symmetrical, it will include a fundamental component and only its odd harmonic multiples. The various bands (tines) of the Harmonic Reject filter will attenuate the fundamental as well as all of the harmonic by- products within the audio spectrum. Should you encounter wide-band line frequency related noise that is asymmetrical, it can produce some even harmonics.
An example of noise containing even harmonics would be that produced by an unsymmetrical sine, trapezoidal or square waveform (e.g.- a square wave having non-symmetrical duty cycle values). Non-linear devices in a signal pathway can introduce this effect. The Harmonic Reject filter can be placed in a mode in which it will attenuate the "evens" rather than the "odds" if you should encounter such noise. Before using the Harmonic Reject Filter, it is often useful to identify the fundamental frequency of the noise signal by using the Spectrum Analyzer in a high frequency resolution mode of operation (found under the "View" menu).
The Harmonic Reject Filter
The following is a summary of the control parameters and range of adjustment provided for the Harmonic Reject Filter:
- Frequency (Fundamental): 20 - 5,000 Hz
- Attenuation: 1 to 100 dB
- Filter Harmonics:
- Odd Only
- Even Only
- Maximum Harmonic (Number): 0 to 500
- Width (or Q): 0.005 to 0.5 Octaves
- Range: (This control optimized the resolution of the Frequency Control) - Choose between 50 Hz, 60 Hz, 400 Hz (aircraft) or Full Range
- Use IIR Checkbox: Changes the mode from an FFT based system to a Resonant IIR Multiple Notch Filter type
This filter also incorporates a "Keep Residue" feature. This allows you to hear or keep only the noise component of the original signal. This feature is useful for "tuning" the filter to the maximum level of noise, so that when you actually run the filter with the "Keep Residue" feature turned off, the noise left behind will be minimized. Lastly, the keep residue function also allows you to produce "slot" filters, provided the slots that you need are harmonically related. See Appendix 1 for more details about slot filters.
Fine Tuning the Harmonic Reject Filter
Let’s assume that you have a .wav file having a buzz which contains roughly a 60 Hz fundamental frequency. This can be determined by applying the spectrum analyzer to the file and making a measurement of the first spike in the series. Set the Range (checkbox) to 60 Hz. Adjust the frequency of the Harmonic Reject filter in the range field to correspond to the reading viewed on the Spectrum Analyzer (direct numerical entry is a good way to do this). Start with a width of 0.2. Set the filter harmonics to 5 or 6 and check the “Keep Residue” mode. Next, click on the preview button and then point your mouse to the frequency slider. You should be hearing mostly buzz and not much signal in this mode. Now, use the up and down keys on your keyboard to move the frequency first above 60 Hz and then below 60 Hz one small increment at a time. Keep on adjusting the frequency in small increments until you hear the loudest buzz coming from your sound system. When you have found the maximum loudness value, switch back to non-keep residue mode. You will hear the signal with the buzz substantially attenuated. The next thing to do is to adjust the Maximum Harmonic number to the proper value required to attenuate the buzz without degrading the quality of your audio signal. Try increasing it until no further improvement is observed. If necessary, try decreasing it too. Do not use maximum harmonic numbers greater than necessary to accomplish noise reduction. The use of too many harmonics will damage your audio signal. Lastly, adjust the Width control to the minimum value that will do the job for you so as to minimize damage to the primary signal of interest.
Sometimes, a better result is achieved using the IIR (resonant notch filter) technique. You can enter that mode via the IIR checkbox. This allows a much higher frequency resolution adjustment capability and so you may be able to more closely hone in on the target signal of interest. However, when using the IIR method, try to minimize the number of harmonics to no more than what is needed to get the job done (Maximum Harmonic). The IIR method is much more math intensive and thus, slower, especially with high values of the Maximum Harmonic settings.
Important Notes:
For severe 50 or 60 Hz buzz situations, run at least two passes of the filter. The first pass should be run with a setting of 50 or 60 Hz, odd, and the desired Maximum Harmonics. On the second pass, run with a setting of 25 Hz (for 50 Hz situations) or 30 Hz (for 60 Hz situations), odd, and a lower Maximum Harmonics number. This sequence can be repeated more than once for further buzz reduction. You should also try running the system in “even only” mode after processing the odds first. Don’t forget that you can stack up (cascade) 2 or more of these filters in the Multi-Filter and apply them all at once as a convenience.
Note: Sometimes, when line-frequency related “buzz” is encountered containing only very high frequency components (spikes) the EZ Impulse or Expert Impulse Filter may perform a better job removing buzz and doing less damage to the main audio signal. Experimentation is the only way to know what filter will produce the best result on any given file.