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Notch Filter
A notch filter attenuates signals that are near its center frequency setting. The degree to which it attenuates frequencies near the center frequency is determined by the bandwidth setting. This filter is useful for removing 50 or 60 Hz “Hum” from a recording. It is also useful for decreasing any sound system acoustic feedback that may be found on some live recordings. A "Slot" filter is also provided within the Notch filter menu item for Forensics applications. Multiple slots can be constructed using the DCArt10/DC Forensics10 Multi-Filter. Slot filters have the reverse response of a Notch Filter; it only passes signals near its center frequency setting.
The Diamond Cut Notch Filter is a digital simulation (IIR based) of a second order notch / slot filter. It attenuates all frequencies near its center frequency setting. The degree to which it attenuates frequencies adjacent to the center frequency is determined by the bandwidth setting. This filter is useful for removing 50 or 60 Hz hum from a recording (or harmonics thereof). In forensics “black-box” recordings, it is useful for reducing 400 Hz aircraft generator noise. It is also useful for decreasing any sound system acoustic resonance that may be found on some live recordings. It can be used to attenuate the heterodyning "whistle" which is sometimes heard on AM broadcast radio reception. Some audio restoration engineers also use this filter to remove some "Hiss" from old 78 RPM recordings. For this application, the filter's center frequency is set somewhere in the 8 to 12 kHz range, with a bandwidth of 0.25 Octave or less. Experimentation is the only way to determine its effectiveness in minimizing "Hiss" from your particular source material. Also, it is important to note that this method is not the most effective for "Hiss" removal. Instead, consider using either the Continuous Noise Filter, Dynamic Noise Filter or the EZ Clean Filter.
The Notch Filter
The following is a summary of the control parameters and range of adjustment provided for the Notch Filter:
This filter incorporates the "Slot Filter" feature. Essentially, a slot filter produces a variable band-pass response to its applied signal. This allows you to hear or keep only the residual component of the original signal. This feature is useful for "tuning" the notch filter to the maximum level of noise, so that when you actually run the notch filter with the "slot filter" feature turned off, the noise left behind will be minimized. The slot filter function is also useful in Forensics applications, wherein one is interested in isolating a very particular sound that exists in a very specific and narrow frequency band. If multiple slots are required, use the Multi-Filter with multiple notch/slot filters in the chain, or consider using the Harmonic Reject filter in "Keep Residue" mode, provided the slots, which are required, are harmonically related.
Note: The frequency range of adjustment up to 18,999 Hz is only effective when utilizing a 44.1 kHz sampling rate. 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.
Notch Filter Procedure (Tutorial)
Note: If only a small section(s) of your .wav file is in need of Notch filtering, as is often the case when acoustic feedback is encountered on a live recording, you can use "sync mode" and just filter the sector which contains the noise which you’re attempting to reduce. "Sync mode" can be selected under the "View Menu" and is associated with the Diamond Cut Classic Edit mode.
A notch filter attenuates signals that are near its center frequency setting. The degree to which it attenuates frequencies near the center frequency is determined by the bandwidth setting. This filter is useful for removing 50 or 60 Hz “Hum” from a recording. It is also useful for decreasing any sound system acoustic feedback that may be found on some live recordings. A "Slot" filter is also provided within the Notch filter menu item for Forensics applications. Multiple slots can be constructed using the DCArt10/DC Forensics10 Multi-Filter. Slot filters have the reverse response of a Notch Filter; it only passes signals near its center frequency setting.
The Diamond Cut Notch Filter is a digital simulation (IIR based) of a second order notch / slot filter. It attenuates all frequencies near its center frequency setting. The degree to which it attenuates frequencies adjacent to the center frequency is determined by the bandwidth setting. This filter is useful for removing 50 or 60 Hz hum from a recording (or harmonics thereof). In forensics “black-box” recordings, it is useful for reducing 400 Hz aircraft generator noise. It is also useful for decreasing any sound system acoustic resonance that may be found on some live recordings. It can be used to attenuate the heterodyning "whistle" which is sometimes heard on AM broadcast radio reception. Some audio restoration engineers also use this filter to remove some "Hiss" from old 78 RPM recordings. For this application, the filter's center frequency is set somewhere in the 8 to 12 kHz range, with a bandwidth of 0.25 Octave or less. Experimentation is the only way to determine its effectiveness in minimizing "Hiss" from your particular source material. Also, it is important to note that this method is not the most effective for "Hiss" removal. Instead, consider using either the Continuous Noise Filter, Dynamic Noise Filter or the EZ Clean Filter.
The Notch Filter
The following is a summary of the control parameters and range of adjustment provided for the Notch Filter:
- Center Frequency: 5 - 18,999 Hz
- Bandwidth: 0.01 Octaves to 1.99 Octaves
- Preview Mode Button: On/Off (The slider controls can be adjusted "live" when the preview mode is on.)
This filter incorporates the "Slot Filter" feature. Essentially, a slot filter produces a variable band-pass response to its applied signal. This allows you to hear or keep only the residual component of the original signal. This feature is useful for "tuning" the notch filter to the maximum level of noise, so that when you actually run the notch filter with the "slot filter" feature turned off, the noise left behind will be minimized. The slot filter function is also useful in Forensics applications, wherein one is interested in isolating a very particular sound that exists in a very specific and narrow frequency band. If multiple slots are required, use the Multi-Filter with multiple notch/slot filters in the chain, or consider using the Harmonic Reject filter in "Keep Residue" mode, provided the slots, which are required, are harmonically related.
Note: The frequency range of adjustment up to 18,999 Hz is only effective when utilizing a 44.1 kHz sampling rate. 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.
Notch Filter Procedure (Tutorial)
- Identify the frequency that you desire to reject from the .wav file recording. The determination of the frequency of interest can be simplified by using the Band-pass filter as an Audible Spectrum Analyzer.
- Highlight the portion of your .wav file on which you desire to apply the Notch filter. (You may choose to highlight the entire file or any portion thereof.)
- Click on "Notch."
- Choose the Center Frequency based on the "problem" frequency that you have observed and desire to reject. The Center Frequency range is from 5 Hz. to 18.999 kHz. (The highest useful frequency is around 15 kHz.) If you desire the finest degree of frequency resolution possible, use direct numeric entry rather than the use of the slider controls.
- Choose the Bandwidth that you find to be the most effective in eliminating the desired frequency. The bandwidth control is calibrated in Octaves, and has a range from 0.01 Octaves to 1.99 Octaves. You should choose the smallest possible bandwidth that still accomplishes the job of rejecting the troublesome frequency. Otherwise, you will start eliminating useful information from your recording. Generally, useful ranges for Bandwidth will be in the 0.5 Octave to 0.1 Octave range.
- If you desire to hear the results of your filter settings before creating a new "Destination File, click on "preview."
- After a short delay, you will hear the effect of the settings that you have chosen. (The system may seem to stutter if your computer is too slow to keep up with the algorithm in real time. However, this repeating pattern will not be present in the final Destination processing of the filter.)
- As the filter is running in either preview mode or normal mode (Destination File Mode), you will see a dialog box that indicates the "% Done" of the filter algorithm on the selected portion of the Source .wav file. Also, at the top of the Dialog box you will see indicated the "Total Samples to Process:"
- Keep adjusting the Frequency and Slope parameters, and testing the various settings using the "Preview" mode until you are satisfied with the results.
- When you are satisfied with a group of settings, you will no longer need to use Preview mode.
- Click on Run, and the filter will process your Source .wav file through the filter algorithm, and create a Destination .wav file containing the output of the filter.
- When this process is complete, you will see the Destination File become highlighted in yellow, at the same time that the Source File becomes unselected.
- Click on "Close."
Note: If only a small section(s) of your .wav file is in need of Notch filtering, as is often the case when acoustic feedback is encountered on a live recording, you can use "sync mode" and just filter the sector which contains the noise which you’re attempting to reduce. "Sync mode" can be selected under the "View Menu" and is associated with the Diamond Cut Classic Edit mode.