De-hum
| Module & Plug-in |
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Overview
De-hum is designed to remove persistent tonal noise, like the AC hum that can be caused by poor electrical grounding. De-hum includes a series of notch filters that can be set to remove both the base frequency of the hum, usually 50 Hz (Europe) or 60 Hz (USA) as well as any harmonics. The De-hum module is effective for removing hum that has up to seven harmonics above its primary frequency.
Controls
- BASE FREQUENCY: Sets the fundamental frequency of the hum to be removed. The Free option unlocks the Notch Frequency control and allows you to manually identify the fundamental. With Preview engaged, adjust the slider until you find the point where the hum is appropriately reduced.
Tip
You can also use the Spectrum Analyzer to help identify the base frequency of the hum.
- SUGGEST: De-hum can intelligently identify the noise profile of the hum in your audio. Make a selection containing the hum in isolation, and click the Suggest button. This will automatically set the Base Frequency based on the learned profile. If you cannot find a selection of hum in isolation, RX can analyze any audio with prominent hum, but the results may not be as useful.
- ADAPTIVE MODE: Allows De-hum to adjust the notch filters based on changes in the audio over time. In this mode, RX will analyze incoming audio to determine what is hum and what is desired audio material. Adaptive mode will work better with hum that changes in pitch throughout the file.
- FILTER Q: Controls the bandwidth of the notch filters for the base frequency and all of the harmonics.
LINEAR-PHASE FILTERS: Linear-phase enables FIR (Finite Impulse Response) filters with a high FFT size. These filters provide very accurate frequency response with no change in phase at the expense of latency and filter pre-ringing.
Disabling Linear Phase (FIR) filters
- When Linear Phase is disabled, De-hum will use minimum-phase IIR filters. These are also very accurate, and are only susceptible to post-ringing, which is usually less noticeable than the pre-ringing introduced by FIR filters.
- Latency Consideration: Disabling Linear Phase Filters will reduce the latency used by De-hum when it is being used as a real-time plug-in.
- When Linear Phase is disabled, De-hum will use minimum-phase IIR filters. These are also very accurate, and are only susceptible to post-ringing, which is usually less noticeable than the pre-ringing introduced by FIR filters.
HIGH/LOW-PASS FILTERS: These traditional filters come ahead of the De-hum notch filters, and allow for frequencies to pass above or below a certain cutoff point. These can be useful for tackling extreme hum or buzz.
- FREQUENCY [Hz]: sets the cutoff frequency for the filter
- Q: Sets the bandwidth of the filter (or dB/octave cut). In the default IIR filter mode with a high Q setting, you may notice a resonance at the cutoff frequency characteristic of traditional analog filters. That resonance can be mitigated by engaging the Linear-phase filters.
- FREQUENCY [Hz]: sets the cutoff frequency for the filter
NUMBER OF HARMONICS: Because harmonics often accompany the fundamental frequency of a hum, De-hum can also attenuate these overtones with notch filters. Using the Number of Harmonics control, you can select up to 7 harmonics above the fundamental. The spectrogram display can make it easy to identify the harmonics. After selecting the number of harmonics, use the Slope control to set how aggressively the higher harmonics are attenuated.
LINK HARMONICS: Connects the gain controls of the notch filters.
- ALL: presents a single node on the display for controlling the gain of all the notch filters. This is the default setting.
- ODD/EVEN: presents two nodes on the display, one for controlling the gain of the fundamental frequency and even harmonics, and another for controlling the 1st harmonic and any following odd harmonics.
- NONE: presents individual gain nodes for the fundamental and each harmonic.
- ALL: presents a single node on the display for controlling the gain of all the notch filters. This is the default setting.
SLOPE: When harmonics are linked, this controls the harmonic slope of the gain nodes for each overtone. As the harmonic order increases, the gain level resolves closer to 0 dB. When the Link Harmonics control is set to Odd/Even, a separate control appears that affords independent control over the slope for both odd and even harmonics.
HARMONIC GAINS [dB]: This section provides a numerical readout of the notch filter gain settings in decibels. You can also manually enter gain settings for the fundamental, or any of the harmonics if Link Harmonics is set to None.
FILTER DC OFFSET: This checkbox will engage a filter to remove any DC (direct current) offset that sometimes occurs in A/D converters or analog circuits used in the recording process.
OUTPUT HUM ONLY: Selecting this check box will isolate the hum that is being removed. This is useful for fine-tuning your settings. Identify a section of your file where the hum is mixed with other material, select this mode, and click Preview. Now adjust parameters like Filter Q and Slope control to maximize hum removal, thus minimizing the effect on the program material.
More Information
Alternative Modules to use for Complex Hum Issues
- Spectral De-noise For hum that has many harmonics that extend into higher frequencies (often described as “buzz”), try using Spectral De-noise. Spectral De-noise features tonal noise reduction controls that can make short work of harmonic hum and buzz across the entire spectrum.
- De-click Some very high frequency buzz can also be removed with the De-click module.
Visual Example
This image shows the spectrogram of a file with 3 harmonics of a 60 Hz Hum:
