Module & Plug-in
Table of Contents
Overview
De-hum is designed to remove persistent tonal noise, like AC noise, electrical buzz or EMF (Electro-magnetic fields). De-hum includes 2 modes: a Static mode that’s best for ground noise or simple hums with a few harmonics, and a Dynamic mode for complex hums or buzz.
Mode Selection
Static:
- Ideal for simple tonal noise with only a few harmonics of the fundamental.
- Hum that doesn’t overlap the speech frequencies.
Dynamic:
- Hum that has more than a dozen harmonics.
- Hum that is inharmonic (contains multiple unrelated tones).
- Buzz or EMF that extend into the upper frequencies.
- Hum/buzz that overlaps speech frequencies.
Dynamic Controls
- LEARN: Captures the full tonal profile for removal of complex hum. To learn in the app, make a selection containing the hum in isolation and click the Learn button. In the plug-in, enable learn and play back the section of audio containing the hum. 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.
- SENSITIVITY: Adjusts the amount of hum that will be removed.
- BANDS: Controls the number of dynamic notch filters, which update their gains based on the input signal to avoid ringing. Adjust to higher values for complex hums, at the expense of transparency.
- FILTER Q: Controls the bandwidth of the notch filters.
- RANGE SELECTORS: Sets the action region of the hum removal by defining the upper and lower bounds of De-hum processing.
Static Controls
- BASE FREQUENCY: Sets the fundamental frequency of the hum to be removed. With Preview engaged, adjust the slider until you find the point where the hum is appropriately reduced.
- FREQUENCY (Hz): Sets the fundamental frequency for the filter.
- FILTER Q: Controls the bandwidth of the notch filters for the base frequency and all of the harmonics.
Tip
During Preview, you can use the Spectrum Analyzer to help identify the base frequency of the hum.
- LEARN: Automatically set the Base Frequency based on the learned profile. To learn in the app, make a selection containing the hum in isolation, and click the Learn button. In the plug-in, enable learn and playback the section of audio containing the hum. 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. Available only in static Mode.
- LINEAR-PHASE FILTERS: Linear-phase enables FIR (Finite Impulse Response) filters with a high FFT (Fast Fourier Transform) 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 (Infinite Impulse Response) 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.
- 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
- 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.
- HARMONICS: Harmonics often accompany the fundamental frequency of a hum. With De-hum you can also attenuate these overtones with additional notch filters. Using the Harmonics control, you can add up to 16 harmonics. The spectrogram display makes it easy to identify the harmonics. After setting 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.
- 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 PANEL [dB]:
- ENABLE: This control allows you to choose which notch filters are active. Disabling unnecessary notches can help preserve the original sound.
- GAIN [dB]: This controls the level for each notch filter. 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
- Guitar De-noise For guitar tracks containing buzz, try using Guitar De-noise. This module can help reduce amp buzz along with string squeaks and picking that is too aggressive.
- 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:
RX 9.3.0
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