Elimination of Harmonic Noise

A typical hum or buzz consists of harmonically related sinusoidal signals - a number of tones with frequencies, which are integer multiplies of the basic frequency, called fundamental. The AC power mains is the most common source of harmonic noise. In this particular case the fundamental frequency is equal to 60 Hz (US) or 50 Hz (Europe). Electric appliances working in the background, switched on accidentally or intentionally (as in the case of some forensic recordings), constitute another group of typical buzz sources.

Using DeHum

The hum-like disturbances can be eliminated using a comb filter - a collection of notch filters designed so as to suppress the fundamental frequency along with the dominant harmonics. To tune the comb filter appropriately, use the window situated in the upper part of the dialog, which displays the local spectrum of the source signal around the cursor position and/or the spectrum of the signal obtained by comb filtering. Spectral plots can be obtained using two different methods - parametric (AR) and nonparametric (FFT). You can change the amplitude of displayed plots by adjusting the minimum spectral floor level (in dB) and resolution - by choosing the range of analyzed frequencies (maximum frequency).

The three radio buttons, combined in the Show group, allow you to display spectrum of the source signal (Source), spectrum of the results of processing (Result) or both spectral plots at once (Both). Finally, if the input signal is stereo, you can use radio buttons combined in the Channel group to display spectrum obtained for the left channel only (Left), the right channel only (Right) or both channels at once (Both).

In order to remove hum you have to turn into detective for a while. Since hum is a combination of harmonically related sinusoidal signals it introduces a number of sharp spectral peaks which are clearly visible on spectral plots. The black vertical lines superimposed on spectral plots mark center frequencies of the narrow-band signal components, which will be removed by the comb filter. After placing the cursor over any of these lines you will be able to drag the entire structure to a new position. By changing the fundamental frequency (Frequency), the number of notches (Multiplicity) and their relative bandwidth (Bandwidth) you can tune the comb filter so as to suppress all dominant harmonic noise components. To perform fine tuning around the selected frequency (+/- 10%), press the Fit button.

If the ‘Show result’ or ‘Show both’ option is switched on, the results of filtering will be not shown until you press the Apply button - this button turns red each time the results require updating.

The tuning procedure can be summarized as follows:

1. Find the fragment of the recording dominated by hum, preferably containing the hum only. Place cursor in the middle of the corresponding area.

2. Localize the dominant harmonic noise components by visual inspection of spectral plots of the analyzed signal. The fundamental frequency usually corresponds to the highest narrow-band spectral peak. Harmonics correspond to sharp peaks localized at frequencies that are integer multiplies of the fundamental frequency, which results in a characteristic comb-like structure. To identify the harmonic noise components, start from the lowest spectral floor (-100 dB) and the highest resolution available (i.e. the smallest value of the maximum frequency setting). If necessary gradually raise the floor and decrease resolution until the entire comb structure of harmonic noise is revealed.

3. Select the fundamental frequency (Frequency), the number of harmonics (Multiplicity) and their location (Odd/Even/All) so as to match the resonant structure of the noise. Since harmonic components scheduled for canceling are marked with black vertical lines all you have to do is align these lines with the corresponding spectral peaks. Instead of changing the Frequency setting you can place the cursor over any of the black notch markers and drag the entire structure to a new position. To hide notch markers “behind” the plot click the right mouse button; to bring them forward click the right mouse button again. When you finish tuning the harmonic reject filter ‘by hand’ press the Fit button to enforce fine frequency tuning. Fine tuning is confined to a narrow frequency band (+/- 10 %) around the selected fundamental frequency.

4. Choose the relative bandwidth of filter notches. Start from the small bandwidth (5%) and gradually increase it to the point the hum is not audible. Remember that the recovered signal is may deteriorate if the bandwidth is too large so your choice must be a reasonable compromise between the degree of hum suppression and degree of signal distortion. Prior to carrying out listening tests you can tune the bandwidth of notch filters using visual inspection. To check spectral plots of the signal at the output of the comb filter, click the Result radio button localized in the Show group and press the Apply button. If the bandwidth is chosen appropriately all spectral peaks corresponding to removed components should disappear from the spectral plots. To inspect both the input and output spectra click the Both radio button.

5. Perform audition tests. If the results are satisfactory press the Process button to start processing. Otherwise return to step 2 above.

NOTICE

There are no free lunches.... On some occasions removal of hum may result in an audible reverb (echo) effect. To minimize this effect try to limit the number of eliminated harmonics and/or reduce the relative bandwidth of notch filters.

Linear phase

When notches comprising the comb structure are very narrow, i.e. if the corresponding filters have a very small relative bandwidth, the effectiveness of dehuming is not satisfactory at the beginning of the recording due to the long-lasting transient effects. In cases like that, check the Linear phase box to enforce filtering in two directions. Linear phase, i.e. shape preserving filtering is also strongly recommended when processing is restricted to a local area.