Adaptive Frequency Domain Filter (AFDF) Forensics only

[Craig Maier]

The Adaptive Frequency Domain Filter (AFDF)

(Forensics Version Only)

The Adaptive Frequency Domain Filter (or AFDF) is the cousin to the Time Domain Adaptive Filter (or TDAF). It uses FFT (Fast Fourier Transform) techniques in conjunction with an adaptive algorithm based on statistical models of noise versus intelligent signals to create a dynamic fingerprint of the noise. When a reference channel is not available, it will often produce better adaptive noise reduction compared to the TDAF. The overall filtering effect can be reduced or increased to varying degrees by the user. This filter is actually a subset CNF Mode of the Continuous Noise Filter, but is directly accessible by the user via the icon. It includes 10 descriptive presets which can be used to get you into the general parametric area when dealing with a noisy forensics audio file. Fine tuning can then be made by the attenuation control in conjunction with the attack and release time constant controls. The frequency resolution of the system can be increased or decreased via the FFT size parameter. Larger FFT sizes result in smaller frequency bins and thus greater frequency discrimination with the tradeoff of greater levels of digital artifact production and poorer time domain transient response.

Note: A good FFT size to use as a starting point is 2048 when working with 44.1 kHz sampled files.


The AFDF in Action

• Attack Time

This is the time required for any of the filters to "open up" on the leading edge of a signal that exceeds the threshold line on the spectral graph.

This represents the time constant normalized value at 1 kHz. The time constant for filter frequencies operating above 1 kHz will be shorter than the setting, and the time constant for filter frequencies operating below 1 kHz will be longer.

The Attack time constant value is weighted with a -1 slope across the audio spectrum. Small values of attack provide excellent transient response, while long values provide a minimization of digital artifacts produced by the system. A good value to start with for the Attack Time parameter when using the AFDF is around 5 Milliseconds. The total range of adjustment for Attack time is 1.0 to 300 milliseconds. Smaller settings will improve leading-edge conversation response but allow more digital artifacts to pass through. Larger values will decrease the leading-edge response, but will reduce the production of artifacts during the noise reduction process. The user will need to balance between these two conflicting factors to obtain the best overall result, which will be found to be very source file dependent.

• Release Time

This is the time allowed for any of the filters to "close down" or "decay" following a signal that falls below the blue threshold line on the spectral graph. All remaining characteristics of the Release time Constant are the same in nature as the Attack time Constant. A good value to start with for the Release Time parameter for the AFDF is around 25 Milliseconds. If there is too much fast filter "breathing" (a digital artifact that is also sometimes referred to as “pumping”), lengthen this time until you are satisfied with the result. The total range of adjustment for Release time is 1.0 to 1000 milliseconds.

• Attenuation

This control sets the degree of attenuation for signals that are present and below the dynamic blue threshold line. The greater that one sets the Attenuation control, the greater will be the degree of noise reduction. However, the greater the degree of noise reduction achieved, the greater will be the loss of the sense of "Ambiance" on the resultant restoration. So you must use careful judgment as to the correct tradeoff between noise reduction and ambiance for the material you are dealing with. A good value for the Attenuation parameter is around 10 to 15 dB as a starting range. If there is too much loss in signal ambiance, decrease this value. If you desire more noise reduction, increase this value. If you increase the attenuation too much, you will begin to introduce some digital aliasing artifacts into the Destination .wav file. The total range of adjustment for Attenuation is 0 to 100 dB.

• Threshold (Blue Graphical Threshold Line)

This feature graphically and dynamically displays how the filter is adapting in real time to the signal being applied to the AFDF. The horizontal axis represents frequency and the vertical axis represents the varying threshold line in dB.

You should find the AFDF factory presets to be a good way to start off when using this adaptive filter. Their names are descriptive in terms of response and degree of aggressiveness. After you find a preset that is close to producing the desired results, you can go back and tweak the attenuation, attack, and release parameters optimally.