There are three important tuning parameters that can be used to influence the process of signal renovation: detection threshold, maximum length of detection alarms and cutoff frequency.
Detection threshold decides about the sensitivity of the outlier detector, a device used to localize impulsive disturbances. It can be chosen in the range <2.8,10.0>. The recommended (default) value of the detection threshold is 3.0 when the frequency selective declicking is turned on, and 3.5 when the frequency selective mechanism is turned off. The values smaller than 3.0/3.5 may result in an overly fussy detection scheme, i.e. scheme which raises too many false alarms introducing noticeable signal distortions and slowing down computation. On the other hand, when relatively large values of detection threshold are adopted, the detector may become too 'tolerant' and fail to isolate small noise pulses.
Maximum length of detection alarms
The maximum length of detection alarms, which can be chosen in the range <1,100>, determines the maximum number of samples in a row that can be scheduled for reconstruction in the automatic detection mode. The recommended (default) value of this constant is 50. When false alarms are raised the system is more prone to introducing audible distortions if the maximum allowable size of reconstructed fragments is large. For this reason one should keep this constant as small as possible.
The length of noise pulses depends on several factors :
Sampling frequency - quite obviously for higher sampling rates a larger number of samples becomes corrupted with a click of a specific duration.
Recording medium (magnetic tape, vinyl record) and the degree of its degradation.
Recording conditions and/or equipment used for playing back an archive recording.
The maximum length of detection alarms should fit characteristics of impulsive disturbances encountered in the processed audio file. For example, if you process audio material from a moderately used vinyl record (with no big scratches on it) you can safely limit the size of reconstructed blocks to 10-20 samples for 22 kHz sampling or to 20-40 samples for 44 kHz sampling. For the real oldies, however, it may be necessary to work with considerably larger block sizes.
Impulsive disturbances distort high frequency components of audio signals, usually leaving its low frequency portion intact. For this reason significant improvements can be often achieved if declicking is restricted to the high frequency component of the renovated signal. DART XP Pro allows you to perform such selective declicking by means of frequency decomposition of the processed audio signal (only the high frequency portion of the signal is declicked). When the frequency selective declicking mechanism is enabled you can safely use small and very small detection thresholds, which guarantees very high sensitivity of the outlier detector. The results are usually impressive.
The cutoff frequency can be adjusted using the Cutoff frequency box. For the best results set the cutoff frequency at the highest value, which guarantees that the lowpass component of the signal remains undistorted. Use the Play lowpass button, situated in the Audition group, to play the lowpass portion of the signal. While playing, gradually increase the cutoff frequency to the point where the click-related distortions become audible. Set the cutoff frequency safely below this point – be aware that all audible distortions present in the lowpass component of the signal will get through to the output file.
In the presence of large scratches consider using a different strategy. Large scratches usually produce audible thumps in the lowpass portion of the signal no matter how low is the cutoff frequency. If scratches are infrequent, consider using the following three-step procedure:
Set the cutoff frequency at the level that guarantees that only the infrequent scratch-related thumps can be heard when the lowpass component of the restored audio signal is played.
Use DeClick Plus to declick audio in the automatic detection mode.
Localize and ‘redo’ scratches in the manual detection mode using the Retouch tool – see Retouching a soundfile .