Expert Impulse Filter

[Craig Maier]

Expert Impulse Noise


As your expertise with the product grows, so too will your demands from the various filters. The Expert Impulse Noise Filter approach allows you to control all of the parameters associated with a basic impulse filter and therefore obtain the most flexible results. Since it contains no adaptive elements, it does require a larger degree of experience to use compared to the EZ Impulse Filter. Multiple Expert Impulse Noise filters placed in the multifilter will often provide the best results, with each instance set up with different modes and/or parameters. Generally speaking, the Expert Impulse Noise Filter is designed to remove impulsive noises from a signal such as clicks, ticks, scratches, pops and static. It can also be used to attenuate harmonically rich “buzz” from a recording. The Expert Impulse Noise filter is primarily intended to be used in extremely unusual phonographic situations or (primarily) for Forensics applications in order to filter out buzz or static from a communications link. You will find that the EZ Impulse filter solves almost all impulsive noise problems that you will encounter, but for the rare few situations that occasionally pop up. So, try the EZ Impulse filter first before experimenting around with the Expert Impulse Filter. As with all of the Diamond Cut Impulse Filters, the minimum sample rate used should be 44.1 kHz. Lower sample rates will produce inferior results. The following is a summary of the Expert Impulse Noise Filter control parameter functionality and their adjustment ranges:


Expert Impulse Noise Filter

• Threshold

This is the Voltage derivative signal level above which the program decides that an impulse noise event has occurred. It has a range of adjustment from 1 to 18,000 (in DAC counts normalized to a 16 bit system). A good starting point for the threshold value is 1/3rd of the full-scale value of the envelope of your .wav file. For example, if the .wav file is almost full scale in amplitude (+/- 32,000 counts) set the threshold at around 10,000.

For 78 RPM records, (with the Tracking control set to 0), start with a Threshold value of 1000, and adjust up or down depending on the results obtained. Lower values of Threshold will produce a higher degree of de-clicking. If it is set too low, however, you will produce distortion on your recording. If you do not want to start at 1000, use the 1/3rd of full-scale rule for your initial setting.

Note: Threshold should be set to its lowest value (slider down) for Vinyl LP/45 RPM applications. Adjustments should be made using the Tracking adjustment for these applications.

• Size

This is the number of samples during which the "click" or "pop" event must remain to be defined as an impulse noise event. It has a range of adjustment from 2 to 60 samples. Short "clicks" require a smaller setting compared to longer "pops." A good range of values to start with for Vinyl LP applications is in the 10 to 15 area. A good range of values to use for non-Vinyl (like 78-RPM records) is between 3 to 7 samples.

• Tracking

This is the value of rectified output Voltage from a High-pass filter that is used to modulate the threshold Voltage of the filter. When there is a lot of high frequency information present on the recording, like the crashing of cymbals, it is desirable to move the threshold higher in value so that the transients contained in such sounds are not mistaken to be impulse noise events. Tracking has a range of adjustment from 1 to 100 (in relative units). Tracking is most useful on "high fidelity" recordings that contain a lot of "real" high frequency information such as loud dynamic cymbal crashes, or exaggerated sibilant sounds, which may be interpreted by the Impulse Filter as impulse transients.
Most 78s de-click best with the tracking turned all the way down (to a setting of 1). For LPs, start with a setting of around 25 to 30, and adjust the value upwards if distortion is heard on high frequency passages or sibilant sounds, until the distortion disappears. However, if the tracking is set too high, adequate de-clicking may not be obtained.

Note: Tracking should be set to its lowest value (slider down) for non-Vinyl LP applications. Adjustments should be made using the Threshold adjustment in these applications. If the threshold control is set too high, the Impulse filter will not completely de-click your recording. If it is set too low, the filter will create distortion on your recording when high frequency signals are present, especially on the higher registers of the audio scale.

• Preview Mode

When enabled, this allows you to hear quickly the results of your chosen settings in real time. If your computer is a slower model, the system may "stutter" when this is enabled, however, the feature can still be quite useful for finding the best settings, since the "stutter" will not appear on the final product when the filter is “Run”. The slider controls can be adjusted "live" when the preview mode is enabled. Preview mode is invoked merely by clicking on the "Preview" button on the filter dialog box.

• Vinyl LP Mode

When this is enabled, a different type of detector algorithm is utilized which is more optimized for the wider bandwidth and narrower clicks and pops encountered with Vinyl LPs. Vinyl mode works most effectively on .wav files that are sampled at 44.1 kHz up to 96 kHz. When this mode is turned off, the detector is more optimized for slower and wider impulse noise. Unlike most of the other controls, you cannot switch Vinyl LP mode on or off "live" when preview mode is invoked. Use the following table for determining the correct mode for this selector based on the type of material you are working with:

Sound Source	Vinyl LP Mode
Vinyl LP (Stereo or Mono)	"On"
45 RPM	"On"
FM Impulse Noise	"On"
FM Stereo Impulse Noise	"On"
Acoustical 78s	"Off"
Electrical 78s	"Off"
Cylinders	"Off"
Hill and Dales	"Off"
Movie Soundtrack "Pops"	"Off"
AM or Short-Wave Static	"Off"
Forensic Audio	"Off"

Note 1: When you run the Expert Impulse Filter, a dialog box will appear which indicates the Clicks/Second and the Total Clicks Processed. The Clicks/Second statistic is relative to the timing of your .wav file, and not the time frame in which your computer is processing the data. This feature is provided to help you determine if you are "trashing" (creating distortion) your .wav file due to Thresholds or Tracking values that are set too low. When the algorithm's parameters are set too aggressively, the Clicks / Second number will become extremely high, which could be an indication of impending distortion of your .wav file (although it depends largely on the condition of your Source material).
Note 2: Multiple runs of the impulse filter(s) set un-aggressively will produce better results than running an impulse filter aggressively. Consider setting up multiple Impulse Filters strung together in the Diamond Cut Multi-Filter to facilitate this approach easily.

• Hind Quaternion Mode (HQ Mode)

DCArt10 and DC Forensics10 have included an enhancement to the Impulse filter called Hind Quaternion mode (a name related to the method used in the click detector portion of the Impulse filter). This mode can be used with 78s, vinyl, or any source containing impulsive type noise or clipping distortion. Its operation couples several related variables in this new detector algorithm together in a logical ratio-metric relationship allowing you to vary them with one control. The Size slider controls these new variables. The other controls on the Impulse filter still perform their previously defined functions. In HQ mode, you will find that you have a much larger degree of control of the detector algorithm, especially through the use of the Size control. Small fast rise time clicks will be detected optimally with small values of Size while larger and slower events will be best detected with larger settings. The advantage of this mode is an improvement in the ability of the detector to reject musical transients while still maintaining good click sensitivity.

• Universal Mode

Universal Mode provides an adaptive element to the detector portion of the Expert Impulse Filter algorithm. It can be used with any type of impulsive noise, whether the source is from records (78s, 45s, 33.3s), radio static, or tape head static discharge. Universal Mode is of particular advantage in some wireless surveillance applications where occasional static needs to be mitigated. The advantage of this mode is its ability to adapt to changing signal environments. The Tracking control is its primary means of adjustment. The Tracking control affects the systems overall centered value of sensitivity while the Threshold control sets the minimum value that the system can apply to the detector. The size control does the same thing as with the all of the other modes associated with the Expert Impulse Filter. The disadvantage of the Universal Mode detector is its diminished transient response. This will cause the system to have some difficulty in dealing with the leading edges of rapidly changing audio material, since it will take some time to re-adjust itself. This could cause some transient leading edge distortion and/or leading edge missed impulse detection.

Important Note: The various Impulse Noise filter modes (LP, 78, HQ, Universal) all take a somewhat different approach toward identifying clicks and pops. Since these types of impulses can come in an almost infinite number of sizes and types, a user may select the mode that provides good results over a very wide range of impulsive noise environments.

• Solo/Brass Mode

DCArt10/DC Forensics10 provides you with a discriminator routine which identifies brass instruments (and certain types of vocal solos) and excludes them as candidates for interpolation. Since brass musical instruments (like solo trumpets) produce waveforms that are quite similar in form to impulsive noise events, this option can be quite useful in preventing distortion from being interjected upon those types of instruments. This mode of operation is also useful on some “close-miked” vocals where the background instrumentation is low in sound level comparatively speaking. The tradeoff with using Solo/Brass mode of operation is reduced de-clicking capability during the actual presence of the brass instrument or vocal in question. This mode should not be used on material that does not contain strong solo brass instruments or up-front lead vocals.

Note 1: Do not mute the beginning or the ending of a .wav file before operating the impulse noise filter. Mute the extraneous noises from the beginning and the ending of your .wav file at the end of all of your audio restoration processes.

Note 2: There will be occasions wherein 78-RPM recordings will benefit from the de-clicking action of the vinyl mode impulse filter in conjunction with the tracking control. If the clicks are small and short, it is worth giving it a try.

Note 3: Multiple passes through the impulse filter, especially in vinyl mode while using the tracking control, can produce ever-improving results, especially when Solo/Brass mode is enabled.
Expert-Impulse Noise Operating Procedure (Tutorial)

1. Highlight the portion of your .wav file on which you desire to apply the Impulse Noise Filter. (You may choose to highlight the entire file or any portion thereof.) Sometimes, when confronted with extremely stubborn clicks or pops, or radio "static" it may be useful to use the Zoom-In feature first before running the Impulse Noise Filter on a "grouping".
2. Click on the "Filter Menu" with the left mouse button.
3. Click on "Expert Impulse Noise".
4. Start with the "Threshold" control at a setting of approximately 1000 for 78s.
5. Start with a "Size" setting of between 3 to 7 samples for non-Vinyl applications, and use a setting somewhere in the 10 to 15 sample range for Vinyl LP and 45 RPM record applications.
6. If you are de-clicking a Vinyl LP record, click "Vinyl LP" on with the left mouse button.

Note: This feature is also utilized for 45-RPM records. If you are de-clicking a 78-RPM record or something similar, make sure "Vinyl LP" is turned-off. (It is important to re-emphasize that Vinyl LP mode works best on .wav files that have been sampled at 44.1 kHz or higher.)

1. If you are de-clicking a Vinyl LP record, set the threshold control to its lowest value, and perform all of your adjustments with the tracking control, starting with a setting of 25 to 30. If you are de-clicking a 78-RPM record or something similar set the tracking control to its lowest value and perform all of your adjustments with the threshold control.
2. Click on "Preview”.
3. Listen to the "Previewed" version of the processing parameters that you have just set. If your computer is too slow, it will "hick-up" or "stutter." (Do not be concerned that your final sound restoration will sound like this, since it will not!) Try to listen "through" the stutter to judge what the Filter is doing. If the "stutter" is too annoying to make a judgment of the performance of the filter settings use Run filter mode on a selected portion of the .wav file directly into the "Destination" workspace. Alternately, run the filter, and then listen to the Destination Workspace in order to make judgments regarding your settings. Iterate until you are satisfied with the results.

Note: Setting the filter too aggressively may cause excessive stuttering during Preview. Even a fast computer will stutter when this filter is finding hundreds of clicks per second. If this is happening, readjust the filter to be less aggressive.

1. When the filter is running, you will see a display of "Clicks / Second" and "Total Clicks Processed." Generally speaking, when the threshold is set too low, the program will begin to react to sound transients rather than just noise transients. If the "Clicks / Second is greater than 30, there is a good chance you are catching sound transients, and creating distortion on the output of the filter. Most records will show less than 10 clicks per second when the settings are correct (except in extreme circumstances). Keep adjusting the threshold setting until the clicks are being removed and distortion is not being produced on the filter output. (The distortion that can occur will be most prevalent on the sibilant sounds.) Keep in mind that lower value settings of the threshold control will cause the algorithm to be more sensitive to removing clicks and pops. However, if it is set too low, distortion will also be produced on the sibilant sounds.
2. If the algorithm is capturing the larger impulses but not the smaller ones, try decreasing the "Size" adjustment, and re-evaluate the results. (You may also have to decrease the threshold control.)
3. When you determine the best setting of the controls for your particular .wav file, click “Run Filter”. When the filter has completed its operation, the results will appear in the "Destination" workspace.

Note: Notes 1, 2 & 3 found at the end of the EZ Impulse Filter Tutorial also apply to the Expert Impulse Noise Filter.