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Filter Sweeper
The Filter Sweeper is a novel system providing a variable frequency response versus time capability. The sweeper can be applied to an entire .wav file or any portion by using the Diamond Cut selective filtering capability. Three filter types are available in the Filter Sweeper including a sweep-able Low-Pass, High-Pass, and Notch. Besides producing interesting special effects, the Filter Sweeper has a number of very useful audio restoration applications. Here are two examples.
The hum frequency of a recording sometimes changes as a function of time. This audio defect may be due to the fact that the original analog tape recorder was battery powered and as the battery became weaker during the recording process, its speed decreased. This mechanism will have caused the resultant hum frequency to increase versus time when played back later on a constant speed system. To correct this defect, you can apply the Filter Sweeper set for operation in “Notch” mode. Set the “Start Frequency” to the appropriate lowest hum frequency value of the defective .wav file, and the set the “Stop Frequency” to the highest value of hum frequency. These two values of frequency can be determined by using the Spectrum Analyzer while using the “looped play” mode during the appropriate sections (beginning and end) of the defective .wav file. The “Sweep Type” will need to be determined empirically, because the appropriate compensation curve will depend on the type of capstan servo motor regulator that was used in the offending tape recorder in conjunction with the type of batteries that had been employed.
The Filter Sweeper
Another application of the Filter Sweeper is what we call “masked” Fade-Ins and/or “masked” Fade-Outs. These “masking” techniques not only can be used to produce very interesting effects, but also provide a very useful noise reduction tool. How can a swept filter produce noise reduction? The principle is based on a simple musical concept coupled with the non-linear nature of the human sense of hearing. Some musical material commences with very simple progressions (including only one or two musical instruments), then moving into a series of complex riffs and refrains (maybe including a few crescendos) and then often diminishing in complexity towards its ending (soft curtain). Much of the noise in such a recording is hidden by the complexity of the musical material found it the central body of the piece. The noise on the recording is heard to be most prevalent to the human ear at its beginning and its ending during the compositions simple parts. (In actual fact, the noise level is probably relatively constant throughout the entire recording.) That is why we refer to this phenomenon as “masking. This perception is a trick that the human ear plays on the listener which you can take advantage of as an audio restoration engineer. By first selecting an area at the beginning or end of the piece and then using the various Filter Sweeper presets labeled “Masked Fade-In” or “Masked Fade-Out”, you can smoothly contour the response of the restoration system during these quiet intervals at the beginning and ending of the musical rendition. If there is excessive hiss present during these time periods, use the Low-Pass Sweeping filter. If there is a high degree of low frequency noise (like rumble and or HVAC noise), use the High-Pass Sweeping filter. If there is a combination of both of these types of noises, you will have to apply two passes of the Swept filter to obtain the optimal results, running one operation with the Low-Pass and another with the High-Pass or by using the Multi-filter. Since some musical material is simple at its beginning and end, reduced frequency response will often not be observed by the listener due to the application of this new technique. The “Sweep Type” that you choose will depend on the type of musical progression that you are dealing with, and is also somewhat subjective.
The Filter Sweeper provides the following controls:
The Filter Sweeper is a novel system providing a variable frequency response versus time capability. The sweeper can be applied to an entire .wav file or any portion by using the Diamond Cut selective filtering capability. Three filter types are available in the Filter Sweeper including a sweep-able Low-Pass, High-Pass, and Notch. Besides producing interesting special effects, the Filter Sweeper has a number of very useful audio restoration applications. Here are two examples.
The hum frequency of a recording sometimes changes as a function of time. This audio defect may be due to the fact that the original analog tape recorder was battery powered and as the battery became weaker during the recording process, its speed decreased. This mechanism will have caused the resultant hum frequency to increase versus time when played back later on a constant speed system. To correct this defect, you can apply the Filter Sweeper set for operation in “Notch” mode. Set the “Start Frequency” to the appropriate lowest hum frequency value of the defective .wav file, and the set the “Stop Frequency” to the highest value of hum frequency. These two values of frequency can be determined by using the Spectrum Analyzer while using the “looped play” mode during the appropriate sections (beginning and end) of the defective .wav file. The “Sweep Type” will need to be determined empirically, because the appropriate compensation curve will depend on the type of capstan servo motor regulator that was used in the offending tape recorder in conjunction with the type of batteries that had been employed.
The Filter Sweeper
Another application of the Filter Sweeper is what we call “masked” Fade-Ins and/or “masked” Fade-Outs. These “masking” techniques not only can be used to produce very interesting effects, but also provide a very useful noise reduction tool. How can a swept filter produce noise reduction? The principle is based on a simple musical concept coupled with the non-linear nature of the human sense of hearing. Some musical material commences with very simple progressions (including only one or two musical instruments), then moving into a series of complex riffs and refrains (maybe including a few crescendos) and then often diminishing in complexity towards its ending (soft curtain). Much of the noise in such a recording is hidden by the complexity of the musical material found it the central body of the piece. The noise on the recording is heard to be most prevalent to the human ear at its beginning and its ending during the compositions simple parts. (In actual fact, the noise level is probably relatively constant throughout the entire recording.) That is why we refer to this phenomenon as “masking. This perception is a trick that the human ear plays on the listener which you can take advantage of as an audio restoration engineer. By first selecting an area at the beginning or end of the piece and then using the various Filter Sweeper presets labeled “Masked Fade-In” or “Masked Fade-Out”, you can smoothly contour the response of the restoration system during these quiet intervals at the beginning and ending of the musical rendition. If there is excessive hiss present during these time periods, use the Low-Pass Sweeping filter. If there is a high degree of low frequency noise (like rumble and or HVAC noise), use the High-Pass Sweeping filter. If there is a combination of both of these types of noises, you will have to apply two passes of the Swept filter to obtain the optimal results, running one operation with the Low-Pass and another with the High-Pass or by using the Multi-filter. Since some musical material is simple at its beginning and end, reduced frequency response will often not be observed by the listener due to the application of this new technique. The “Sweep Type” that you choose will depend on the type of musical progression that you are dealing with, and is also somewhat subjective.
The Filter Sweeper provides the following controls:
- Filter Type:
- Low Pass (First Order Butterworth)
- High Pass (First Order Butterworth)
- Notch (Second Order with 0.1 Octave Bandwidth)
- Sweep Type: (Determines the shape of the Filter Sweeper’s frequency versus time relationship)
- Linear
- Logarithmic
- Exponential
- Start Frequency: (Determines the frequency value in Hz from which the sweep begins.) Range: 10 – 19,999 Hz
- Stop Frequency: (Determines the frequency value in Hz at which the filter’s sweep terminates over the determined highlighted area of the .wav file.) Range: 10 – 19,999 Hz