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Median Filter
The Median Filter can be used to substantially reduce "crackle" (small impulse noise) from a recording. Use a sample setting of 3 to 7 for this application as your starting point range. A “weighting” control is also provided, which affects the “timbre” of the processed sound.
The Median Filter
There is no analog circuit equivalent to the Median Filter. This filter defines a window of samples, and for that window, determines which sample is the median value within the grouping (median meaning middle amplitude value). That value is the one that is passed along to the destination file, and then the window moves over 1 sample and re-evaluates the median, again passing the new median value to the destination file. This filter is useful for improving the intelligibility of severely distorted signals and it is also useful for pulling signals out of a very poor signal-to-noise ratio situation (pulling signals out of the mud). It is somewhat similar in sound performance to a high-order Low-pass filter. (The median value of a string of sorted numbers is the one in the middle of the string. In other words, if you have seven sorted numbers by amplitude, the fourth number will be the median value.) The DCArt10/DC Forensics10 Median Filter allows you to choose the number of samples over which the median value is determined by the algorithm. The range is from 3 to 20 samples. The higher that you set this value, the greater will be the attenuation of high frequency signals and/or noise. Also, the higher the samples setting, the longer will be the processing time required for the filter. The most useful settings will generally be found to be from 3 to 7 samples. Outside of that range, you may hear a significant degradation of the top end of your recording, depending on its bandwidth. Even at 7 samples, you will notice some "fuzziness" inter-modulating with the upper end of the spectrum on some recordings. So always start with 3 to 5 samples when using the Median filter, and choose the smallest value that produces an effective de-crackling result. It is also important to note that the higher the number of samples selected, the longer it will take your computer to calculate the Median values to process your .wav file. At some high settings, your system may not be able to process the file in "real-time" due to the heavy demands that it places on your CPU.
Weighting Function
A “Weighting” function is provided with this filter that shifts the position on the number line as to which value will be chosen as the “Median.” This feature is particularly useful in forensics audio applications wherein extremely inarticulate speech needs clarification. For example, the absence of consonant or sibilant sounds can render a recording undecipherable. Recording bandwidth limitations can severely distort or eliminate the “hissing-consonants” which render speech understandable - - - a fact that has been known since the earliest ventures by Thomas Edison into the field of recorded sound. With the use of the Median filter in conjunction with the weighting control, this problem can often be corrected. The weighting control will essentially affect the “timbre” of the processed sound. Trial and error will determine the best combination of “Samples” and “Weighting” to cure a particular forensics sound problem.
The following is a summary of the control parameters and range of adjustment provided for the Median Filter:
Note 2: Best results on Forensics files will be had when they are first converted to a 44.1 kHz sample rate.
Median Filter Operating Procedure (Tutorial)
Note: The “Weighting” control is used to affect the Timbre of the resultant sound of the Median filter.
The Median Filter can be used to substantially reduce "crackle" (small impulse noise) from a recording. Use a sample setting of 3 to 7 for this application as your starting point range. A “weighting” control is also provided, which affects the “timbre” of the processed sound.
The Median Filter
There is no analog circuit equivalent to the Median Filter. This filter defines a window of samples, and for that window, determines which sample is the median value within the grouping (median meaning middle amplitude value). That value is the one that is passed along to the destination file, and then the window moves over 1 sample and re-evaluates the median, again passing the new median value to the destination file. This filter is useful for improving the intelligibility of severely distorted signals and it is also useful for pulling signals out of a very poor signal-to-noise ratio situation (pulling signals out of the mud). It is somewhat similar in sound performance to a high-order Low-pass filter. (The median value of a string of sorted numbers is the one in the middle of the string. In other words, if you have seven sorted numbers by amplitude, the fourth number will be the median value.) The DCArt10/DC Forensics10 Median Filter allows you to choose the number of samples over which the median value is determined by the algorithm. The range is from 3 to 20 samples. The higher that you set this value, the greater will be the attenuation of high frequency signals and/or noise. Also, the higher the samples setting, the longer will be the processing time required for the filter. The most useful settings will generally be found to be from 3 to 7 samples. Outside of that range, you may hear a significant degradation of the top end of your recording, depending on its bandwidth. Even at 7 samples, you will notice some "fuzziness" inter-modulating with the upper end of the spectrum on some recordings. So always start with 3 to 5 samples when using the Median filter, and choose the smallest value that produces an effective de-crackling result. It is also important to note that the higher the number of samples selected, the longer it will take your computer to calculate the Median values to process your .wav file. At some high settings, your system may not be able to process the file in "real-time" due to the heavy demands that it places on your CPU.
Weighting Function
A “Weighting” function is provided with this filter that shifts the position on the number line as to which value will be chosen as the “Median.” This feature is particularly useful in forensics audio applications wherein extremely inarticulate speech needs clarification. For example, the absence of consonant or sibilant sounds can render a recording undecipherable. Recording bandwidth limitations can severely distort or eliminate the “hissing-consonants” which render speech understandable - - - a fact that has been known since the earliest ventures by Thomas Edison into the field of recorded sound. With the use of the Median filter in conjunction with the weighting control, this problem can often be corrected. The weighting control will essentially affect the “timbre” of the processed sound. Trial and error will determine the best combination of “Samples” and “Weighting” to cure a particular forensics sound problem.
The following is a summary of the control parameters and range of adjustment provided for the Median Filter:
- Samples: 3 - 20
- Preview Mode Button: On / Off (The slider control can be adjusted "live" when the preview mode is on.)
- Weighting: +/- 100 (%) from the neutral value of 0. (0 provides no weighting to the Median value calculated by the filter.)
- Bypass Button: Allows you to compare between the filtered and the raw signal.
- Keep Residue: Allows you to hear what the Filter is removing from the raw signal.
Note 2: Best results on Forensics files will be had when they are first converted to a 44.1 kHz sample rate.
Median Filter Operating Procedure (Tutorial)
- Highlight the portion of your .wav file on which you desire to apply the Median Filter.
- Click on the “Median Filter” (Filter Menu).
- Choose the number of samples over which you desire the median calculation to be performed. The higher the number of samples selected, the greater will be the attenuation of the higher frequency portion of the audio spectrum. You can choose any integer value from 3 to 20 samples. (The most useful values will be found in the 3 to 7 samples range.) The higher the chosen number of samples, the longer will be the process time requirement for the algorithm. Changes in value are accomplished utilizing the slider control.
- If you desire to hear the results of your filter settings before creating a new "Destination" file, click on "Preview".
- You will hear the effect of the calculation of the median value over the chosen number of "samples".
- As the filter is running in either preview mode or normal mode (Destination File Mode), you will see a dialog box that indicates the "% Done" of the filter algorithm on the selected portion of the Source .wav file. Also, at the top of the Dialog box you will see indicated the "Total Samples to Process:"
- Keep adjusting the number of samples until you achieve your desired effect.
- When you are satisfied with a setting, click on “Run”, and the filter will process your source .wav file through the filter algorithm, and create a Destination .wav file containing the output of the filter.
- When this process is complete, you will see the Destination File become highlighted in Yellow, at the same time that the Source File becomes unselected.
- Click on "Close".
Note: The “Weighting” control is used to affect the Timbre of the resultant sound of the Median filter.