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A very common use of the SOFControl module is within a perceptual volume control. As the volume of the system is reduced, overall spectral balance should be maintained. Due to the sensitivity of the human auditory system, low frequencies and high frequencies appear to drop off more quickly than mid frequencies. Thus, to maintain the overall spectral balance, boost low and high frequencies as the volume level decreases. The VolumeControl module accomplishes this with a fixed boost table. For finer control over the boost, use a TableInterp module together with a SOFControl filter as shown below. The control signal “Volume” specifies the listening level and ranges from 0 (loud) to -80 (soft). The lookup tables convert the Volume setting into low frequency and high frequency boosts which are applied using the SOFControl module. The low frequency SOFControl module implements a peaking filter at 40 Hz and the gain is taken from the control pin. The high frequency SOFControl module implements a high shelf in which the gain is taken from the control pin.
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Filters with Raw Coefficients
Audio Weaver contains several filter types which operate on raw coefficients. These filters are for expert users who understand DSP and know how to calculate the filter coefficients[1]. .
Note: MATLAB is often used by expert Audio Weaver users to compute coefficients and then update them in the block diagram.
There are two types of filters – Finite Impulse Response (FIR) and Infinite Impulse Response (IIR). Although Audio Weaver supports both types of filters, the majority of the filters used in audio applications are IIR due to their computational efficiency.
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 | FIR
| Time domain FIR filter Specify filter length in module properties |
 | Biquad
| Second order IIR filter. 5 filter coefficients are specified. No smoothing. |
 | BiquadCascade
| Multiple Biquad filters in series. The number of filters is specified in module properties. The same coefficients are used per channel. |
 | BiquadSmoothed
| Second order IIR filter. 5 filter coefficients are specified. Smoothed on a block-by-block basis |
 | BiquadNCascade
| Multiple Biquad filters in series. The number of filters is specified in module properties. Different coefficients are used per channel. |
  | FIR Sparse
| Sparse FIR filter in which most values are zero. Less convolution cycles than normal FIR |
  | FIR Sparse Reader
| Sparse FIR that connects to a delay state writer. Convolution is based on a pointer rather than a separate FIR buffer.
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  | FIR Sparse Reader Fract16
| Like FIR Sparse Reader except half the memory. Data is converted to fract16 for computations and has a conversion for the output if necessary. |
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The High Precision modules are designed to be drop in replacements for the non-high precision filters. That way, numerical problems can be resolved by replacing the offending filter with its high precision version.
  | BiqudSmoothedHP | Smoothly varying Biquad |
  | ButterworthFilterHP | Butterworth lowpass, highpass, and allpass filters |
  | BiquadCascadeHP | Cascade of N Biquad stages |
  | GraphicEQBandHP | Single band of a graphic equalizer |
  | SOFControlHP | Controllable second order filter with design equations |
  | SOFCascadeHP | Cascade of second order filters each with design equations |
  | SecondOrderFilterHP | Single second order filter with design equations |
  | VolumeControlHP | Fletcher Munson volume control with loudness compensation |
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Pass Through filterType = 0
Applicable parameters: none.
Biquad coefficients are set to b0=1, b1=0, b2=0, a1=0, and a2=0. The filter runs and consumes processing but the output equals the input. |
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Gain filterType = 1
Applicable parameters: gain
A simple gain with coefficients set to b0=undb20(gain), b1=0, b2=0, a1=0, and a2=0 |  |
1st order Butterworth lowpass filter filterType = 2
Applicable parameters: freq |  |
2nd order Butterworth lowpass filterType = 3
Applicable parameters: freq |  |
1st order Butterworth highpass filterType = 4
Applicable parameters: freq |  |
2nd order Butterworth highpass filterType = 5
Applicable parameters: freq |  |
1st order allpass filterType = 6
Applicable parameters: freq |  |
2nd order allpass filterType = 7
Applicable parameters: freq and Q |  |
2nd order low shelf filterType = 8
Applicable parameters: freq and gain
Use as a low frequency tone control |  |
2nd order low shelf with Q filterType = 9
Applicable parameters: freq, gain, and Q |  |
2nd order high shelf filterType = 10
Applicable parameters: freq and gain
Use as a high frequency tone control
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2nd order high shelf with Q filterType = 11
Applicable parameters: freq, gain, and Q |  |
2nd order peaking / parametric filterType = 12
Applicable parameters: freq, gain, and Q
Commonly used for generic equalization since it has controllable frequency, gain, and Q settings. |  |
2nd order notch filterType = 13
Applicable parameters: freq and Q |  |
2nd order bandpass filter filterType = 14
Applicable parameters: freq and Q |  |
1st order Bessel lowpass filter filterType = 15
Applicable parameters: freq |  |
1st order Bessel highpass filter filterType = 16
Applicable parameters: freq |  |
1st order asymmetrical low shelf filterType = 17
Applicable parameters: freq and gain |  |
1st order asymmetrical high shelf filterType = 18
Applicable parameters: freq and gain |  |
1st order symmetrical low shelf filterType = 19
Applicable parameters: freq and gain |  |
1st order symmetrical high shelf filterType = 20
Applicable parameters: freq and gain |  |
Note: The Butterworth filter from SecondOrderFilterSmoothed is the same as the ButterworthFilter module of equal filter order. However, SecondOrderFilterSmoothed only implements
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1st and
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2nd order Butterworth filters. Higher order Butterworth filters can only be implemented by the ButterworthFilter module.
The SecondOrderFilterSmoothed implementation of the first order Butterworth filter is more computationally efficient than the ButterworthFiltermodule.
Low/high shelf filter and low/high shelf filter Q are identical if Q is set to 0.707 (
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√0.5).
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Second Order Filter Smoothed Cascade
This module contains several SecondOrderFilterSmoothed modules in series. This can be used to implement a more complicated EQ with only a single module. Under module properties, specify the number of stages of filtering. If the number of stages is set to 1, then this module is equivalent to the SecondOrderFilterSmoothed module. When there are multiple stages, the inspector expands as shown right:
[1] Matlab is often used by expert Audio Weaver users to compute coefficients and then update them in the block diagram
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Table of Filter Modules
Type | Filter Type | Order | Usage Tip |
Adaptive | LMS | (FIR length) | signal to be learned' goes into reference pin. |
Calculated Coeffs | Allpass Pair | Variable | set Rs for better band separation |
Calculated Coeffs | Audio Weighting | Cascaded 2nd | change weighting with the dropdown variable. |
Calculated Coeffs | Crossover Filter | Variable up to 10th | frequency will split into upper and lower bands |
Calculated Coeffs | Emphasis Filter | 1st | used typically by weighting formulas |
Calculated Coeffs | Graphic EQ | Variable up to 12th | automatic frequency setting with module variables |
Calculated Coeffs | Graphic EQ Band | Variable up to 12th | up to 12th order, useful. |
Calculated Coeffs | Hilbert | 12th | +-90 degrees added to phase |
Calculated Coeffs | Pink Filter | 8th | turns white noise pink. Turns pink noise red. |
Calculated Coeffs | Three Band Tone Control | 2nd | Cheap solution for handling "full spectrum" |
Calculated Coeffs | Tilt Filter | Variable 2nd | Makes noise more 'blue' or 'pink' based on slope. |
Controllable | First Order Filter Control | 1st | immediate control |
Controllable | LPF1 Control | 1st | immediate control |
Controllable | Second Order Filter Control | 1st/2nd | delayed control |
High Precision | Biquad Cascade (Precise) | Cascaded 2nd | High Precision, use for handling low freq, high SR, or sensitive ears. |
High Precision | Biquad Smoothed (Precise) | 2nd | (same) |
High Precision | Butterworth Filter (Precise) | Variable up to 10th | (same) |
High Precision | Graphic EQ Band (Precise) | Variable up to 12th | (same) |
High Precision | Second Order Filter (Precise) | 1st/2nd | (same) |
High Precision | SOFCascade (Precise) | Cascaded 1st/2nd | (same) |
High Precision | SOFControl (Precise) | 1st/2nd | (same) |
Raw Coeffs | Biquad | 2nd | clicks/pops with changes, this is for constant filter. |
Raw Coeffs | Biquad Cascade | Cascaded 2nd | same as above, higher order. |
Raw Coeffs | Biquad N Cascade | 2nd | cascade with different filter responses. |
Raw Coeffs | Biquad Smoothed | 2nd | smoothly varying, use for end-user control. |
Raw Coeffs | FIR | (FIR length) | convolution FIR |
Raw Coeffs | FIR Sparse | (FIR length) | FIR with mostly zeros, computationally efficient. |
Raw Coeffs | FIR Sparse Reader | (FIR length) | Hook to delay state writer, unique iterator feeds input for convolution |
Raw Coeffs | FIR Sparse Reader Fract16 | (FIR length) | twice the memory efficiency as above. |
Floating Modules | Butterworth Filter | Variable up to 10th | Only odd allpasses are supported. Use SOF for even allpasses. |
Floating Modules | Second Order Filter Smoothed | 1st/2nd | 20 different filter types, varying orders. |
Floating Modules | Second Order Filter Smoothed Cascade | Cascaded 1st/2nd | same as above, but cascaded. |