These filters are all suitable for use as a Voltage Controlled Filter (VCF) in synthesizers, and as such can have their cutoff and resonance frequencies modulated rapidly.
Notes: It should be taken as read that all these filters have their control voltage internally limited to +10 Volts, and that they operate within the normalizer SynthEdit audio voltage range. Some may have the audio internally clipped so will distort at high input levels.
Ladder Filters.
TD_DiodeLP24_A
Type: 24dB per Octave frequency roll-off diode ladder lowpass filter type A
Notes: A simplified 4 pole diode-ladder lowpass filter with a single symmetrical non-linearity. Partial passband gain-loss compensation. Diode-ladder filters are unbuffered which leads to the cutoff frequency being a product of cutoff amount, input level and resonance.
TD_DiodeLP24_B
Type: 24dB per Octave frequency roll-off diode ladder Lowpass Filter Type B
Notes: A 4 pole diode-ladder lowpass filter, the first pole is an octave higher, with an additional unbuffered high pass filter in the feedback path that partially damps the resonance at low cut-off frequencies.
An extra buffered high pass filter is on the input too. So it is more like a 5 pole + 1 pole filter. The filter cutoff slope is not a constant vs frequency. There are asymmetrical non-linearities, and partial passband gain-loss compensation. Diode-ladder filters are unbuffered which lead to the cutoff frequency being a product of cutoff amount, input level and resonance. Because of the first pole being an octave higher, maximum cutoff is 9.5 Octaves (~9.95kHz) if the sample rate is equal or less than 48kHz, 10.5 octaves (~19.9kHz) otherwise.
TD_DiodeLP24_C (EMS VCS3 Emulation)
Type: 24dB per Octave frequency roll-off diode ladder Lowpass Filter Type C
Notes: A 4 pole diode-ladder lowpass filter inspired by the paper “Efficient polynomial implementation of the EMS VCS3 filter” by Stefano Zambon and Federico Fontana. However it uses ‘Mystan’s Pivot’ method to approximate the non-linearities instead, which in this case are asymmetrical. Full passband gain-loss compensation. Diode-ladder filters are unbuffered which lead to the cutoff frequency being a product of cutoff amount, input level and resonance.
TD_Ladder5out_HP
Type: Linear 4 pole High pass filter with five outputs
Note: This module is useful as the basis for pole-mixing filters.
Note: The filter does not self oscillate.
Traditionally, 4-pole pole-mixing filters only mix the 4 poles.
Two of the poles are inverting – either 1 and 3 or 2 and 4. Some short/set-high/bypass the first pole under some circumstances to get more variations.
With this version, instead of bypassing – all you have to do is move the mixing-coefficients up i.e. hp1 coefficient, hp2 coefficient, hp3 coefficient becomes in-hp4 coefficient, hp1 coefficient, hp2 coefficient.
TD_Ladder5out_LP
Type: Linear 4 pole Lowpass Filter with 5 outputs
Notes: This module is useful as the basis for pole-mixing filters.
Note: The filter does not self oscillate.
Traditionally, 4-pole pole-mixing filters only mix the 4 poles.
Two of the poles are inverting – either 1 and 3 or 2 and 4. Some short/set-high/bypass the first pole under some circumstances to get more variations.
With this version, instead of bypassing – all you have to do is move the mixing-coefficients up i.e. lp1 coefficient, lp2 coefficient, lp3 coefficient becomes in-lp4 coefficient, lp1 coefficient, lp2 coefficient.
TD_LadderHP24_A
Type: 24dB/Octave Transistor-Ladder High pass Filter
Asymmetrical non-linearities. Partial passband gain-loss compensation. Self oscillates.
TD_LadderLP18_A
Type: 18dB per Octave Transistor-Ladder Lowpass Filter
Simplified filter version with a single symmetrical non-linearity. No passband gain-loss compensation. Self oscillates.
TD_LadderLP24_A
Type: 24dB/Octave Transistor-Ladder Lowpass Filter Type A
Simplified version with a single asymmetrical non-linearity. No passband gain-loss compensation (classic behaviour). Self oscillates.
TD_LadderLP24_B
Type: 24dB per Octave Transistor-Ladder Lowpass Filter Type B
Multiple asymmetrical non-linearities. Partial passband gain-loss compensation. Note: The filter does not self-oscillate.
TD_LadderLP24_C
Type: 24dB/Octave Transistor-Ladder Lowpass Filter Type C
This filter has a buffered high pass filter in the feedback path. This prevents self oscillation at very low frequencies and also prevents bass loss at high resonance levels. Asymmetrical non-linearities.
Oberheim X-Pander type filter.
TD_Panda (Inspired by the Oberheim X-Pander filter)
Type: 4pole OTA Inspired Filter
Notes: The filter poles are mixed internally to create 25 different frequency responses. Produces predominantly 3rd harmonic distortion. Self oscillates.
List of Filter frequency responses; LP4, LP3, LP2, LP1, HP4, HP3, HP2, HP1, BP4, BP2, BPS, HP3LP1, HP2LP1, HP1LP3, HP1LP2, HP1BR2, BR2LP2, BR2LP1, BR4, BR2, BRBP2, LP1BR2HP1, LP1PK2, BR2PK2, HPX.
Sallen-Key filters.
TD_SK3P
Type: Buffered 12dB/Octave Lowpass Sallen-Key Filter
Frequency cutoff 12dB per octave slope. The extra pole is in the feedback path and limits resonance at higher frequencies and alters tracking a bit. Self-resonates up to about 3.7kHz. Symmetrical non-linearities.
TD_SK4P
Type: 12dB/Octave Lowpass Sallen-Key Filter
Experimental, has extra poles. Asymmetrical non-linearities.
TD_SK_A
Type: Multi-Mode Sallen-Key Filter; Low Pass 12dB per Octave, High Pass 6dB per Octave, Band Pass 6dB per Oct.
Buffered. This saves some CPU cycles compared to the Steiner modules. Asymmetrical non-linearity.
Steiner-Parker filters.
TD_Steiner_A
Type: Buffered Steiner-Parker multi-mode filter Type A
A Steiner-Parker filter is simply a multi-input Sallen-Key filter.
Symmetrical non-linearities. Does not self-oscillate. Maximum input is +/- 10 volts. If the sample rate is less than, or equal to 48 kHz, then the maximum cutoff frequency is approximately 14 kHz otherwise it is approximately 19.9kHz.
TD_Steiner_B
Type: Steiner-Parker Filter Type B
An unbuffered version of a Steiner-Parker filter (the feedback loop is still buffered), which simply is a multi-input Sallen-Key filter.
Asymmetrical non-linearities. Does not self-oscillate. Resonance decreases in higher octaves. Maximum input is +/- 10 volts.
If the the sample rate is less or equal to 48kHz, maximum cutoff frequency is approximately 17.3 kHz otherwise it is approximately 19.9 kHz.
TD_Steiner_C
Type: Steiner-Parker Filter Type C
A buffered version of a Steiner-Parker filter, which simply is a multi-input Sallen-Key filter. Bright asymmetrical non-linearities. Self-oscillates.
If the sample rate is less or equal to 48kHz, the maximum cutoff frequency is approximately 17.3kHz otherwise it is approximately 19.9kHz.
State-Variable filters.
TD_SV24_A
Type: Cascaded State Variable Filters Type A; 24dB per Octave Low pass, 24dB per Octave High pass, and 12dB per Octave Band pass.
Induced passband gain-loss to prevent too high levels with high resonance.
This is a non-linear filter but it is not able to self-oscillate.
This module uses less CPU than if two standard SE SV filters were cascaded.
TD_SV24_B
Type: Cascaded State Variable Filters Type B; 24dB per Octave Low pass, 24dB per Octave High pass, 12dB per Octave Band pass.
Induced passband gain-loss to prevent signal levels becoming too high with high levels of filter resonance.
This is non-linear filter but it is not able to self-oscillate. The resonance level decreases as the filter cutoff frequency increases.
This module uses less CPU than if two standard SE SV filters were cascaded.
TD_SV_C
Type: State Variable Filter Type C; 12dB per Octave Low pass, 6dB per Octave Band pass.
Experimental State Variable Filter that is modified by adding an extra feedback path around the structure, so it’s not really a true SV filter any more. Resonance decreases at higher frequencies. Asymmetrical non-linearities. Does not self oscillate.
TD_SVmo2
Type: Multiple Output State Variable Filter; 12dB/Octave Low pass, 6dB/Octave Band pass, 12dB/Octave High Pass
The filter outputs can be mixed, combined or otherwise processed to achieve other filter shapes. Non-linear filter.
TD_SVmo2_ST
Type: Stereo Multiple Output State Variable Filter 12dB/Octave Low pass, 6dB/Octave Band pass, 12dB/Octave High Pass
The filter outputs can be mixed, combined or otherwise processed to achieve other filter shapes. Non-linear filter.
TD_SVxfade
Type: Cross-fading State Variable Filter. Three pre-programmed selectable crossfade sequences.
Select the filter type and cross-fade sequence from;
Low pass /Band Reject /High pass,
Low pass /Peak /High pass,
Low pass /All pass /High pass.
The cross-fade voltage range is from -5 Volts to +5 Volts.
