Tube Filter

Filters

1. Frequency Knob Sets the filter’s root frequency.
2. Decay Knob Changes the filter decay time.
3. Tone Knob Controls the brightness of the filter.
4. Open/Close Switch Selects between a tube open on both sides or closed on one side.
5. Trim Knob Adjusts output volume.
6. Audio Input Signal to be filtered (stereo).
7. Audio Output Filtered signal (stereo).

Overview ⚓︎

The Tube Filter module uses physical modeling to simulate sound propagation in a cylindrical tube. The tube acts as a filter, shaping the sound by emphasizing or attenuating specific frequencies based on the module’s settings.

In Depth ⚓︎

The Frequency knob adjusts the fundamental frequency of the tube. This is the lowest frequency that will be emphasized by the filter. In the physical model of the tube, this knob adjusts its length.

The Decay knob acts like the resonance control on a traditional filter: a higher decay will increase the length of the filter’s impulse response and make it sound more resonant.

For greater creative flexibility, we made the decay parameter extend well beyond what would be possible in a real-world physical tube. When using the Tube Filter to simulate a real tube, keep the Decay knob in the first half of its range.

The Tone knob controls the brightness of the tube. Fully clockwise, the filter sounds brighter, simulating a tube with a small radius. Fully counterclockwise, the filter sounds darker and has a shorter decay, simulating a tube with a large radius.

The Open/Close switch controls whether the tube is open on both ends or closed on one end. This changes the frequency response of the tube: an open tube will emphasize the full harmonic series above the fundamental frequency of the tube, while a closed tube will only emphasize odd harmonics.

Astute readers may notice the similarity between the frequency response of the Tube Filter and that of the Comb Filter in feedback mode. The physical model of a tube is indeed a specialized form of a feedback comb filter, incorporating a built-in tone control and parameter ranges that are better suited for this application.

Depending on the harmonic content of the source signal and the Tube Filter settings, the input and output signal levels may vary significantly. Use the Trim knob to compensate for these differences.

If the volume varies too much when the input signal frequency changes, try decreasing the tube decay.