Hilbert Filter | Transforms the input into two signals with 90° phase difference
|  | A Hilbert transformer takes an input signal and calculates two output signals that have a constant 90° phase difference across the whole spectrum. Applications are single sideband modulation (SSB) also known as frequency shifting and envelope generation.
| Input file Output file: The 90° outputs can be thought of as the cosine and the sine part of the signal or as the real and imaginary part alternatively. When doing the pure transform without modifications you'll get two output files. When performing frequency shifting or envelope generation the output is real again (single file). Operation: What should be done after the hilbert transform. You can leave the signal unchanged, shift the spectrum up or down or calculate the sound's envelope. Antialiasing: When frequency shifting is applied we will normally want to have a protection against aliasing. Aliasing occurs for example when you shift a signal downwards by 100 Hz and the signal has significant energy below 100 Hz. Think of a partial at 30 Hz. When you downshift it by 100 Hz the mathematical outcome is -70 Hz and physically you'll hear it at +70 Hz. By checking this gadget all signal content that would generate aliasing is eliminated. There are applications where you want to have aliasing. For example try to shift a signal up or down half the sampling rate (22050 Hz for a 44.1 kHz sound) - when you allow aliasing the result will be an inverted spectrum! (1000 Hz becomes 21050 Hz, 2000 Hz becomes 20050 Hz etc.). Note that because of the antialiasing filter the bass frequencies are slightly attenuated. Try to feed this module with white noise and see what comes out. If you don't like it apply a bass compensation filter afterwards. Shift amount: The the "shift up" and "shift down" operation. Note that frequency shifting is not pitch transposition. That means that harmonic relationships are distroyed resulting in bell-like spectra with non-linear partial spacing, the kind of sound you know from ring modulation. Shift envelope: Not yet implemented! Toolbar: Popup menus for loading and saving settings, presets and options. Refer to a the basic chapter on process windows. Processbar: Buttons for closing the module, starting and stopping processing. Process gauge. Refer to a the basic chapter on process windows. Known bugs: None To be done: Envelope implementation! Option for bass compensation.
Contents | last modified: 25-Feb-02