Alias Free Oscillators

fathom spectrum.png

Many synthesizers are advertised as alias free. If you place a spectrum analyzer on the plugin track it reveals a frequency profile which gradually tapers off into the upper frequencies, proof that your synth is using raw waveforms and internally filtering the oscillator.

For an oscillator to be called alias free, the output must be constructed from partials, a series of sin waves, which control the frequency content prior to filtering.

Filtering should be your choice, not built into your oscillators.

Shown above is the Fathom saw tooth at 128 partials in the frequency domain. This unedited screen capture shows a high frequency noise floor well below -130 dB in relation to the amplitude, or less than one part in one million. Notice that there is no aliasing, imaging or artifacts after the last partial.

We achieve this taking your edited waveform, running it through an FFT (Fast Fourier Transform) to derive the component partials, and then storing the result accurately in a buffer of 16384 samples for each oscillator single cycle. Then at run time the oscillator fetches samples from this buffer for any varying detune frequency using nonlinear interpolation between buffer points. 

The result is a clean waveform like the one shown above.

No Clicking

Clicking results from starting a waveform in mid-cycle which creates an abrupt change in amplitude from one sample to the next. Many software synth’s have the clicking problem because they do not take steps to avoid it. Clicking also happens when any modulation which effects the waveform passes through the vertical edge of an envelope while the oscillator is in mid-cycle.

Fathom contains real time logic which virtually eliminates clicking and enforces perfectly smooth oscillator note transitions and modulations.

Even with oscillator voices detuned, the processor keeps track of the relative phase of each oscillator, and at the start of each note plays each oscillator virtually with no volume for a few microseconds until the beginning of its waveform. In this way all voices maintain their relative phase but never start in the middle of a waveform.

Smooth Note Transitions


Shown above is a screen capture of a Fathom note transition with a single oscillator sin wave and detune set to one voice in free running mode. If a sequencer note stops and a new one starts at the same point in the host sequence, Fathom will make the frequency transition in mid-cycle, emulating a single oscillator hardware synth, even when running in polyphonic mode. 


Above is a Fathom note transition, single oscillator sin wave, with detune set to eight voices in retrigger mode. All voices finish their cycle in the terminating note and start at the beginning of their cycle in the new note.


Single oscillator sin wave, detune 8 voices, free running mode, polyphonic.


Single oscillator sin wave, detune 8 voices, free running mode, monophonic with gilde.