In Serum, the native-mode (default) playback of oscillators operates with an ultra high-precision resampling, yielding an astonishingly inaudible signal-to-noise (for instance, -150 dB on a sawtooth played at 1 Khz at 44100)! This requires a lot of calculations, so Serum’s oscillator playback has been aggressively optimized using SSE2 instructions to allow for this high-quality playback without taxing your CPU any more than the typical (decent quality) soft synth already does. Many popular wavetable synthesizers are astonishingly bad at suppressing artifacts - even on a high-quality setting some create artifacts as high as -36 dB to -60 dB (level difference between fundamental on artifacts) which is well audible, and furthermore often dampening the highest wanted audible frequencies in the process, to try and suppress this unwanted sound. Artifacts mean that you are (perhaps unknowingly) crowding your mix with unwanted tones / frequencies. Without considerable care and a whole lot of number crunching, this process will create audible artifacts.
Playback of wavetables requires digital resampling to play different frequencies.
