Serum further allows you to specify the mode for the OCT and SEM controls.

Right-click either control and choose the mode in the context menu.

You can select from the following options:

• Semitones — Adjust the pitch in semitones, which are the intervals between two adjacent keys on a piano tuned to 12-tone equal temperament used in American/European musical tradition.
This allows for fine-tuning, transposing, or creating intervals such as minor/major thirds, fifths, and more.
• Harmonics — Change the pitch by multiplying the base frequency using whole number harmonics.
This generates pitches based on the harmonic series, which is useful for creating overtone-rich sounds, like organ tones or harmonic layers.
• Ratio — Set the pitch of the oscillator in relation to the base frequency using ratios, which is common in FM synthesis.
In this case, the oscillator is tuned to a specific ratio relative to another oscillator to create complex timbres. When you select this mode, you can then set the specific source (SRC) and ratio.
• Step — Adjust the pitch up or down in periods and steps, as defined by the active MTS-ESP tuning.
This option only appears when you are using microtuning and have an MTS-ESP tuning source available.