Increasing FM modulation index transfers energy from the carrier into a growing number of sidebands

When the modulation index I is zero, all energy resides at the carrier frequency. As I increases from zero, sidebands appear symmetrically above and below the carrier at integer multiples of the modulating frequency: c±m, c±2m, c±3m, etc. Crucially, this energy comes from the carrier itself: the carrier amplitude diminishes and sidebands grow. The number of sidebands with significant amplitude grows with I, producing an increasingly wide and bright spectrum. This explains the perceptual experience of turning up FM ‘depth’ or ‘index’: the sound goes from a pure tone to progressively richer, brighter timbres. A misconception is that raising index simply adds new frequencies; in fact it redistributes a fixed total energy across more components.

Examples

In Dexed (DX7 emulator): set operator output level to minimum (I≈0) — you hear only the carrier. Gradually increase it and watch sidebands emerge while carrier amplitude fluctuates, per Bessel function curves.

Assessment

Explain why an FM patch can produce a near-pure tone at very low index and a dense, noise-like spectrum at very high index, using the concept of energy redistribution from carrier to sidebands.