Complex natural sounds (e.g., animal vocalizations or speech) can be characterized by specific spectrotemporal patterns whose components change in both frequency (frequency modulation, FM) and amplitude (amplitude modulation, AM). The neural coding of AM and FM has been widely studied in humans and animals, but typically with either pure AM or pure FM stimuli. The neural mechanisms employed to perceptually unify AM and FM acoustic features remain unclear. Using stimuli with simultaneous sinusoidal AM (at rate f_AM=37Hz) and FM (with varying rates f_FM), magnetoencephalography (MEG) is used to investigate the elicited auditory steady state response (aSSR) at relevant frequencies (f_AM, f_FM, f_AM ±f_FM ). Previous work demonstrated that for sounds with slower FM dynamics (f_FM <5Hz), the phase of the aSSR at f_AM tracked the FM; in other words, AM and FM features were co-tracked and co-represented by 'phase modulation' encoding. This study explores the neural coding mechanism for stimuli with faster FM dynamics (up to 30 Hz), demonstrating that at faster rates (f_FM>5Hz), there is a transition from pure phase modulation encoding to a single-upper-sideband (SSB) response (at frequency f_AM+f_FM ) pattern. We propose that this unexpected SSB response can be explained by the additional involvement of subsidiary amplitude modulation encoding responses, simultaneously to, and in quadrature with, the ongoing phase modulation. These results, using MEG to reveal a possible neural encoding of specific acoustic properties, demonstrate more generally that physiological tests of encoding hypotheses can be performed non-invasively, and on human subjects, complementing invasive, single-unit recordings in animals.