Detection of combined frequency and amplitude modulation

Area of Science:

• Auditory Perception
• Psychoacoustics
• Signal Processing

Background:

• Mixed modulation (MM), involving simultaneous amplitude modulation (AM) and frequency modulation (FM), presents complex auditory signals.
• Understanding the independent or interactive detection mechanisms of AM and FM is crucial for auditory scene analysis.

Purpose of the Study:

• To investigate the detection thresholds and mechanisms of mixed modulation (AM and FM).
• To determine how the presence of one modulation type affects the detection of the other.
• To examine the role of modulator phase in the perception of combined AM and FM.

Main Methods:

• Adaptive two-alternative forced-choice tasks were employed to measure detection thresholds.
• Psychometric functions were measured for AM alone, FM alone, and combined AM and FM.
• Stimuli varied in modulation rates (4-256 Hz) and modulation depths, with controlled AM presence during FM detection.

Main Results:

• FM detection thresholds were significantly influenced by the magnitude of coexisting AM.
• At high modulation rates (256 Hz), modulator phase strongly affected FM detection, aligning with sideband detection models.
• At low modulation rates (4-16 Hz), AM and FM detection appeared non-independent, with detectability of combined MM exceeding independent coding predictions.

Conclusions:

• Auditory detection of AM and FM is not strictly independent, particularly at lower modulation frequencies.
• Modulator phase plays a significant role in MM perception, especially at higher rates, suggesting complex spectral processing.
• Existing models do not fully capture the observed phase effects in combined AM and FM detection.