Interfacing PDM MEMS microphones with PFM spiking systems: Application for Neuromorphic Auditory Sensors

TL;DR

A new architecture of spike-based band-pass filter to reject DC components and distribute spikes in time is introduced, allowing the time response of the NAS to be improved, allowing its use in more time restrictive applications.

Abstract

In neuromorphic engineering, computation is commonly performed asynchronously, mimicking the way in which nervous systems process information: spike by spike. The Neuromorphic Auditory Sensor (NAS) has been implemented under this principle: applying different spike-based Signal Processing blocks. Computation in the spike domain requires the conversion of signals from analog or digital representation to the spike domain, which could present a speed constraint in many cases. This paper presents a spike-based system to convert audio information from low-power pulse density modulation (PDM) MicroElectroMechanical Systems (MEMS) microphones into rate coded spike frequencies. These spikes represent the input signal of the NAS, avoiding the analog or digital to spike conversion, and therefore improving the time response of the NAS. This conversion has been done in VHDL as an interface for PDM microphones, converting their pulses into temporal distributed spikes following a pulse frequency modulation (PFM) scheme with an accurate Inter-Spike-Interval, known as "PDM to spikes interface" (PSI). This was tested in two scenarios, first as a stand-alone circuit for its characterization, and then integrated with a NAS for verification. The PSI achieves a Total Harmonic Distortion (THD) of -39.51dB and a Signal-to-Noise Ratio (SNR) of 59.12dB, demands less than 1\% of the resources of a Spartan-6 FPGA and has a power consumption below 5mW.

Figures (9)

• Figure 1: NAS Architecture: Audio to spikes, spikes processing banks, and AER output interface.
• Figure 2: PDM to spikes interface circuit.
• Figure 3: Filtered spikes evolving from an increasing PDM audio signal.
• Figure 4: Spike Band-Pass Filter (SBPF) internal blocks.
• Figure 5: Test scenario. Sound is played by a response speaker, exciting PDM microphones. Finally, the information is sent to a computer through an AER-to-USB interface.