Asynchronous Delta Modulation Research Articles

Asynchronous Event-driven Encoder With Simultaneous Temporal Envelope and Phase Extraction for Cochlear Implants.

Conventional cochlear implants using periodic sampling are power consuming and incapable of capturing the amplitude and phase of the input acoustic signal simultaneously. This paper presents an asynchronous event-driven encoder chip for cochlear implants capable of extracting the temporal fine structure. The chip architecture is based on asynchronous delta modulation (ADM) where the signal peak/trough crossing events are captured and digitized intrinsically, which has the advantages of significantly reduced power consumption, reduced circuit area, and the elimination of dedicated data compression circuitry. An 8-channel prototype chip was fabricated in 0.18 μm 1P6M CMOS process, occupying an area of 0.125 × 1.7 mm2 and has a power consumption of 36.2 μW from a 0.6V supply. A 16-channel stimulation encoding system was built by integrating two test chips, capable of processing the entire audible frequency range from 100 Hz to 10 kHz. Experimental characterization using the human voice is provided to corroborate functionality in the application environment.

Analysis of Encoding Degradation in Spiking Sensors Due to Spike Delay Variation

Spiking sensors such as the silicon retina and cochlea encode analog signals into massively parallel asynchronous spike train output where the information is contained in the precise spike timing. The variation of the spike timing that arises from spike transmission degrades signal encoding quality. Using the signal-to-distortion ratio (SDR) metric with nonlinear spike train decoding based on frame theory, two particular sources of delay variation including comparison delay $T_{\mathbf {DC}}$ and queueing delay $T_{\mathbf {DQ}}$ are evaluated on two encoding mechanisms which have been used for implementations of silicon array spiking sensors: asynchronous delta modulation and self-timed reset. As specific examples, $T_{\mathbf {DC}}$ is obtained from a 2T current-mode comparator, and $T_{\mathbf {DQ}}$ is obtained from an M/D/1 queue for 1-D sensors like the silicon cochlea and an $\text {M}^{\mathrm {\mathbf {X}}}$ /D/1 queue for 2-D sensors like the silicon retina. Quantitative relations between the SDR and the circuit and system parameters of spiking sensors are established. The analysis method presented in this work will be useful for future specifications-guided designs of spiking sensors.

A 0.5 V 55 $\mu \text{W}$ 64 $\times $ 2 Channel Binaural Silicon Cochlea for Event-Driven Stereo-Audio Sensing

This paper presents a $64 \times 2$ channel stereo-audio sensing front end with parallel asynchronous event output inspired by the biological cochlea. Each binaural channel performs feature extraction by analog bandpass filtering, and the filtered signal is encoded into events via asynchronous delta modulation (ADM). The channel central frequencies $f_{0}$ are geometrically scaled across the human hearing range. Two design techniques are highlighted to achieve the high system power efficiency: source-follower-based bandpass filters (BPFs) and asynchronous delta modulation (ADM) with adaptive self-oscillating comparison. The chip was fabricated in 0.18 $\mu \text{m}$ 1P6M CMOS, and occupies an area of $10.5 \times 4.8$ mm2. The core cochlea system operating under a 0.5 V power supply consumes 55 $\mu \text{W}$ at an output rate of 100k event/s. The measured range of $f_{0}$ is from 8 Hz to 20 kHz, and the BPF quality factor ${Q}$ can be tuned from 1 to almost 40. The 1 $\sigma $ mismatch of $f_{0}$ and ${Q}$ between two ears is 3.3% and 15%, respectively, across all channels at ${Q}\approx $ 10. Reconstruction of speech input from the event output of the chip is performed to validate the information integrity in event-domain representation, and vowel discrimination is demonstrated as a simple application using histograms of the output events. This type of silicon cochlea front end targets integration with embedded event-driven processors for low-power smart audio sensing with classification capabilities, such as voice activity detection and speaker identification.

A Dynamic Vision Sensor With 1% Temporal Contrast Sensitivity and In-Pixel Asynchronous Delta Modulator for Event Encoding

A dynamic vision sensor (DVS) encodes temporal contrast (TC) of light intensity into address-events that are asynchronously transmitted for subsequent processing. This paper describes a DVS with improved TC sensitivity and event encoding. To enhance the TC sensitivity, each pixel employs a common-gate photoreceptor for low output noise and a capacitively-coupled programmable gain amplifier for continuous-time signal amplification without sacrificing the intra-scene dynamic range. A proposed in-pixel asynchronous delta modulator (ADM) better preserves signal integrity in event encoding compared with self-timed reset (STR) used in previous DVSs. A 60 $\times$ 30 prototype sensor array with a 31.2 $~\mu\hbox{m}$ pixel pitch was fabricated in a 1P6M 0.18 $~\mu\hbox{m}$ CMOS technology. It consumes 720 $~\mu\hbox{W}$ at a 100k event/s output rate. Measurements show that a 1% TC sensitivity with a 35% relative standard deviation is achieved and that the in-pixel ADM is up to 3.5 times less susceptible to signal loss than STR in terms of event number. These improvements can facilitate the application of DVSs in areas like optical neuroimaging which is demonstrated in a simulated experiment.

Description of a Differential Pulse-Frequency Modulator

The operation of a novel differential pulse-frequency modulator based on the Asynchronous Delta Modulation principle is described in this paper. The complete modulator consists of a Voltage Controlled Oscillator (VCO) in a feedback configuration and generates an output pulse train whose frequency is proportional to the slope of the input signal. It is shown that the proposed system allows substantially larger input signal amplitudes for linear operation than those permitted by open loop VCO schemes or other voltage-to-frequency converters. In addition, the feedback path provides “locally” a signal that tracks the input, so that the feedback arrangement can be readily used as a demodulator at the receiver end.

General aspects of an asynchronous delta modulated inverter with RL loads

The application of an asynchronous delta modulation principle to the operation of pulse width modulated inverters with resistive induction loads is presented. The main inverter with its load is within the loop of an asynchronous delta modulator. With this arrangement, the load current can follow the desired input waveform with a predetermined accuracy. General relations are given with associated data curves applicable to fundamental performance characteristics useful for AC induction motor loads. An experimental system was built and the measurements taken are in good agreement with theory.