|The AS3421/2 speaker driver from ams features an ambient noise cancelling function for headsets, headphones or ear pieces. It is intended to improve quality of music listening, phone conversations, or other audio applications by reducing background ambient noise. The fully analog implementation allows the lowest power consumption, lowest system BOM cost, and most natural received voice enhancement available with DSP implementations. The device is designed to be easily applied to existing architectures.
An internal OTP-ROM can be used to store the microphone gain calibration settings, as well as all application specific settings. The AS3421/2 can be used in different configurations for the best trade-off of noise cancellation, required filtering functions, and mechanical designs. The simple, feed-forward topology is used to effectively reduce frequencies typically up to 2-3 kHz. The strength of this feed-back topology with either 1 or 2 filtering stages is especially evident at very low frequencies. The typical bandwidth for a feedback system is from 20 Hz up to 1 kHz, which is lower than with feed-forward systems.
The filter loop is determined, by measurements, for each specific headset individually, and depends very much on mechanical designs. The gain and phase compensation filter network is implemented with low cost resistors and capacitors for reduced system costs. The AS3421/2 also features an audio playback-only mode which allows the user to easily switch between ANC on and off modes. In ANC off mode, unused blocks are automatically switched off to guarantee the lowest power consumption in each operating mode.