Analog Audio Subsystem with Integrated Noise Reduction
National Semiconductor Corporation has released the industry's first analog audio subsystem with integrated noise reduction for use in smartphones and full-featured phones. The PowerWise® LM49155 dramatically improves the talker's voice-signal-to-background-noise ratio while retaining natural speech quality. Phone users can now enjoy clearer voice calls in noisy environments on both transmit and receive ends of the communications link. The LM49155 also enables increased accuracy of voice recognition systems by minimizing background noise. National's LM49155 analog audio subsystem delivers noise reduction at the lowest power consumption, consuming only 3.7 mA of quiescent current for the earpiece amplifier and microphone amplifier signal path. The subsystem's ease of use enables the rapid integration of noise suppression performance in phone handsets, without the added development time required to write and test voice-processing programming code for a DSP or microprocessor. The LM49155 provides uplink noise suppression and downlink signal-to-noise ratio (SNR) enhancement in an ultra-small 36-bump micro SMD package measuring 3.4 mm by 3.4 mm. The subsystem includes a high-efficiency, 1.35W Class D speaker driver with automatic level control (ALC), ground-referenced headphone amplifier with click-pop suppression, and mixing and volume control. The LM49155's uplink noise suppression technology rejects far-field noise through a unique two-microphone amplifier implementation. Downlink voice intelligibility is improved by enhancing the SNR between the handset's earpiece signal level and the ambient noise level. The LM49155 preserves uplink near-field voice signals within close range, while rejecting acoustic noise more than 0.5 meters away from the microphones. In addition, the LM49155 provides continuous downlink signal adjustments so the caller's voice is always clearly heard regardless of the listener's environment. National's latest noise suppression technology is also available in a discrete noise-processing IC, the LMV1099. It delivers 34 dB uplink far-field noise suppression at 1 kHz, near-field SNR enhancement of 6 dB to 18 dB, and downlink SNR improvement of 16 dB. The LMV1099 provides uplink and downlink total harmonic distortion plus noise (THD+N) of 0.1 percent, shutdown current at 0.07 uA, and supply current of 3.7 mA. The IC operates from a supply voltage range of 2.7V to 5.5V, and is supplied in a 25-bump micro SMD package.