Speakerphones
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HANDS-FREE SPEAKERPHONES Hands-free performance has become a standard function on most desktop and residential phones as well as car kits. The quality of hands-free communication depends on the technology used for speakerphone function, as well as plastic enclosure design choices. HALF-DUPLEX AND FULL-DUPLEX OPERATION Half-duplex allows only one party to talk, cutting off the other party in the call. Only the party with the highest volume is heard. A half-duplex speakerphone causes the conversation to be broken as the algorithm determines the difference in volume and changes to the active party. Although the performance is poor, half-duplex algorithms tolerate low-quality plastic enclosure designs. Achieving a high quality speakerphone requires a full-duplex algorithm, which allows both parties to talk at the same time and only cancels the returned echo. This system is susceptible to impairment in the echoed audio path. The performance of a full-duplex algorithm depends on the linearity of the echo signal. Non-linearities in the audio path degrade the voice performance. Echo Return Echo return is the amount of reflected audio from the speaker that is picked up by the microphone. The reflected signal is composed of the direct acoustic coupling (the amount of audio that the microphone picks up directly from the speaker) and the audio reflections from the surfaces in the room. The goal of the speakerphone designer is to minimize the echo return. This is done in part through the plastic enclosure design. Even with a well designed plastic enclosure it is impossible to entirely eliminate echo, hence the need for an acoustic echo canceller (AEC). To achieve the highest performance, an AEC tries to minimize this returned signal. Gain in the Echo Path The speaker and microphone gain pads affect the echo path. These gains are necessary but should be set appropriately to achieve high performance echo cancellation. For optimal performance, an AEC should be selected based on its ability to cancel the highest level of acoustic echo return without freezing adaptation (should continuously track changes in the echo path) or using switched attenuation (should not fall back to half-duplex).
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ZARLINK'S HANDS FREE VOICE PROCESSING SOLUTION Zarlink Semiconductor has developed a family of single-chip devices targeting high-performance hands-free communications, including hands free car kits. The company's newest hands-free solution, now commercially available, is a dedicated voice processor that combines integrated dual-channel wideband codecs with multiple interfaces. The device supports advanced echo cancellation, psychoacoustic noise reduction, full-duplex operation and is flexible and field-upgradeable. The chip's patented software algorithms continuously track changes in the echo path, even during double-talk conditions, and reduce background noise while preserving high voice quality. The solution is based on a flexible platform that targets introductory systems with basic features and high-end systems with advanced features. It's a field upgradeable solution that allows the designer to build on platform designs with future system upgrades without re-designing the hardware.
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