While other audio implementations have limited support for simple array microphones, Intel HD Audio supports larger array microphones. By increasing the size of the array microphone, users get incredibly clean input through better noise cancellation and beam forming. This produces a higher-quality input to voice recognition, VoIP, and other voice-driven activities.

An array microphone provides a speakerphone-like "open audio" usage solution for softphone VoIP calls. It does not require the user to wear headphones or a headset microphone; rather the user can speak directly into the device and listen to the platform speaker at the same time. The array microphone provides higher-quality audio and an improved user experience.

An array microphone is a set of multiple microphone elements integrated on the mobile platform to provide better microphone input quality and features, such as Acoustic Echo Cancellation (AEC) and repetitive noise attenuation and filtering or Noise Canceling .

Figure 9 depicts Intel Array Microphone architecture with four microphone elements.

Figure 9: Intel Array Microphone architecture
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Figure 9 depicts four key processes associated with Array Microphone processing. Speaker Tracking allows tracking of speaker source and direction to be able to replicate directionality at the rear endpoint if needed. Beam Forming implements speaker localization and sound attenuation. AEC ensures isolation between the speaker and the microphone for elimination of annoying echoes during the conversation. Finally, Repetitive Noise Attenuation and Filtering ensures effective noise cancellation for clearer audio speech.

For more information about Intel High Definition Audio design and implementation please refer to http://www.intel.com/design/chipsets/hdaudio.htm.

Bluetooth* Headset for Hands-Free VoIP

Bluetooth headsets are commonly used as an audio device with softphones. Bluetooth headsets enable users to have a hands-free VoIP conversation and to manage their connection, for example, accept/terminate a call, adjust volume, etc. A few softphone vendors have chosen to integrate Bluetooth headsets to enable ease of use with wireless headsets. The softphone and Bluetooth stack integration architecture on Microsoft Windows* operating system is depicted in Figure 10.

The Bluetooth stack is composed of the API interface; middleware protocols, such as the Service Discovery Protocol (SDP); Logical Link Control and Adaptation Protocol (L2CAP); RFComm (for serial communication); and Bluetooth profiles such as the headset profile, which is used for wireless headsets.

The softphones need to be integrated with the Bluetooth API to achieve service discovery and to interact with Bluetooth headset devices, i.e., be able to send and receive commands to and from Bluetooth headsets. Intel has worked closely with the Bluetooth stack and softphone vendors to define a common profile for VoIP control commands for headsets/handsets. Going forward, Intel will work with the Bluetooth stack vendors to define a common API for both headsets and handsets to enable softphones to interact with Bluetooth stacks from different vendors in the same way. This will significantly simplify the softphone integration efforts to support Bluetooth headsets for VoIP.

Figure 10: Softphone and Bluetooth stack integration
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The coexistence of Bluetooth and WLAN radios on a notebook could impact the VoIP quality due to the interference between the two radios. Intel Centrino 2006 platforms implement the Wireless Co-existence Solution (WCS) to solve the problem of Bluetooth-WLAN coexistence by sending WLAN channels to Bluetooth. The WCS describes the interface between two radios running simultaneously in the same laptop to transfer information on the channel occupied by WLAN to a Bluetooth NIC. As a result, Bluetooth will skip the WLAN channel for non-critical events, and WLAN performance will be recovered in such cases. This solution significantly increases the throughput of WLAN when Bluetooth is presented. Additionally, the WCS specification provides better protection of critical Bluetooth communications (voice for example). This is accomplished by the following method:

• When Bluetooth expects to receive or transmit a high-priority packet on a Bluetooth channel that is located inside the WLAN channel, Bluetooth raises a BT_Priority signal.
• WLAN radio will then defer or kill its transmission if a BT_Priority signal is raised to avoid collision with Bluetooth packets.

For more information about Intel WCS solutions, please refer to [6].