Full-Duplex H.323/H.324 Video and Audio Conferencing

WDM audio supports the following features for full-duplex video and audio conferencing under Windows 98 and Windows NT 5.0:

Native 32-bit DirectSound support for simultaneous audio input and output, not dependent on 16-bit MMSYS components
Input and output position reporting mechanisms for synchronization of speaker and microphone streams, accurate to 1 ms or better
WDM Stream class driver that provides access to acoustic echo cancellation (AEC) reference interfaces supported by hardware codecs
WDM system audio filter graph that supports insertion of a non-Microsoft kernel-mode AEC filter module adjacent to the WDM Stream class driver in both the input and output streams

Baseline PC 98 audio hardware support for H.323/H.324 video and audio conferencing requires full-duplex audio capability. For a list of the PC 98 requirements, see the “Basic Audio Requirements” section later in this chapter.

The system designer might choose to include optional hardware to provide the following capabilities:

Full-duplex operation at independent sample rates for simultaneous 8-kHz microphone input and 44.1-kHz speaker output
Hardware AEC references for echo cancellation filtering

The following table presents actual measurements collected by IAL for H.323/H.324 audio on Intel Pentium II processors with MMX technology.

Performance Measurements for H.323/H.324 Audio

Function	233-MHz Pentium II processor¹
Full-duplex G.723.1 encode and decode, mono at 8 kHz	10%
Full-duplex AEC, mono at 8 kHz	6%
Total H.323/H.324 audio	16%
¹Actual performance is dependent on specific processor, chip-set, memory, and I/O subsystem implementations, as well as the software implementation (integer, FP, or MMX technology). To estimate the performance on other processor speeds with Intel MMX technology, use a ratio based on the processor’s MHz rating.