USB Audio Support for Non-PCM Formats
Microsoft's USBAudio class system driver, Usbaudio.sys, does not currently support USB Audio Type III formats with padded AC-3. For more information, see the Universal Serial Bus Device Class Definition for Audio Data Formats specification at the USB Implementers Forum website.
USBAudio can accept packed, "raw" AC-3 (as opposed to the padded, AC-3-over-S/PDIF format accepted by the PortCls driver). USBAudio supports the internal format of DirectShow's DVD-splitter filter (see DVD Decoder Support in Windows), which can be connected directly to USBAudio under the control of KsProxy (see Kernel Streaming Proxy). Specifically, the nonpadded AC-3 data range exposed by USBAudio is KSDATAFORMAT_SUBTYPE_AC3_AUDIO, which is the same GUID value as MEDIASUBTYPE_DOLBY_AC3.
USBAudio currently does not support DirectSound playback of non-PCM audio data.
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