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Microsoft's USBAudio class system driver (usbaudio.sys) does not currently support USB Audio Type III formats with padded AC3. (See the Universal Serial Bus Device Class Definition for Audio Data Formats for more information.)
USBAudio can accept packed, "raw" AC3 (as opposed to the padded, AC3-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. Specifically, the non-padded AC3 data range exposed by USBAudio is KSDATAFORMAT_SUBTYPE_AC3_AUDIO, which is the same GUID as MEDIASUBTYPE_DOLBY_AC3.
USBAudio currently does not support DirectSound playback of non-PCM audio data.