Multifunction Device Limits
The number of audio functions per multifunction device is limited by the following factors:
When the adapter driver calls PcAddAdapterDevice, the function's fourth parameter, MaxObjects, specifies the maximum number of miniport driver objects that the driver can support. The sample adapter drivers in the Microsoft Windows Driver Kit (WDK) set this parameter to the integer constant MAX_MINIPORTS, which is typically defined to be a small value (five or less). You might need to increase this value if you plan to support multiple stereo pairs or other types of audio subdevices.
Windows 2000 allows a maximum of ten device interfaces on each of the following Windows Multimedia APIs: waveIn, waveOut, midiIn, midiOut, mixer, and aux. Windows XP increases the limit to 32 interfaces per API. These limits do not apply to Windows Me/98, which allow arbitrary numbers of device interfaces. For more information, see Windows 2000 Device Interface Limits.