WDM provides considerable leverage to audio applications. It provides an audio mixing and resampling component that runs in kernel mode, known as "KMixer." KMixer facilities multiclient access to the same hardware and provides the illusion of limitless audio streams that are mixed in realtime. Due to its layered architecture, WDM also provides automatic support for the MME and DirectX APIs. A vendor simply needs to implement a WDM mini-port driver, and other layers in the system’s driver stack provide MME and DirectX support. Unfortunately, this power comes at a price. Due to internal buffering KMixer nominally adds 30 msec of latency to audio playback streams. (At present, Microsoft does not provide a method to allow host applications to bypass KMixer.)

WDM Adoption WDM is relatively new to most audio hardware vendors. It is much more like the NT driver model than the Win9x VxD model. This means that the many hardware vendors who haven’t yet developed NT drivers will have to learn new technology in order to support WDM. (Note that difference between the NT driver model and VxD driver model is the primary reason for the current dearth of NT drivers, including NT ASIO drivers. Building a driver for a new platform requires learning the DDK for the platform, regardless of which API the driver is meant to support or how crisply defined the API happens to be.) We see the newness of WDM as an opportunity for the Windows audio community at large. We are at a point in time where many hardware and software vendors are simultaneously taking their first serious look at WDM. At times when everybody is "on the same page" it is easiest focus on working together towards building the best solutions for everybody. Another opportunity presented by WDM is its synergy with Win2k. We have already suggested that Win2k is a more desirable operating system for audio because of its well-bounded interrupt latency. Win2k also has the benefit of support for advanced file management such as asynchronous disk I/O. Since Win2k has great potential for professional audio, and WDM is the driver model for Win2k, a large trends towards the adoption of WDM seems inevitable.

Is WDM the Answer? The following table places desirable traits of an idealized audio streaming solution alongside the capabilities of WDM.

Design Goal	Supported by WDM?
Requires only a single, well-defined, kernel mode component?	Yes. One kernel mode component provides MME and DirectX support automatically.
Works equally well on Win9x and Win2k?	Yes. WDM drivers are source (and often binary) compatible across these 2 operating systems.
Easy to implement?	Yes. WDM drivers are designed in a simple mini-port model, where the vendor needs only to provide details that are specific to their hardware. This model removes much of the excess "glue code" in building a driver.
Free of political or legal baggage?	Yes. WDM has no baggage. It is a standard technology built into Windows.
Provides < 5 msec latency for all hardware and software?	No. KMixer adds 30 msec of latency.

Resolving the Limitations of WDM

Fortunately, the WDM has a provision for driver extensions. The WDM DDK provides a function named IoRegisterDeviceInterface through which any kernel mode driver can "advertise" that it has custom behavior. A user-mode application can then query the system for registered drivers, and communicate directly with them via the user-mode DeviceIoControl function by sending I/O controls (IOCTLs).

This mechanism suggests a way to work around the KMixer issue in WDM. If all hardware vendors can agree on a common set of IOCTLs, and expose these IOCTLs as standard WDM registered device interfaces, then we have an ideal solution that all software vendors can use.

A further benefit of IOCTLs is that drivers use them as the underpinnings of their support for ASIO or EASI user-mode APIs. So a kernel-mode solution based on IOCTLs will be compatible with existing applications that support ASIO or EASI. In fact, any hardware vendor who is considering ASIO or EASI support under Win2k is going to have to build a mechanism like this anyway.

To summarize, an IOCTL solution provides low-latency audio across the board for all Windows audio applications. A solution based on WDM plus IOCTLs has all of the desirable properties mentioned elsewhere in the paper:

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