In Windows 95, 98 and Me, the DirectSound mixer component and the sound card drivers were both implemented as a kernel-mode VxD driver (Dsound.vxd), allowing direct access to the primary buffer used by the audio hardware and thus, providing the lowest possible latency between the user-mode API and the underlying hardware, but in some cases causing instability and blue screen errors.
Windows 98 introduced WDM Audio and the KMixer, which enabled digital mixing, routing and processing of simultaneous audio streams with a higher quality sample rate conversion as well as kernel streaming. Under WDM, DirectSound sends data to the software-based KMixer. Windows 98 Second Edition improved WDM audio support by adding DirectSound hardware buffering, DirectSound3D hardware abstraction, KMixer sample-rate conversion (SRC) for capture streams, multichannel audio support and introduction of DirectMusic. If the audio hardware supports hardware mixing (also known as hardware buffering or DirectSound hardware acceleration), DirectSound buffers directly to the rendering device. If DirectSound streams use hardware mixing, KMixer and its latency delay are bypassed. On Windows 98 and Windows Me, WDM audio drivers were preferred but compatibility with VxD driver model was preserved.
Although Windows Driver Model (WDM) was available starting with Windows 98, few audio card manufacturers used it. Due to internal buffering, KMixer introduced significant processing latency (30 ms on then-current systems). Windows 98 also includes a WDM streaming class driver (Stream.sys) to address these real time multimedia data stream processing requirements. When the sound card uses a custom driver for use with the system supplied port class driver PortCls.sys or implements a mini-driver for use with the streaming class driver, applications can bypass the KMixer completely and use the kernel streaming interfaces instead to reduce latency.
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