5
Configuration 1
inputs and outputs, which is slightly different to its behaviour under
Windows XP where each input/output pair had its own mixer device. How
this will work with any particular application that uses the mixer API can
only be determined by experimentation.
Audio Engine
The audio engine in Windows Vista / 7 / 8 runs at a fixed sampling rate
and bit depth. The Digitorc 4 driver sets this to be the same as the card’s
hardware sampling rate and with 24-bit resolution on all inputs and
outputs. The Windows sampling rate converter is used whenever a different
sampling rate or bit depth is requested by applications.
The Digitorc 4 supports the event-driven pull mode on Windows 7 and 8,
providing lower latency and more robust performance.
DirectSound and WDM Kernel
Streaming
Microsoft’s DirectSound and Windows Driver Model (WDM) are designed
to work hand in hand to take full advantage of audio card hardware and
provide kernel-mode software emulation of features not directly supported.
Kernel streaming allows audio data to be passed directly between driver
modules resulting in a robust, high performance integrated sound
processing system.
With its WDM driver, the Digitorc 4 can play MIDI files and audio CD’s,
in addition to Wave files. The kernel mixer allows multiple sounds from
the same or different applications to be played simultaneously, while the
kernel splitter automatically splits an input source to multiple applications.
Sampling rate conversion is automatically inserted if any of the multiple
playback or recording streams have different sampling rates.
Extended Bit Depth Audio
Traditionally, PC-based audio capture and playback has been restricted to
either 8 bit or 16 bit encoding, with the WAVE_FORMAT_PCM descriptor
used to define the audio parameters such as samples per second, bits per
sample and number of channels.
With the advent of 24 bit A/D and D/A converters, there was a need to
support extended bit depth. To achieve this, Microsoft introduced a new
data format descriptor called WAVE_FORMAT_EXTENSIBLE. This provides
additional information, including the number of valid bits per sample and
support for specifying channel placement in multichannel streams.
The WAVE_FORMAT_EXTENSIBLE descriptor may be used in place of
the normal WAVE_FORMAT_PCM in setting up playback and recording
through either the Winmm (wave) API or the DirectSound API. For further
information refer to the Microsoft DirectX 8 documentation.
Some audio editing applications, such as Syntrillium’s Cooledit 2000 and
Adobe Audition, support extended bit depth audio and work well with the
Digitorc 4 using 24 or 32 bit recording.
