Virtual Surround Sound Support
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8/27/2008
While the number of receivers that support Microsoft Windows Media Audio Professional (WMA Pro) is steadily increasing, the large existing base of receivers that provide surround support through Dolby Pro Logic means that Multi-channel Audio via S/PDIF alone would not be enough to ensure wide support of multi-channel audio.
Virtual surround sound, or LtRt, is a special encoding system in which multi-channel audio is encoded to two channels. These two channels are sent to a receiver that has surround sound decoders, such as Dolby Pro Logic, that are capable of decoding the surround sound signal back to play on multi-channel speakers.
If a surround sound decoder is not available, the signal is still rendered normally with no surround sound effects present.
Windows Embedded CE supports virtual surround sound by using of a separate library, the Virtual Surround Component (wmachmtx.lib). This library is wrapped into the WMA decoder DMO object, and is transparent to users of Microsoft DirectShow.
Performance
Virtual surround sound is optimized for x86 processors (floating point with SSE optimizations). Virtual surround sound consumes approximately 5 percent of the processing power of a 600 megahertz (MHz) Intel Celeron M processor on the Intel 854 platform.
Because the code is floating point, it would perform very poorly on a fixed point processor. We do not recommend that you enable virtual surround sound on a platform that has a fixed point processor.
Virtual surround sound is disabled by default on all platforms other than the Intel 854 platform. Other platforms revert to the traditional fold down mechanism but do not use virtual surround sound. If the CPU on your platform has sufficiently resources, you can activate virtual surround sound processing by changing settings in the registry of the computer.
Virtual surround sound supports the following content and bit rates, with LtRt On and Off and with Dolby Pro Logic and stereo output:
- WMA1 content (44.1k)
- WMA2 content (44.1k and 16k)
- WMA7 content (44.1k)
- WMA8 content (44.1k)
- WMA9 Std (32k, 44.1k, 48k)
- WMA9 Pro 2 Channel (44.1k/24 bit, 48k/24 bit, 88k/24 bit, 96k/24 bit)
- WMA9 Pro 5.1 Channel (44.1k/16 bit, 44.1k/24 bit, 48k/16bit, 48k/24 bit, 96k/24 bit)
- WMA9 Pro 7.1 Channel (48k/24 bit, 96k/24 bit)
Enabling the Virtual Surround Component
You can enable the virtual surround component by using the registry subkey HKEY_LOCAL_MACHINE\Software\Microsoft\DirectX\DirectShow\WMADecoder\LtRt
The following table shows the named values for this subkey.
| Value : type | Description |
|---|---|
Mode: DWORD |
Set this value to 0 to turn off virtual surround sound processing. Set this value to a value other than 0 to turn on virtual surround sound processing. |
Virtual surround sound processing is disabled if the number of audio channels is two or less than two.
See Also
Reference
Multi-channel Audio via S/PDIF