Virtual Surround Sound Support (Windows CE 5.0)
While the number of receivers that support WMA Pro is steadily increasing, the large existing base of receivers that have surround support through Dolby Pro Logic® means that Multi-channel Audio via S/PDIF alone would not be enough to enable wide multi-channel support.
Virtual surround sound or LtRt is a special encoding system where multi-channel audio is folded down to two channels. These two channels are sent to a receiver that has surround decoders (such as Dolby Pro Logic) that are capable of decoding the encoded signal to multi-channel speakers.
If a surround decoder is not available, the content is still rendered normally, without any surround effects present.
The Windows CE 5.0 Networked Media Device Feature Pack adds virtual surround sound support to Windows CE through the use of a separate library, the Virtual Surround Component (wmachmtx.lib). This library is wrapped into the existing WMA decoder DMO, and so is transparent to DirectShow users.
Performance
Virtual surround is optimized for x86 processors (floating point with SSE optimizations). For reference, it consumes approximately 5% of the processing power of a 600MHz Intel Celeron M processor on the Intel 854 platform.
Given that the code is floating point, it would perform very poorly on a fixed point processor. Enabling Virtual Surround on a platform with a fixed point processor is not recommended.
Virtual surround sound is disabled by default on all platforms other than the Intel 854. Other platforms revert to the traditional fold down mechanism without using virtual surround sound. If the CPU on your platform is sufficiently powerful you can activate virtual surround sound processing by changing settings in the registry.
Virtual Surround supports the following content and bit rates, with LtRt On and Off with 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 with 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 2 or fewer.
See Also
Networked Media Device Architecture | Multi-channel Audio via S/PDIF
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