Wireless Display, In Depth
If you read some of the preliminary coverage on Wireless Display, then you undoubtedly saw gripes about lots of latency between the host notebook and client output. This is absolutely the case. According to Intel, the technology was never meant to be interactive (allowing you to play a game on the laptop through the TV, for example). Instead, it enables a “kick back and watch” usage model, compelling you to fire up a television show on Hulu or a PowerPoint presentation and take your hands off of the laptop. Understanding why this is the case requires a little background on what’s happening behind the scenes.
At the heart of this technology you’ll find Intel’s My WiFi and Wireless Display software, running on the notebook. Wireless Display grabs the desktop frame buffer and encodes it at 30 frames per second. In the first software release, the technology supported resolutions of up to 1280x800. But the latest version adds 1366x768 to the list—the native resolution of our Dell Studio test subject. To the video, Intel’s encoder adds two-channel audio, multiplexing it into an MPEG-2 stream transmitted over WiFi to the Netgear adapter using Real Time Streaming Protocol (RTSP). The adapter decodes the stream and renders it out to the HDMI output.
Now, on the television side, extended display identification data (EDID) establishes the compatible display resolution, audio capabilities, and so on of a display device. Netgear’s Push2TV adapter reads that data structure and scales the output accordingly.
The stream itself is variable bitrate. When you’re looking at the desktop, it’s around 6 Mb/s. If you’re playing back video with lots of motion, it scales up to about 9 Mb/s. Audio adds another 1 Mb/s or so. Intel keeps the stream at this level on purpose. You see, the connection between the notebook and adapter employs 802.11g, with theoretical bandwidth of 54 Mb/s. But as with all theoretical throughput rates, the real world is a lot different. Practically, the connection is good for somewhere between 22 and 24 Mb/s. Keeping Wireless Display below 50% of that pipe leaves room for other devices on the Personal Area Network (PAN). Or, the difference can be used to cope with interference.
In addition to enabling the PAN utilizes by Wireless Display, the same Centrino networking module also connects to the Internet over a standard WLAN. Intel’s My WiFi software is the key to making this dual-network scheme possible. It allows the hardware adapter to be timeshared between serving as an access point to the PAN and a client on the WLAN simultaneously. Complex though it might sound, the Wireless Display software component controls the My WiFi SDK, hiding the networking aspect of the PAN. You enter that four-digit security code during initial setup, and that’s it. The network is enabled and disabled based on the presence of the Netgear adapter transparently.
Update: A while back, Nvidia asked me about Intel's plans with regard to the relationship between Wireless Display and its own Optimus technology. Nvidia already claims the two technologies work well together, so I can only assume it's afraid Intel will block the use of Wireless Display on Optimus-enabled notebooks. I asked Intel to weigh in, but was answered back with "we can't comment on other companies' products." In other words, Wireless Display may or may not work with Optimus-enabled platforms moving forward.