Webcasting With Windows Media
Webcasting enables the live TV experience via streaming—people get to watch the event as it’s happening. There’s a lot of content that makes sense for this technology, such as news and sports. Webcasting is also popular for presentations where there’s an opportunity for people watching remotely to ask the presenter questions via email or IM.Webcasting is really only appropriate for content that needs to be broadcast live. Even when a simultaneous multicast event is desired, if the source is available ahead of time, better quality and a lower antacid budget can always be obtained by encoding the file in advance and then broadcasting it in real time.
Why Windows Media for Webcasting
Reach—Windows Media is the most broadly distributed technology for watching webcasts. It’s preinstalled on most of the world’s computers and available for other systems, like Mac OS X and a variety of mobile devices. And the forthcoming Silverlight will increase the reach of Windows Media playback and open up new opportunities for richer integration of other media with the video and audio.
Performance—Windows Media has the fastest encoders in the business. We have lots of optimization and can split encoding over four processor cores at the same time. And when even that isn’t enough, the Tarari card is available to offer much faster acceleration, supporting even HD encoding in very high quality.
Efficiency—The flip side of good performance is efficient encoding. Since webcasting has to be in real time, a faster codec means it can get more done on each frame of video, enabling it to deliver better quality in fewer bits.
Our Windows Media Video 9 implementation of the VC-1 codec is also extremely efficient for low bit rate encoding. On the audio side, Windows Media Audio 9 is the most efficient of the commonly used webcasting codecs, and the new low bit rate modes of Windows Media Audio Professional lead the industry in quality delivered below 128Kbps.
To get the best results when webcasting with Windows Media, or any other format for that matter, you need to pick the optimal settings. Here’s a look at those—some Windows Media-specific, some applicable to any format.
Video processing is everything that happens between the source video frames and the codec itself.
Scaling—Video sources are typically 720x480 or 720x576, but most web streaming is at lower frame sizes—you need north of 2Mbps to get value out of more than 640x480. So the video has to be scaled down. Scaling does use some CPU power, but you can get some more juice for compression by using a capture card with built-in hardware scaling.
Another important aspect of scaling is to get the aspect ratio right. Scaling isn’t just a matter of reducing frame size by a fixed value. For instance, 720x480 never gets scaled to 360x240. Instead, the output resolution should match the aspect ratio of the source. With a 4:3 source, 320x240 is a good choice. With 16:9, 432x240 will leave circles looking like circles.
Deinterlacing—Traditional analog video is interlaced, where the even and odd lines are captured half a frame’s duration apart. In interlaced video, thin horizontal lines are seen wherever there’s motion. These look bad in their own right, and they’re hard to encode, leaving a lot of artifacts.
Ideally, the webcasting source will simply be shot progressive (supported by all modern cameras), but when that’s not possible, the video will have to be deinterlaced. Again, this uses some CPU power, and also degrades detail when outputting more than half as many lines as in the source (like more than 320x240 with interlaced 720x480 source).