Streaming Media - August/September 2007 - (Page 76)

webcasting with windows media The other time you want to crop is with a letterboxed or windowboxed source. Those big black rectangles use some bits and, more importantly, a lot of CPU time as the codec looks for something in them to encode. Better to crop them out and let all the horsepower apply to the parts of the content that matters. Codecs VIDEO CODEC The mainstream live encoding codec in Windows Media today is Windows Media Video 9, which is widely compatible with existing, installed players. While older codecs are available, they don’t offer the compression efficiency and performance tuning of WMV 9. If interlaced streaming is required, WMV 9 Advanced Profile should be used. Note, however, that decent performance with interlaced encoding required a highend system like one with a Tarari board or an Inlet Spinnaker unit. AUDIO CODEC pipe in the link between the encoder and the final viewer, be it between encoder and server, or server and user. And this should be determined by predicting the worstcase scenario—video and audio will be lost with even a short dip in available bandwidth. Since webcasting is a real-time broadcast, there isn’t any provisioning. Windows Media also includes Intelligent Streaming technology, which allows a single encode to include video and audio at different data rates [see sidebar, “Intelligent Streaming”]. FRAME RATE The mainstream audio codec in Windows Media today is Windows Media Audio 9.2. Backwards compatible to players from the ’90s, it offers better quality at lower bit rates than MP3. For very low bit rate voice-centric audio, the WMA 9 Voice codec can provide better quality than WMA below 32Kbps. And despite its name, in “mixed” mode WMA 9 Frame rate determines the smoothness of the motion of the video. In webcasting, normally either the rate of the source or half that rate is used (so 29.97 for NTSC, or 14.98 for lower bit rates from NTSC). Cutting the frame rate in half reduces CPU requirements by nearly half and reduces required bit rate by a fair amount as well. But motion is much smoother at the source frame rate, which should be used when there are enough bits available. I prefer to get my frame rate up to 29.97 before going to more than 320x240 resolution. COMPLEXITY Complexity is often a forgotten control in encoding, but it’s a critical one. Complexity controls the tradeoff between speed and quality of the encode—each value up is roughly two times slower, but can provide better The Tarari Encoder Accelerator PCI-X card currently offers the fastest high-resolution and interlaced standard-definition encoding speeds with the latest Windows Media codecs. For the overall fastest live encoding speeds, you’ll want to look at Inlet’s Spinnaker, which is designed for high-availability broadcast applications. Voice can provide decent reproduction of music cues (although you’d certainly never dance to it). When efficiency is critical, the new WMA 10 Professional codec’s modes below 128Kbps can provide exceptional efficiency—at 32Kbps it matches WMA at 64Kbps. However, users need to have Windows Media Player 11 to get full fidelity out of it. You should note that only WMA is supported in Silverlight 1.0. Professional and Voice are in consideration for future versions. DATA RATE quality at a given data rate. By default, Windows Media Encoder uses a complexity of 1, which was a reasonable default when it first shipped. But for simple 320x240 encoding, today’s dual-core laptops are several times more powerful than the biggest encoding box available back then. So a typical machine today should be able to handle Complexity 3 instead of 1 for 320x240 29.97 fps. As frame size and frame rate go up, complexity needs to come down on the same hardware. BUFFER SIZE Data rate measures how many bits per second the video uses. This is mainly determined by the smallest 76 STREAMING MEDIA August/September 2007 The buffer size is the window over which the data rate needs to be averaged. For example, encoding at 200Kbps with a two-second buffer means than any two

Table of Contents Feed for the Digital Edition of Streaming Media - August/September 2007

Table of Contents Sweet Sixteen Letters to the Editor Technology and Business Trends IBM Takes Webcasting to the Desktop ROI in the EDU Bank Serves Espresso, Enterprise Video Online Video Advertising: Hit or Miss MLB’s Foul Bawl A Crash Course in Flash Video Newspapers in Multimedia Metamorphosis Webcasting Large Entertainment Events: Seeing Through the Hype Webcasting With Windows Media Can’t Start A Fire . . . A WiMAX Primer Mobile Video at Last Telestream Episode Pro Grass Valley ProCoder 3 Silverlight Is No Flash Killer

Streaming Media - August/September 2007

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