Streaming Media - December 2007/January 2008 - (Page 69) producer, it’s worth having your attorneys take a look. Still, for the purposes of this discussion, the nomenclature change looks like a branding thing that has no practical implications for most producers. Other encoding controls are relatively standard. You can set the keyframe interval manually, but the Encoder will also insert keyframes manually at all scene changes irrespective of the chosen interval. In CBR mode, the Encoder offers a smoothness control that balances frame rate against frame sharpness. Paradoxically, low values ensure a higher frame rate, which promotes smoothness, while higher values allocate more bits to the individual frames, potentially resulting in dropped frames. In one of the multiple free tutorial videos available from the Silverlight website, Waggoner recommends boosting this value to 90 when producing at relatively high data rates. Note that you can select one- or two-pass encoding with CBR (all VBR is two-pass), but the checkbox is located below the audio bitrate controls. Interestingly, all presets use a peak video bitrate roughly 2X the average bitrate. My rule of thumb has always been a peak rate of 1.5X (and minimum rate of .5X) and I’ve seen these numbers as high as 4X/.4X (Sorenson Squeeze). Again, you should use VBR only when distributing progressive video via a regular web server, but if your viewers complain about streamus interruptus during high motion sequences, you may want to lower the peak rate value. Smoothness and Packet Controls The final video-related compression control relates to Video Complexity and Packet Size (Figure 5). According to the Help file and very useful and detailed tool tips, complexity controls encoding speed, with 0–2 used for live, depending upon your encoding hardware, and 3–5 optimized for offline encoding. Technically, this value controls functions that can extend encoding times, like search patterns for interframe redundancies and the like. To get a feel for the time difference produced by the various values, I ran two encoding test-runs on my HP xw8400 Dual Processor, Quad-Core computer, one at a complexity setting of zero, the other at five. In both tests, I encoded a 1-minute source file with variable low and high motion content to VC-1 Main Profile format at 895Kbps (480x360@30fps) using 2-pass CBR. Variable Bitrate Encoding With variable bitrate video, the only VBR option is VBR peak constrained (Figure 4), and though you can set peak bitrate values, you can’t set minimums. I didn’t see any degradation in the low motion video that I tested, but it’s always a concern when you can’t set a minimum. WWW.STREAMINGMEDIA.COM 69 http://www.streamingmediahosting.com http://www.streamingmediahosting.com http://WWW.STREAMINGMEDIA.COM
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.