Streaming Learning Center

The most important point to remember while shooting is that any motion degrades video quality. The second point is that the amount of visible degradation relates to the date rate of the compressed clip, and the selected codec.

The motion/data rate connection

The first point is illustrated in Figure 3, where on the left the image is encoded at 100kbps, and looks awful, while on the right, it's encoded at 500kbps, and looks great. Basically, irrespective of the codec that you use, if the video has too much motion at a low enough data rate, the video gets ugly.

Figure 3. The same sequence at 100 kbps and 500 kbps (click the image to view the full frame).

Conversely, throw enough data rate at the problem and motion-related degradation minimizes and then largely disappears. At higher data rates, this means that you really don’t have to do anything special to produce high quality video.

How high do you have to go to avoid visible artifacts?Well, for their 1080i Miami Vice trailer, Universal Studios encoded at 9.89 mbps, which is higher than a Hollywood DVD. At that data rate, though there is some graininess during dark sequence, and banding around lights, most motion is very well preserved and artifacts are limited.

Figure 4. Why do Hollywood movie trailers look so good? It's the data rate, of course!

Obviously, however, most of us can’t stream video over our intranets or out our Internet sites at close to 10 mbps, so what rules should we follow? While general, these are shown in Table 1.

Table 1.Green means that excessive motion

At 56 kbps, even at a resolution of 160x120, you need to carefully manage the motion in the video, irrespective of the video resolution. Trying to produce at higher resolutions would produce very ugly video.

At 100 kbps, video quality at 160x120 should be very good, even with lots of motion, though the video will be very small, even if doubled during playback. If you limit the motion in your video, and plan your background carefully, you can render at 320x240 resolution and produce very good quality video, as shown in Figure 5 on the right. Too much motion results in the blurriness and artifacts seen on the left.

Figure 5.  Too much background motion degrades the quality on the left, while limited motion and plain background looks good on the right. Both videos at 320x240@100 kbps.

At 1 mbps, assuming a 640x480 video resolution and 30 fps, you should be able to produce very good quality irrespective of the amount of motion in the video. Ditto for 320x240 video at 30 fps at 500 kbps.

At 300 bkps, even relatively high motion video should look pretty good at 320x240, while lower resolutions show the caution orange because you’re being unnecessarily conservative. At 500 kbps, 320x240 video looks absolutely pristine, but you’re still pushing the envelop a bit for 640x480 video, which really starts to look quite good at 750 kbps and above.