Decoding the Truth About Hi-Def Video Production

Finding the Lowest Possible Data Rate
The following is an excerpt from Critical Skills for Streaming Producers, a mixed-media tutorial resource available at www.streamingmedia.com.

As we’ve seen, encoding at too high a data rate not only costs extra bandwidth but also can slow the display rate on lower-power computers and increase the processor load required to play the file on all computers. There’s another factor you should consider as well.

Specifically, have a look at the slide shown in Figure 1. The slide is from a workshop given by Hendrik Knoche, a graduate student at University College London, who has published a host of articles on streaming quality that you can see at www.cs.ucl.ac.uk/staff/H.Knoche/publications.htm.

Briefly, the slide assesses viewer satisfaction relating to news content containing legible text on mobile TV as displayed on a number of devices. The left axis is video quality acceptability while the bottom axis is the encoding bitrate. As you can see, viewer satisfaction increases sharply at all viewing resolutions as the data rate jumps from 0 to 100Kbps, but then the rate slows dramatically, especially for the 320x240 video displayed on the HP iPAQ, where tripling the data rate increases quality acceptability by about 10%. For the 240x180 and 208x156 videos, viewer satisfaction actually decreased with additional data rate. In financial terms, your initial investments in bandwidth enjoy a high rate of return, but once quality is "acceptable," the rate of return drops and can even go negative.

How do you find the tipping point where quality is good enough? In their article "Can Small Be Beautiful," Knoche and his co-authors asked their participants to specify what made the video they were watching unacceptable. With news videos, almost 100 of 128 viewers commented about text detail, which, according to the article, included the news ticker, headlines, clock, logos, and captions. Of relevance to this discussion, facial and general detail drew about 30 comments each, as did audio fidelity.

This tells me two things. First, when planning your shoots, eliminate as much detail as possible, since these stress the codec, display artifacts readily, and are a significant source of complaints. Second, when choosing a data rate, be sure it’s high enough to deliver artifact-free faces as well as text and other details. Once these components are artifact free, you get dramatically decreasing returns from further bandwidth investment.