When Worlds collide: DTV and the Olympics
By Paul Boyden
The Winter Olympics have come and gone. But KSL-TV, the NBC affiliate in Salt Lake City, was transformed when faced with the monumental task of managing their digital spectrum during the Olympics. The task involved broadcasting NBC-provided high-definition coverage, transmitting a standard-definition signal, continuing service to their traditional analog viewers, and leasing extra bandwidth to WOW Digital TV.
Just before the Olympics, NBC announced that it would team with HDNet to provide eight hours each day of original HD coverage of competition and the opening and closing ceremonies. This meant that KSL would have the NBC/HDNet high-definition feed and their simulcast standard-definition feed multiplexed on their DTV channel.
KSL did not have a reliable way to quantify the differences in the encoding processes, so for comparison they left the original system in place in constant bit rate (CBR) mode, running SD at 3.5Mb/s and HD at 12 Mb/s.
By contrasting the two systems, they immediately realized that the MV50 SD encoders from Harmonic instantly responded to non-demanding content and dropped the data rate of the encoded output, even to its minimum rate of around 0.8Mb/s, with no discernible loss of quality. The HD was a little more challenging. They found the least forgiving monitor to be a native 720p (all of their HD is 1080i), 52-inch plasma display and used this as the most sensitive way to search for artifacts. In the de-interlacing process, this monitor appeared to enhance any MPEG artifacts. With no datacasting, they would have been running HD at 15Mb/s, which is about 100:1 compression. With data, the CBR encoded stream dropped down to 12Mb/s, revealing tiling artifacts in dissolves and detailed motion shots.
In the variable bit rate (VBR) configuration, the KSL team found the encoded HD video would frequently hover below 10Mb/s, lending a lot of space for quality SD video. They expected this, but were surprised at how much quality improvement the HD could recover by negotiating with the SD service — a true give-and-take.
There were other factors at work that enabled bandwidth efficiency. KSL used a large, 30-frame GOP on SD. The only disadvantage to a large GOP in transmission is the time required for a channel change. The SD encoder offers edge-processing options that allocate less bandwidth to the edges of the picture that would normally be hidden in overscan. The HD encoder, not as mature a product as its SD counterpart, still reduced bandwidth consumption using horizontal filtering. Incidentally, the configuration process proved that these other measures were important in achieving the highest perceptible video quality.
The end result of the test was that KSL met its objectives and demonstrated the viability, feasibility and the increased revenue potential associated with digital broadcasting.
At the time this article was written, Paul Boyden was a DTV engineer at KSL. He has since joined WOW Digital TV as broadcast operations manager.