Sounding Off on Digital Simulcasts

While improved digital pictures are one of the more obvious benefits to customers in digital simulcasts, audio is also an important factor in the equation. Dolby Laboratories Broadcast Product Manager Jeff Riedmiller says his company found out five or six years ago that the way viewers measure TV sound was through human speech patterns, as opposed to music or sound affects. Dolby came up with the AC-3 audio coding system, which was adopted by the Society of Cable Telecommunications Engineers for use in digital cable systems in the United States. "Dial norm, which is short for dialogue normalization, is a parameter that is encoded by either the programmer, someone at the headend in digital, or by someone in Hollywood who passes it along to CBS or HBO," Riedmiller says. "Dial norm is the most important parameter in every AC-3 stream because it’s meant to indicate the speech level of the program to the decoder in the set-top box. The set-top box automatically sets the level to normalize speech." Learn the parameters Every digital set-top box sold in North America has a two-channel AC-3 decoder that can be operated in two modes, Line and RF. Each of these modes has a specific application, and a cautious approach is in order when the set-top is both designed and deployed to make sure the intended mode is used by default. Riedmiller points towards some recent work he did with Charter Communications in its Madison or Southern Wisconsin division as one example of getting audio tuned up for digital simulcast. (See the July issue of Communications Technology for more on Charter’s digital simulcast deployment.) "It’s a lot to get your head around," Riedmiller says. "I was there (Madison) for one entire day sitting in a room with PowerPoints on the math, the theory, on how to set up the set-top boxes, and how the parameters affect the listeners’ experiences. A lot of people don’t understand the parameters in the encoders. A big part of what I did in Madison was training on the theory behind this." A level playing field for audio Pat Hourigan, Time Warner Cable’s vice president of engineering and technology in its Raleigh, N.C. division, says audio was a challenge during Time Warner’s first digital simulcast deployment in the Raleigh system last month. While it’s a given that cable operators will have to contend with re-encoding local broadcaster content, Hourigan says programmers that use satellites need to make sure the parameters are well encoded with equal audio levels. Riedmiller agrees that audio can and should improve. "We can make it a lot better than it is," he says. "The errors are so gross, I would say 80 to 85 percent, but we can reel it in to just the occasional complaint. I’ve seen it work first hand; it’s just a matter of getting everyone on the same page."