Dialnorm: A good idea gone bad?
Dec 1, 2007 12:00 PM, By Bruce Jacobs
Fix dialnorm
“Dialnorm doesn't have to be complicated,” says Jim Starzynski, principal audio architect with NBC Universal and chair of the ATSC S6-3 audio working group on DTV loudness.
Currently, the NBC Universal DTV network performs ongoing tests of legacy and commercial content mixed using traditional meters. The LEQ(A) average dialog level ranges from -20dBFS to -24dBFS. By simply setting station AC-3 encoder dialnorm remotely from the network to the average value of 22, reasonably consistent dialog levels in the home are achieved, matching other dialnormed sources. This averaging method is effective when specific content-matched metadata is not available and can work for local station content as well. It is not necessary to remix all the old shows. In the case of local commercials with extremely varied levels, a simple, properly adjusted audio compressor and limiter can be successful in reaching a targeted dialog level. PBS has adopted a similar approach, using a value of -24.
The 2007 edition of the PBS Technical Operating Specifications requires all new content to be delivered with an Leq(A) dialog level of -24dBFS. “This specification is key to our effort to improve audio for the public,” Kutzner said. (The specifications can be found at www.pbs.org/producers/redbook.) NBC Universal uses a similar value of -23dBFS. Currently, there is no need for a complex metadata system for every production, only a new approach to metering the final mix and layback.
Knowledge of this fixed dialnorm approach allows stations to consider simple steps to fix the dialnorm problem. (For more, read “Four simple steps to successful dialnorm.”) As awareness and technology progresses, an agile metadata system that tags matched dialnorm to content can be a goal, fulfilling the original intent of the DTV audio designers.
What about cable?
As the cable industry continues to reduce the number of analog channels and increase the digital ones, properly set dialnorm becomes even more important. In the old analog days, many cable systems ran all audio channels through their own compression equipment to reduce consumer complaints, making actions of a station irrelevant.
As cable converts to digital and passes supplier-provided dialnorm to analog and digital services, it is vital that the value in our broadcast stream is correct. Cable providers prefer to pass the signals without processing them.
The future
The ATSC is leading an industry-wide effort to improve the situation. The organization's subgroup is considering a recommended practice to help the industry properly implement dialnorm. According to Starzynski, the subgroup is evaluating adoption of the new ITU LKFS loudness measuring standard, which is similar to the current ATSC Leq(A) specification but better suited to the measurement of music and other content without dialog.
By adopting ATSC A-53, the FCC applied the force of law to the proper value of dialnorm: “The value of the dialnorm parameter in the AC-3 elementary bit stream shall indicate the level of average spoken dialogue within the encoded audio program.” If the industry doesn't obey the law, count on consumer complaints and congressional hearings.
As consumers increasingly rely on AC-3 audio delivery through digital broadcasting, cable and DBS, it is imperative that the production, network, station and cable industries work together to make dialnorm effective.
Bruce Jacobs is chief technologist with Twin Cities Public Television and a long-standing member of the PBS Enterprise Technology Advisory Committee.
Four simple steps to successful dialnorm
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Measure the long-term average A-weighted dialog level for different types of content and ensure that the dialnorm parameter is set to the measured value.
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If the correct meter is not available, set the dialnorm value 14dB lower than the typical peak value and compare with other stations. If your channel seems too loud, move the dialnorm value toward the value of -1. If your channel seems too soft, move the dialnorm value toward the value of -31.
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If the average dialog level going into the encoder varies by more than 4dB from show to show, install a gentle audio automatic gain control.
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Encourage your peers to follow these same rules.
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