Audio level control
Oct 1, 2008 12:00 PM, By Aldo Cugnini
Correct modulation requires a fine-tuned ear.
Figure 4. Dialnorm at the DTV receiver adjusts loudness on a program-to-program basis.
Click to enlarge
One way to provide consistency is to produce all internal content to a common standard and then transmit a fixed dialnorm level. For example, at NBC Universal (NBCU), network stereo and 5.1 deliveries are required to be mixed at -23dBFS by the provider. Most commercials deliver at a -22dBFS average, making a nice transition with programs when they meet the spec, according to Jim Starzynski, principal engineer and audio architect for NBCU. The network's dialnorm parameter is set to -23dBFS to match the programs and commercials and comply with the ATSC standard by targeting to -23dBFS without the need for agile metadata, but yielding exactly the same results. All contribution material is therefore produced to the -23dBFS level, and the network metadata is sent to the stations at this same fixed level. Of course, adjusting dialnorm for each program is equally appropriate, provided it's done correctly.
Internet chatter is rife with anecdotes about hot or weak audio across different broadcasters, despite the use of dialog normalization. This is not a direct consequence of different broadcasters using different values of dialnorm, because the system allows this.
Unfortunately, some broadcasters are still playing a loudness war by intentionally setting dialnorm to -31dBFS and then producing audio at a higher level. This of course distorts the intent of dialog normalization by reproducing a higher loudness level. A similar problem exists when broadcasters leave their Dolby encoders set to the factory -27dBFS level and then produce audio without regard to the actual loudness levels.
Establishing common procedures
The ATSC S6-3 “Ad Hoc Group on Loudness Issues” has been looking at this problem for more than a year. The group is in the process of establishing a recommended practices document, which will include guidance on contemporary measuring, monitoring, metadata and dynamics control techniques, as they relate to the loudness portion of DTV. Expect this work to be available in 2009.
Aldo Cugnini is a consultant in the digital television industry.
Send questions and comments to: aldo.cugnini@penton.com
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