MANAGING AUDIO LEVELS
Sep 1, 2005 12:00 PM, BY JIM STARZYNSKI
Enter DTV, DVD and Dolby Digital
Dialnorm is one of 29 metadata parameters that are part of the Dolby Digital bit stream required for all ATSC DTV streams and worldwide DVD audio. Dolby AC-3 is the term identifying the codec for this technology.
Metadata is steering information that is encoded with the audio when it's bit-reduced by Dolby Digital. Metadata is essential data that controls a home decoder to do many things, such as turn on the proper number of channels (e.g., 5.1 or two-channel), let the consumer pick a dynamic range from choices provided by the show's producer (e.g., wide, normal or narrow) and, for dialnorm, automatically adjust the perceived loudness from show to show, commercial to show and so on.
Using a device, such as Dolby LM-100 loudness meter, show audio is measured and dialnorm readings are created using the LEQ-A audio measurement practice that's part of Dolby's Dialog Intelligence. This provides a long-term A-weighted summation in real-time, yielding a figure for the perceived loudness of the audio within minutes. This practice (in conjunction with the proper and complete distribution of the metadata) has the capability to maintain the same perceived loudness for all show, commercial and promotional audio during all parts of a station's DTV broadcast day. Dialnorm is intended to eliminate the NTSC issue of inconsistent loudness during broadcasts. How it works: The audio engineer labels the program or commercial audio with a measured numeric value between -1 and -31 as indicated by an LM-100. This figure is entered as part of the overall metadata stream with a Dolby 570 authoring tool and muxed directly into the VANC of the HD-SDI stream or into the Dolby E metadata bit stream. Dolby E is used to pass multichannel audio and metadata through broadcast gear with limited audio channel capacity.
DTV stations can pass this compatible metadata to home decoders using their Dolby 569 AC-3 encoders, set up to receive the signal on their external metadata input. The audience's set-top boxes or home theater receivers extract the dialnorm metadata from the Dolby Digital bit stream transmitted from the station.
This information is used to dynamically adjust the perceived loudness for all audio to the same -31 level no matter the actual audio amplitude a show or commercial may have. With dialnorm, no limiting or compression is used and the audio maintains all of its dynamic range.
Moving from acoustic energy to electricity, analog to digital, digital to dialnorm, the modern mixing engineer has an arsenal of tools and established practices. Proper set-up, use of headroom and measurement of perceived loudness all help to ensure the accuracy and consistency of the audio from its origin to delivery. The broadcaster's awareness and use of these practices will ensure that today's digital savvy home theater audiences receive the extraordinary sonic experience they now demand.
Jim Starzynski is principal engineer in advanced technology for NBC-Universal.
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