Published: TV Technology magazine

Issue: 20 April 98



by  Joe Fedele



Defining the Issues — What is Dolby Metadata?



  The word “metadata” means data about the data. Metadata is non-audio information that is delivered with audio 

in the Dolby Digital bitstream. This information is used by the decoder to control the level and/or dynamics of the 

reproduced audio. The Dolby system allows audio information to be sent to the encoder without any changes in 

level or dynamics. This allows for all changes to be made at the decoder end and at the option of the listener. 

  The advantage of using metadata is that individual users may adjust the program’s audio characteristics to their 

own liking. Currently broadcasters make adjustments to dynamic range and levels of compression that affect all 

viewers in a similar fashion.

  Three types of metadata control the reproduced average volume level and dynamics of audio: Dialnorm, Dynrng 

and Compr.



Dialnorm — Dialog Normalization

  Dialnorm adjusts the volume setting within the decoder. Designed as a 5-ms-(millisecond)-per-audio-block-

control word, Dialnorm is permanently enabled in all consumer decoders and ensures that program items con-

taining speech at different levels (with respect to digital full-scale) emerge at a more constant acoustic level.

  Music with wide dynamic ranges, which are usually lower than average levels, are controlled to ensure that they 

do not emerge subjectively quieter than music with a narrow dynamic range

  Dialnorm is best generated during post production and transmitted in parallel with the audio via a separate 

metadata distribution path. 

  Being a measure of the average loudness, Dialnorm is used as a reference for compression thresholds in the 

Dynrng and Compr metadata signals.



Dynrng — Dynamic Range Compression

  Dynrng can reduce the dynamic range of the reproduced audio when the decoder is in the line-out mode. This 

control word occurs once per audio block and remains active within program segments.  It may be generated to 

produce a result similar to the audio dynamics processors used in analog broadcasting facilities. 

  Like Dialnorm, Dynrng is ideally generated during post production. This allows audio engineers to preview and 

modify their effects. The Dynrng data also passes to the encoder in parallel form with the associated audio.

  Some consumer decoders allow the user full access to its control capabilities. Maximum flexibility in controlling 

dynamic range is thus achieved.



Compr — Compression Control

  Occurring once every 32 ms, Compr alters the dynamic range of audio when the decoder is in its RF mode. The 

RF mode output is used to drive RF modulators that feed TV sets and VCRs. Like Dynrng, Compr remains active 

during program segments and produces a dynamic rage similar to analog transmissions.

  Compr is generated during post production but must be distributed through its own distinct path to the encoder. 

Several user profiles are available to the consumer that allow for changes in its operating parameters.