Dialnorm is the meta-data parameter that controls decoder gain within the proprietary Dolby Laboratories Dolby Digital (AC-3) audio compression system. It ranges in integer values from 31, where decoder gain remains at unity, to a value of 1, where decoder gain is reduced by 30 dB.

Dolby recommends that the dialnorm value be determined by measurement of average dialog level in the encoded audio of every program. The recommended approach derives the absolute value of a power sum of the A-weighted audio level in all digital channels. If every producer and distributor uses this method, consumer dialog levels will be consistent from program to program and channel to channel. The dialog levels will be "normalized" - hence the abbreviation of "dialog normalization".

Historical Basis

Audio levels within analog disk, tape and broadcasting have conventionally been adjusted to keep peak levels within the physical and legal modulation limits of the medium. While this practice has resulted in comparatively consistent dialog levels and minimized audibility of channel noise, it has come with a severe penalty in the form of excessive audio compression and limited dynamic range.

Digital recording has eliminated the concern for noise, but the lack of a standard for digital audio level has resulted in compact disk recordings and portable music player files with widely varying levels. Increasingly, digital music is being excessively compressed, to insure that the selection is not perceived by the listener as being too soft in comparison.

Dolby Laboratories sought to give for both wide dynamic range and consistent audio levels in the Dolby Digital encoding system. One approach would have been to require audio levels at the encoder input to be adjusted to a fixed value. The approach used instead by Dolby was to allow producers to choose any input level and require them to include a dialnorm value representing the measured dialog level of the input signal.

Use Within US Digital TV

The Dolby Laboratories recommendation for dialnorm has the force of law for US Digital Broadcasting, through the FCC's adoption of ATSC A/53 Annex B, section 5.5 Dialogue Level: "The value of the dialnorm parameter in the AC-3 elementary bit stream shall designate the level of average spoken dialogue within the encoded audio program."

However, many broadcasters were unaware of this requirement and did not initially configure their encoding equipment for a proper dialnorm value. Many broadcasters have no means to measure average dialog level. Compliance remains spotty, with CBS Television in specific operating their network with dialog levels 6 dB higher than the transmitted dialnorm value would suggest. The result is a wide disparity in audio levels when changing between digital broadcast channels through over-the-air and cable distribution.

Dolby recommends that the dialnorm value be determined for all legacy content and be carried along with the audio signal throughout the entire distribution chain. However, this is a significant challenge when dealing with legacy standard definition systems and content. Many broadcasters instead seek to bring the benefit of dialnorm to consumers by fixing the dialnorm to a value approximating the typical average dialog levels of programs mixed using legacy methods.