DTV Audio: Dynamic Range Control

Dynamic Range Control (DRC)

Dolby Digital (AC-3) requires that every decoder implement Dynamic Range Control (DRC). This allows the program producer to exercise control over the kind of audio compression applied within the consumer's decoder. Depending upon the device and output used, the consumer may also have control over the amount of compression applied. All this happens in the decoder after any dialnorm gain reduction has been applied.

Consumer RF Modulator
If the consumer device includes an RF modulator, Dolby requires the decoder feeding it to use whatever compression profile has been assigned to the "RF Mode" within the encoder. The level of the signal is also raised 11 dB and peaks are limited. If Dialnorm was set correctly, dialog will be raised to a level of -20 dBFS, leaving headroom of 20 dB.

Consumer Stereo Output
For these outputs, Dolby requires that consumer device offer compression that has been applied within the encoder to the "Line Mode". The consumer device may also optionally offer a switch for the "RF Mode" compression if the user desires less dynamic range. The device may allow scaling of the low level boost compression, but does not allow scaling of the high-level gain reduction.

Consumer 5.1 Surround Output
Devices with these outputs can, and often do, provide the consumer with a choice of compression: Line Mode ("Light"), RF Mode ("Heavy") or None. Some devices also provide a range of choices, scaling between "Line Mode" and no compression.

Encoder Compression Profiles
For both "Line Mode" and "RF Mode" compression, every Dolby Digital encoder offers the choice of six different compression "profiles" as defined by Dolby:

"None: No dynamic range compression is applied unless downmixing could cause overload, in which case protection [compression] is automatically applied.

"Speech: Appropriate for programs with predominantly dialogue.

"Music Light: Applies light compression to music that is already compressed and does not require excessive dynamic range restriction.

"Music Standard: Applies more compression to music that is not compressed and requires dynamic range restriction.

"Film Light: Applies light compression to a subjectively quiet film that does not require excessive dynamic range restriction.

"Film Standard: Applies more compression to a subjectively loud film that requires dynamic range restriction."

These profiles are illustrated below and defined in Appendix A of the Dolby Digital encoder instruction manual.

Dolby DP569 Encoder

Implications
If a broadcaster chooses "none" for both "line mode" and "RF mode" within their encoder, DRC will be deactivated for all consumers. While DRC is not a multiband processor and may be more audible than a multiband broadcast limiter, it seems logical to use minimal compression prior to the Dolby Digital system and enable moderate DRC for "line mode" and more aggressive DRC for "RF mode". This will provide the consumer with convenient automatic and manual control over the amount of compression used.

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