Dolby Laboratories Multichannel Audio Tool DP570 - Manual (Page 104)

88 Dolby ® DP570 Multichannel Audio Tool User’s Manual
A Guide to Dolby Metadata
The Dolby Digital stream carries metadata for the two possible operating modes in the
decoder. The operating modes are known as Line mode and RF mode due to the type of
output they are typically associated with. Line mode is typically used on decoders with six‐
or two‐channel line‐level outputs and RF mode is used on decoders that have an
RF‐remodulated output. Full‐featured decoders allow the consumer to select whether to
use DRC and if so, which operating mode to use. The consumer sees options such as Off,
Light Compression, and Heavy Compression instead of None, Line mode, and RF mode.
Advanced decoders may also allow custom scaling of the DRC metadata.
All that needs to be done during metadata authoring, or encoding, is selection of the
dynamic range control profiles for Line mode and RF mode. The profiles are described in
the following sections.
Line Mode
Line mode offers these features:
• Low‐level signal boost compression scaling is allowed.
• High‐level signal cut compression scaling is allowed when not downmixing.
• The normalized dialogue level is reproduced from the decoder at a constant loudness
level of –31 dBFS ITU‐R Rec. BS.1770, assuming the Dialogue Level parameter is set
correctly.
Line‐level or power‐amplified outputs from two‐channel set‐top decoders, two‐channel
digital televisions, 5.1‐channel digital televisions, Dolby Digital A/V surround decoders,
and outboard Dolby Digital adapters use Line mode.
Consumer control of the dynamic range is limited when downmixing. Products with stereo
or mono outputs do not usually allow consumer scaling of Line mode. This is because these
devices are usually downmixing (for example, when receiving a 5.1‐channel signal).
However, in these products, the consumer may have a choice between Line mode and RF
mode.
RF Mode
In RF mode, high‐ and low‐level compression scaling is not allowed. When RF mode is
active, that compression profile is always fully applied.
RF mode is designed for products (such as set‐top boxes) that generate a downmixed signal
for connection to the RF/antenna input of a television set; however, it is also useful in
situations where heavy DRC is required—for example, when small PC speakers are used
for DVD playback. In RF mode, the overall program level is raised 11 dB, this results in
dialogue being reproduced at a level of –20 dBFS , while the peaks are limited to prevent
signal overload in the D/A converter. By limiting headroom, severe overmodulation of
television receivers is prevented. The 11 dB gain provides an average loudness level that
compares well with existing analog television broadcasts.
Note: While the use of DRC modes during decoding is a consumer‐selectable feature, the
Dialogue Level parameter setting is not. Therefore, setting the Dialogue Level
parameter properly is essential before previewing a DRC profile.