Downmixing - Linear Acoustic MetaMAXLA-5180 Operation Manual

Linear Acoustic LA-5180 Metadata Frame Sync/Generator Operation Manual
7.3.3

Downmixing

Downmixing allows a multichannel program to be reproduced over fewer speaker chan-
nels than for which the program is optimally intended. Downmixing allows consumers to
enjoy a multichannel program without requiring a full-blown home theater setup.
As with stereo mixing where the mix is monitored in mono on occasion to maintain com-
patibility, multichannel audio mixing requires the engineer to reference the mix to fewer
speaker channels to ensure compatibility in downmixing situations. In this way, metadata
allows every consumer who receives the program and proper metadata to enjoy the best
audio reproduction possible, regardless of the playback system.
For a set-top box or DVD player, the analog stereo output is a downmixed version of the
audio data stream. In these devices, the analog stereo output is one of two different stereo
downmixes. One type is a stereo-compatible Matrix Surround downmix (left-total/right-
total, or Lt/Rt) of the multichannel source program that is suitable for Pro Logic, PL II,
dts neo:6, or other matrix decoding.
The other type is a simple stereo representation (called a left-only/right-only, or Lo/Ro)
suitable for playback on a stereo hi-fi or on headphones, and from which a mono signal is
derived for use on an RF/Antenna output. The difference between the downmixes is how
the surround channels are handled. The Lt/Rt downmix sums the surround channels and
adds them, out-of-phase to the left channel and in-phase to the right channel. This allows
a consumer matrix decoder to reconstruct the L/C/R/S channels for a Pro Logic home
theater. The Lo/Ro downmix adds the right and left surround channels discretely to the
left and right speaker channels. This preserves the stereo separation for stereo-only mon-
itoring and produces a mono-compatible signal.
NOTE: Note:The LFE channel is not included in any downmix- it is ignored.
On most home equipment, the consumer can use the product's user interface to choose
the appropriate stereo output for their playback system. There are separate metadata pa-
rameters for the adjustment of the Lo/Ro and Lt/Rt downmix conditions. Certain meta-
data parameters allow the engineer to select how the stereo downmix is constructed and
which stereo analog signal is preferred, but Lt/Rt is the default selection in all consumer
decoders. During downmixing the adjustment of dynamic range control parameters is lim-
ited. Broadly speaking, the stereo outputs use the Line Mode compression profile and the
mono signal uses RF Mode compression. As with dynamic range control, downmixing is
ultimately dependent upon each consumer's unique listening environment.
While the engineer must optimize the multichannel mix for reproduction in an ideal listen-
ing environment, it is also important to preview the mix in downmixing conditions to en-
sure compatibility with different playback systems when selecting the downmixing
metadata parameters. These previews can be achieved in real time using the Dolby DP570
Multichannel Audio Tool, the Linear Acoustic LAMBDA, and other products.
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Metadata