Dolby Laboratories Multichannel Audio Tool DP570 User Manual

Page 107

Downmixing

Dolby

DP570 Multichannel Audio Tool User’s Manual

Set‐top boxes, used to receive terrestrial, cable, or satellite digital television, typically offer
an analog mono signal modulated on the RF/antenna output, a line‐level analog stereo
signal, and an optical or coaxial digital output. DVD players offer an analog stereo and a
digital output, and some offer a six‐channel analog output (for a 5.1‐channel presentation).
Portable DVD players offer analog stereo, headphone, and digital outputs. DVD players in
computers and game consoles offer a digital output as well as analog stereo, headphone,
and possibly six‐channel analog outputs. 5.1‐channel amplifiers, decoders, and receivers
have six‐channel analog outputs and possibly six speaker‐level outputs.

In all of these cases, a Dolby Digital decoder creates the analog audio output signal.

In the case of the set‐top box or DVD player, the analog stereo output is a downmixed
version of the Dolby Digital data stream. The digital output delivers the Dolby Digital data
stream to either a downstream decoder or an integrated amplifier with Dolby Digital
decoding.

In each of these devices, the analog stereo output is one of two different stereo downmixes.
One type is a stereo‐compatible Dolby Surround downmix, of the multichannel source
program that is suitable for Dolby Surround Pro Logic

decoding. This kind of downmix

is also called Pro Logic or Left total/Right total (Lt/Rt). The other type is a simple stereo
representation (called 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, in‐phase to the Left
channel and out‐of‐phase to the Right channel. This allows a Dolby Surround Pro Logic
decoder to reconstruct the L/C/R/S channels for a Pro Logic home theater. The Lo/Ro
downmix adds the Left Surround and Right Surround channels discretely to the Left and
Right speaker channels, respectively. This preserves the stereo separation for stereo‐only
monitoring and produces a mono‐compatible signal. In all downmixes, the LFE channel is
not included.

On most home equipment, the consumer can use the product’s user interface to choose the
appropriate stereo output for his playback system. The mono signal feeding the
RF/antenna output is usually derived from the Lo/Ro downmix.

High‐end DVD player

DVD player

Includes games consoles

(some units)

High‐end set‐top box

Often HDTV

Set‐top box

Usually SDTV

IDTV

TV set with an integrated
digital TV tuner

High‐end TV

Large screen TV with a
5.1‐channel speaker system

Table D-2

Outputs from Different Equipment Types

Equipment

Output

Digital

5.1-Channel

Analog

2-Channel

Analog

Remodulated