Origins Of Dolby Surround; The Dolby Mp Matrix - Toshiba CN27E90 Technical Training Manual

1. ORIGINS OF DOLBY SURROUND

Dolby Stereo movies and Dolby Surround video and television
programs include an additional sonic dimension over
conventional stereo productions. They are made using a Dolby
MP (Motion Picture) Matrix encoder, which combines four
channels of audio into a standard two-channel format, suitable
for recording or transmitting the same as regular stereo programs.
To recapture the dimensional properties brought by the
additional channels, a Dolby Surround decoder is used. In
the theatre, a professional decoder is part of the Dolby Stereo
cinema processor used to play 35 mm stereo optical prints.
The decoder recovers the left, center, and right signals for
playback over three front speakers, and extracts the surround
signal for distribution over an array of speakers wrapped
around the sides and back of the theater. (These same
speakers may also be driven from four of the six discrete
tracks on 70 mm Dolby Stereo magnetic prints, but in this
case no decoder is needed.)
Home viewing of movies on video has become extremely
popular, and with the advent of stereo VCR's, stereo television
and digital video discs, the audio side of the video presentation
has improved considerably, inviting the use of full-range
sound reproduction. The ability to deliver high quality audio
in these formats made it easy to bring MP Matrix-encoded
soundtracks into the home as well, thus establishing the
foundation for Dolby Surround.

2. THE DOLBY MP MATRIX

One of the original goals of the MP Matrix was to enable
Dolby Stereo soundtracks to be successfully played in theaters
equiped for mono or two-channel stereo sound. This allows
movies to be distributed in a single optical format, and
furtheremore results in complete compativility with home
video media without requiring separate soundtrack mixes.
Since the three front channels of the MP Matrix are assembled
in virtually the same way as a conventional stereo mix --- left
+
Left
-3dB
Center
Right
+
+
Surround
+
Fig. 15-1 Conceptual Dolby Stereo/Dolby Surround encoder
into left, center equally into left and right, and right into
right-playing a Dolby Stereo soundtrack over two speakers
reproduces the entire encoded soundtrack. There is but one
exception: the surround signal, though audible, is not
reproduced in its proper spatial perspective. When the first
home decoder was developed in 1982, its goal was to restore
this lone missing dimension.
Before we discuss decoders, it is necessary to see how the MP
Matrix encoder works. Referring to the conceptual diagram
in Fig. 15-1, the encoder accepts four separate input signals;
left, center, right, and surround (L, C, R, S), and creates two
final outputs, left-total and right-total (Lt and Rt).
The L and R inputs go straight to the Lt and Rt outputs without
modification, and the C input is divided equally to Lt and Rt
with a 3 dB level reduction (to maintain constant acoustic
power). The S input is also divided equally between Lt and Rt,
but it first undergoes three additional processing steps:
a. Frequency bandlimiting from 100 Hz to 7 kHz.
b. Encoding with a modified from of Dolby B-type noise
reduction.
c. Plus and minus 90-degree phase shifting is applied to
create a 180-degree phase differential between the
components feeding Lt and Rt.
It is clear there is no loss of separation between the left and right
signals; they remain completely independent. Not so obvious
is that there is also no theoretical loss of separation between the
center and surround signals. Since the surround signal is
recovered by taking the difference between Lt and Rt, the
identical center channel components in Lt and Rt will exactly
cancel each other in the surround output. Likewise, since the
center channel is derived from the sum of Lt and Rt, the equal
and opposite surround channel components will cancel each
other in the center output.
The ability for this cancellation technique to maintain high
separation between center and surround signals requires the
amplitude and phase characteristics of the two transmission
channels to be as close as possible. For instance, if the center
DOLBY NR
-3dB B.P.F
ENCORDER
110
+
Lt
+
+90 DEG
-90 DEG
+
+
Rt