.: Matrix Surround Sound Format
Dolby
Surround (also known as Dolby Setero)
 The
popular Dolby Surround technology, is featured in millions of home worldwide
in amplifiers, A/V receivers, separate decoders, and televisions. Besides
the many films available on video, increasing numbers of sports, dramas, talk
shows, and even commercials are being produced for television in Dolby Surround.
The
Dolby Setero encoder takes 4 channels (L, R, C, S) and creates two output
channels (Lt- Left total, Rt- Right total). L goes to Lt and R goes to Rt
without alterations. C, after a 3dB level reduction, gets divided equally
between Lt and Rt. The bandwidth of S gets limited to 100Hz - 7 kHz range,
it is encoded with a form of Dolby B noise reduction. After that it gets divided
between Lt and Rt but with a +90 and -90 degree phase shifts.
To
recapture the dimensional properties brought by the additional channels, a
Dolby Surround decoder is used during playback. Surround decoder artificially
creates the front center channel by by finding the sounds the left and right
channels have in common, and the rear channel is created by extracting the
differences between the left and right channels.
Thus
the setero release is decoded into 4 playback channels– 2 discrete and
2 matrixed
- 2 discrete, full-bandwidth channels (left front, right front),
- 1 matrixed full-bandwidth center channel,
- 1 matrixed, limited-width channel that goes to 2 surround speakers.
The
rear surround channel is a monaural channel; although it can be routed to
multiple speakers--no rear stereo surround is possible. Moreover, if two rear
speakers are used, the mono sounds can lead to a small phantom image at the
listening position between them. This can be mitigated by using dipole speakers
or THX decorrelation techniques to create a diffuse surround sound.
On
a normal stereo audio system, matrix encoded program sounds like conventional
stereo—the center channel information comes out , in equal portions,
through both the left and right channels, as does the surround channel information.
This compatibility between mono, stereo, and full four-channel playback is
one of the main reasons for the popularity of this format.
Dolby Pro Logic (DPL)
 Though
Dolby Surround provides channel separation, but the matrixing is subject to
flaws. For example, if there is simultaneously dialogue in the center channel
and music in the front channels, both the dialogue and music can be steered
toward the center. To overcome this flaw Dolby Pro logic decoder was introduced
in 1987 in consumer products.
Pro
Logic uses directional enhancement circuitry to continuously monitor the sound
field, and actively steer sound toward the dominant position.
In
particular, it determines the dominant signal among the four outputs by comparing
the signal level of the left-total and right-total signals while simultaneously
comparing the level of the sum and difference signals. An active matrixing
circuit uses this information to output the dominant signal to the appropriate
channel while canceling other channels. In addition, the amount of enhancement
applied is relative to the level of dominance.
The
use of the adaptive matrix circuit into the playback side brings out a clearer
dialogue and allows the right and left front speakers to be spaced farther
from the center for a wider, richer distribution of sound.
As
with Dolby Surround, the single Dolby Pro Logic surround channel is often
fed to two surround speakers, which both play the same track. Additionally,
there is no discrete subwoofer channel specified in the standard, so subwoofer
channel can be extracted from other channels rather than specified explicitly.
Dolby Pro Logic II (DPL2)
 Dolby
Surround Pro Logic II is an improved matrix decoding technology that provides
better spatiality and directionality to the decoded sound field created by
the five speaker arrangement. The PL2 decoder achives directional enhancement
through better channel speration with the use of a feedback loop around the
steering circuitry which facilitates in closer matching of anti-pashe signals
being feed to the matrix with the unwantd cross-talk signals; thereby yileding
improved image stability.
The
other noticiable improvements of Dolby Surround PL2 compared to the former
matrix version of surround system are:
- The Surround (Rear) channels are in stereo (only mono with Pro Logic)
- Playback
covers the full frequency range (only up to 7 kHz with Pro Logic)
- PL2 includes bass management feature for controlling the spatial dimensionality
and frontal sound field imaging– it allows bass to be reproduced from
the main speakers, or it can derive an LFE output.
- Pro Logic II decoding can be implemented in either analog or digital circuitry.
In
addition to that, Dolby Pro Logic II is designed to get 5.1-channel sound
effect from any 2-channel software, which means you can enjoy perfect surround
sound even if the software is in conventional stereo recording. "Phantom
mode" is also available in which quasi 5.1- channel sound field is reproduced
from 4speaker system, without center speaker.
Dolby Pro Logic IIx
 Dolby
Pro Logic IIx technology provides home theater enthusiasts with a solution
for 7.1-channel playback from any 2-channel or 5.1 source: DVD, VHS, television
broadcasts, radio, and CDs; while maintaining all the
richness
and clarity inherent in any Dolby audio
technology.
The
new technology also provides a flexible upgrade path that allows users the
choice to connect a 5.1 speaker system initially and add extra channels and
speakers to their 5.1- or 6.1 systems in the future.
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