Vincent
79
78
Vincent
are calculated. All Dolby Surround software offered
up to now contains decoders for Prologic II. As
opposed to Prologic the full HiFi frequency range is
covered for all channels and there are two surround
channels.
Dolby Digital 5.1 (AC-3)
Dolby Digital 5.1 (firstly named AC-3) is a digital
audio multi channel format developed in 1992 for
the movie sound recording and playback in the
Dolby Laboratories. It contains up to six indepen-
dent channels (two front channels, one center chan-
nel, two surround channels and one low-frequency
channel (LFE)). The first mentioned five channels can
transfer the full audible frequency range (20 to
20,000 Hz), the LFE channel only the low bass fre-
quency range (20 bis 120 Hz). All separate digital
signals are interleaved into one digital data stream
that can be transferred via ONE digital cable.
Compression techniques are used to reduce the
huge data rate necessary for this high quality signal
data transfer by the factor of eight. The effective
data rates are 384 and 448 kbit/s. Since 1997
Dolby Digital 5.1 is the standard DVD audio for-
mat.
Dolby Digital Surround EX 6.1
Dolby Digital Surround EX 6.1 (extended surround)
reproduces film sound using signals for seven spea-
ker channels. Compared to Dolby Digital 5.1 a
rear centre channel is added. The stored signal still
contains 5.1 channels and is recognised and play-
ed back by earlier decoders as Dolby Digital 5.1.
An additional, phase-shifted Centre Surround
Signal is mixed in with the two rear signals. As a
result of the phase shift, the three signals can be
properly separated from each other for playback.
The additional signal is used as a pure effect chan-
nel to support the two regular rear speaker chan-
nels. Even if EX-coding is not present in the sound
material to provide the extra channel, the special
algorithm derives a signal for the rear centre chan-
nel. This sound process was developed jointly by
Dolby Laboratories and Lucasfilm and used for the
first time in May 1999 in “Star Wars – Episode 1”
and was previously marketed only in conjunction
with THX. However, this coupling has since been
dissolved. From 2001 it was also possible to set an
EX flag in the data stream using the Dolby Digital
Professional Encoders, which automatically swit-
ches a corresponding decoder detecting this flag
into EX mode. This switching previously had to be
done manually, because there was no EX-identifier
in the data stream and Dolby Digital 5.1 and Dolby
Digital Surround EX signals have no recognisable
difference.
DTS Digital Surround 5.1
„Digital Theater Systems Digital Surround“ (called
DTS) is a digital multichannel audio system similar
to Dolby Digital 5.1, had been developed for
movie theaters first and uses the same channels and
similar frequency ranges (20Hz to 20kHz at 20 bit,
LFE channel: 20Hz to 80Hz). It uses higher data
rates than Dolby Digital (768kbit/s) and thus requi-
res more space on the media. As data reduction
techniques for the picture improve reducing space
for the picture data, more and more DVD’s are pro-
vided with dts. It is believed that dts provides better
sound quality because of less data compression
(4:1). Compared to cinema DTS (20 bit resolution
and 5-channel-system) the LFE is integrated in the
surround channels) the Home Cinema dts standard
is a true 5.1 system with up to 24 bit resolution. Dts
may only be added to the DVD as bonus audio stre-
am. The standard still is Dolby Digital.
DTS ES Digital Surround 6.1
Extension of DTS Digital Surround 5.1. There are
different variants of DTS ES (extended surround). In
DTS ES Matrix 6.1, as with Dolby Digital Surround
EX 6.1, there is one additional Centre Surround
Channel compared to dts; here, too, this additional
channel is produced in the recording, is mixed into
the two Surround channels and is not stored sepa-
rately on the disc. As a result, older dts decoders
can also play this seven-channel sound material.
On playback, the Surround Centre signal is calcu-
lated from these two rear channels by ES decoders
using a special matrix-decoding process. In DTS ES
Discrete 6.1, as the name suggests, the Rear
Surround Centre channel is stored as an equal
channel data stream. As DTS was designed for a
total of 8 channels, the DTS-ES Discrete 6.1 process
was introduced in the year 2000. For downward
compatibility, soundtracks of this format also have
to be playable on old DTS-Matrix decoders. That is
why in “DTS-ES Matrix 6.1”, the Rear Centre
Channel has been mixed into the two stereo Rear
Surround channels using Matrix coding. In old DTS
decoders the additional channel is simply ignored
and one actually hears a DTS-ES Matrix 6.1 format.
The acoustic pattern is even more real, because
new combinations of effects, which cannot be pre-
sented just with the aid of a matrix, can now be
compiled: therefore combined effects can, for
example, come from the left Surround speaker and
the Centre Surround speaker, while the right
Surround speaker is silent. Likewise, the dynamics
of the discrete channel are far higher than those of
the matrix channel, as the additional channel
covers the full frequency spectrum from 20 Hz to 20
kHz. If an AV unit displays the “DTS ES Compatible
6.1” logo and not the DTS-ES logo on the front
plate of the unit, it contains a matrix decoder for the
Back Surround channel developed by the manufac-
turer in question and fitted without an official DTS
licence.
PCM
The PCM format is the original format of all digital
sound signals. It is the term for the conventional
digital representation of a signal, having sampled
values using a constant resolution in bits and a con-
stant sampling frequency. Therefore every analo-
gue signal, regardless of whether an audio or a
video signal, can be represented and stored on
digital media. The original analogue signal is sam-
pled at a specific frequency in equal time intervals;
this produces a pulsed amplitude modulated signal
initially with any desired number of amplitude valu-
es. This is now quantised in the AD-converter; to
that end the amplitude values are divided into a
limited number of quantisation stages (= sampling
resolution). If linear quantisation is used in the digi-
tising process (differences in signal values of
neighbouring stages are of equal size), which is
generally the case, the resulting data stream is cal-
led LCPM (Linear Pulse-Code Modulation). A code-
word containing the amplitude data is calculated
from every sampled value. A digital signal is produ-
ced from the time sequence of the binary code-
words. However, PCM is not the name for digitised
video signals, as in this case multiple component
signals, which can be blended to a data stream,
are generally used for the representation of a
colour picture. The stereo audio signal stored on
audio CD’s or in WAV file formats is generally
meant by this. The resolution (sampling depth) in a
CD is 16-bit, which equates to 65,536 gradations
in volume at a scanning rate of 44.1 kHz, which
allows a maximum frequency resolution of 22.05
kHz, but is limited internally to 20 kHz. A sampling
frequency of up to 96 kHz is possible in the DVD
Video, which therefore enables a maximum fre-
quency resolution of 48 kHz, and in a resolution of
24-bits, corresponds to 16.7 million volume values.
DVD Audio even allows sampling frequencies of up
to 192 kHz and therefore achieves a frequency
resolution of up to 96 kHz. Digital PCM sound can
also contain more than two channels and have sam-
pling frequencies of 44.1 kHz, 96 kHz and 192
kHz.
DSP
stands for „Digital Signal Processor“. This is a small
microcomputer (IC, chip) designed for the proces-
sing of digital signals (audio or video). It can, for
example, be used to add reverberation or echo to
an audio signal. One of the most important factors
in the perception of sound is reverberation, that ist
the way how the sound is reflected continuously by
different objects and walls (similar to echoes) in the
room.
HDCD
(High Definition Compatible Digital)
Listening tests showed that an increase in the reso-
lution of digital CD recordings starting from 16 bits
represents improvements in sound. HDCD is a rec-
ording/playback process for standards CD’s with a
sampling depth of 20 bits and data compression. In
this patented procedure the sound material is recor-
ded at a 20-bit sampling resolution. The sound qua-
lity compared with standard Audio-CD’s is impro-
ved as a result. These discs labelled HDCD are
downward-compatible to standard players, but can
only be played to their full quality in special play-
ers. HDCD-signalling is stored in the lower value
bits of the conventional sound sample, which repre-
sents inaudible alterations there. Only very few CD
titles are obtainable in this extended format.
