Hitachi CP2996TAN Service Manual page 35

24
equalization for the Dolby-B level and frequency calcula-
tions. Dolby-B filtering is performed on the equalized sig-
nal.
In the Dolby Pro Logic decoder, there are 3 different center
channel modes: wide, normal and phantom.
Wide mode
If all three front speakers are capable of reproducing low
bass frequencies and they have enough amplification
power, then the system can be in wide mode. Wide is an
abbreviation of wideband, which means the whole fre-
quency band is reproduced from the center speaker.
Normal mode
If two full range loudspeakers for the L and R channels are
available, the easiest way to set-up the Dolby Pro Logic
system is to add a small center channel speaker somewhere
near the television screen. The small loudspeakers are
normally incapable of reproducing low bass information.
In normal mode the center channel response is limited to
100 Hz and the bass content is redirected to the L and R
channels (or alternatively to the subwoofer). Another name
for this mode is "bass splitting mode".
Phantom mode
If there is no center channel speaker available, then the
decoder can be set to phantom mode. In this mode, all
center channel information is added to the left and right
speakers.
For the surround channel there are two modes: normal and
3 stereo.
In 3 stereo mode, the surround output is cleared. The sur-
round information is ignored and not added to the other
channels.
To summarise, there are, in principle, four different loud-
speaker configurations.
Full configuration (5 loudspeakers)
- organised as two external front speakers (L / R), center
speaker, and two external rear speakers
Phantom mode (4 loudspeakers)
- where the TV's own loudspeakers (L / R) operate as front
speakers, with no center speaker, and two external rear
speakers
3 stereo (3 loudspeakers)
- with two external front speakers (L / R), center speaker,
but no rear speakers
Basic configuration (2 loudspeakers)
- where only the TV's own loudspeakers (L / R) are used.
In configurations where the center speaker is used, it can
be either an external speaker or the TV's own loudspeak-
ers connected in series. A subwoofer can be used in addi-
tion to the above loudspeakers.
The Dolby Surround Pro Logic system consists of a clock
oscillator icar1, digital sound processor icar2, delay
memory icar3, phase locked loop icar4, reconstruction fil-
tering / digital to analog conversion icar5 and icar6, power
amplifier ICar7 and loudspeaker selection switch SWA1.
3D Mono
The 3D Mono system is designed for mono transmissions.
It is an artificial surround sound system, that creates the
surround signal from the mono signal (or from the sum of
the left and right channels) after the graphic equalizers.
This mode is suitable for non Dolby Pro Logic programmes
when a surround channel is still desired. 3D mono mode
requires no external speakers.
The artificial surround effect is created from the mono sig-
nal. The effect is mainly an echoing, where the size of the
space can be artificially changed by the delay control and
the richness by the effect control.
The 3D mono system is also called the 3D pseudo or Semi-
Tech surround system.
The signal is taken from the graphic equalizers and fed to
the attenuation block to avoid an overflow. The coefficient
stage 1 and delay buffer 2 have constant values. The mul-
tiplication value is 0.12 and the delay time is limited, due
to the size of the delay memory, to 104 ms. The control
range for delay buffer 1 is between 50 ms and 150 ms and
the multiplication value of coefficient stage 2 is between
0.1 and 0.2. In the sound menu these controls are called
effect and delay.
Finally, the multiplied and delayed signal is fed to the HRTF
(Head Related Transfer Functions) filter, which artificially
shifts the audible image of the loudspeaker from its physi-
cal placement to another location.
L
+ +
-18dB
coef 2
R
Principle of the 3D Mono system
3D Stereo
3D Stereo is an artificial system which is used to widen the
stereo image of the TV's loudspeakers, thus rendering ex-
ternal speakers unnecessary. 3D Stereo mode only empha-
sizes the stereo information (no delays or echoing), thus
the 3D Stereo has no effect on a non-stereo transmission.
The effect control in the sound menu changes the amount
of emphasis on the stereo information.
The algorithm is based on a band reject filter (BRF). The
output of this filter is multiplied by an adjustable coeffi-
cient. This signal is then added to the original channel and
subtracted from the other channel
L
BRF
Adj
BRF Adj
R
Principle of the 3D Stereo system
+
coef 1
1
-1
Z
H + H
i c
delay 1
+
-1
Z
delay 2
+
+
coef
coef
+
Left
Right
Left
Right