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Audio Feature modules, AR700 and

AR701

General

The audio feature module AR700 is based on the so-called
3D-audio system, which covers the audio systems called
3D Mono, 3D Stereo and 3D Movie systems.
The audio feature module AR701 consists not only the
above mentioned systems, but also the Dolby Surround
Pro Logic system.

Signal process in the DSP

Both systems are produced using digital sound process-
ing, whose program consists of seven main blocks: Dolby
Pro Logic, 3D Mono, 3D Stereo, 3D Movie, graphic equal-
izer, tone control, and noise generator.

Input
Balance

Bass &
Treble

Graphical
Equalizer

Adaptive
Matrix

Combining
Network

GLE

R
L

Equ Control

R
C

3D
Stereo

Volume
ST
Stereo

Volume
DPL

Volume
ST Sur-
round

Equ Control

S_ Equ (from

GLE / GE

)

S_ Orig (from L/R)

7kHz
LPF

Dolby B
Level

Dolby
B

Delay

3D
Movie
on/off

3D
Mono
on/off

Echoing
for 3D
Mono

L
R

C
S

_GLE_GE

Noise
Gene-
rator

L
R

C
S

Principle of the signal process in the DSP

Dolby Surround Pro Logic

The Dolby Surround Pro Logic is a four channel surround
sound system.
In the transmission phase, the four channel information is
encoded into two channels (L’ and R’). The center (C) and
the surround (S) channels are added to the left (L) and right
(R) channels according to the following formula:

- L’ = L + C x (-3 dB) + S x (-3 dB)
- R’ = R + C x (-3 dB) - S x (-3 dB)

The center channel information is added to the L and R
channels in the same phase using a factor of 0.7. The sur-
round channel information is added to the L and R chan-
nels using the same factor of 0.7, but in the opposite phase,
as +90

into the L channel and -90

into the R channel.

In the receiver, the Dolby Pro Logic decoder decodes the
two channels, L’ and R’, back to four separate channels,
left, center, right and surround. If the transmission includes
Dolby Pro Logic sound, the software automatically switches
the receiver to Pro Logic mode, assuming that the receiver
is in AUTO mode, the transmission is in stereo (Nicam or
A2) and the WSS (Wide Screen Signalling) data includes
the Dolby bit. In addition, the rear speakers have to be con-
nected, otherwise the software selects 3D Movie mode.
During digital signal processing, the audio data (L’ and R’)
is first fed to the input balance control, which compensates
for possible errors in the level between the two input sig-
nals. The balance control is designed to operate automati-
cally, which is a very user-friendly feature. The automatic
system continuously analyzes the level difference between
the left and right channels and tries to keep it as small as
possible.

Next the signal is fed to the adaptive matrix stage, whose
first block is a bandpass filter. The highpass frequency of
this filter is 200 Hz and the lowpass frequency is 5 kHz.
After bandpass filtering the data (Lbp and Rbp) is fed to a
calculation block, which generates the center and surround
channels. The calculation formula is Cbp = Lbp + Rbp and
Sbp = Lbp - Rbp.
The absolute values are then taken from the Lbp, Rbp, Cbp
and Sbp signals. The next step is to calculate the relation-
ship between the L and R channels, and also between the
C and S channels.
Next comes the time constant calculation. In the Dolby Pro
Logic decoder, there are two time constant filters. The sig-
nal level determines which one is chosen. If the level dif-
ference between L and R channels or between C and S
channels is more than 5.5 dB, then long time constant fil-
tering is performed. After that, the short time constant fil-
ter is calculated. If the level difference is less than 5.5 dB in
both directions, then only short time constant filtering is
performed.
Finally the LR and CS signals are fed through a polarity
definition block, the last operation block in the adaptive
matrix stage, to the combining network stage.

After the combining network stage, only surround chan-
nel processing continues. In the surround channel there is
a 7 kHz low pass filter, a delay and a modified Dolby-B de-
coder. The lowpass filter is simply a second order filter
whose cut-off frequency is 7 kHz. The delay can be a vari-
able delay from 15 ms to 30 ms or a constant 20 ms delay.
The modified Dolby-B decoder is a complex part of the
processing. The gain of the Dolby-B filter is dependent on
the frequency of the input signal and also on the level of
the input signal. The program uses a surround channel sig-
nal calculated without graphic (GE) and loudspeaker (GLE)