SSM-2005 Analog Devices, SSM-2005 Datasheet - Page 7

• Manufacturer Part Number: SSM-2005
• Description: Search ---> SSM2005
• Manufacturer: Analog Devices

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L
R
L
R
SIMPLIFIED THEORY OF OPERATION
General
The SSM2005 Circle Surround decoder processes stereo input
signals, and outputs 5 channels of surround sound, plus an L+R
output for a subwoofer low-pass filter. The SSM2005 provides
signal processing, steering control, input autobalance, and a digital
interface for mode control. This device uses analog circuits such as
amplifiers, rms detectors, VCAs and digital logic to carry out the
circle surround algorithm in real time. No artificial reverberation
or delay effects are used, preserving the natural sound of the
original stereo recording.
The SSM2005 can decode any existing media including CD,
VCD, DVD, cassette tapes, VHS, FM radio and television stereo
broadcasts. For convenience in balancing the system, an on-chip
digital noise generator is available. The net result is outstanding
5.1 channel surround sound from all stereo sources, which re-
veals the hidden ambiance already contained in existing music
and cinematic recordings. The listening experience is greatly
enhanced and made more enjoyable when compared to ordinary
stereo. In the following description, please refer to Figure 6.
Input Signal Processing
Stereo inputs L and R are fed to both the Steering Control
Generator, and the Autobalance circuitry. The balanced signals
are passed through the Center Channel Cancel circuitry to the
Channel Steering VCAs.
The autobalance output signals also feed the Precision Sum and
Difference Amplifiers. The sum (L+R) and difference (L–R)
signals form the basis for the center and surround channels,
respectively. The center channel signal is fed to the center chan-
nel cancel circuitry, and to the channel steering VCAs. The
surround channel signal is separated by an external Crossover
Network into the Surround High and Surround Low bands and
fed to the channel steering VCAs.
Steering Control Generator
The purpose of the steering control generator is to analyze the dy-
namic characteristics of music, dialog, or special effects, using pro-
prietary high speed analog computing circuits. Control voltages for
all VCAs are then computed, and the soundfield expansion per-
formed in accordance with the circle surround decoding algorithm.
The control signals depend upon the SSM2005 modes selected, and
will differ for video mode vs. music mode, 5.2.5 mode vs. 4.2.4
mode, etc.
Channel Steering VCAs
Command signals from the steering control generator are fed to
the channel steering VCAs, which control the amplitude of the
five output channels. High performance, low distortion VCAs
with typically 12 dBd headroom are used for all channels.
T
T
Left
Right
Surround Encoded Left Input
Surround Encoded Right Input
Table I. Abbreviations and Notations Used in the Text
LF
RF
LS
RS
Left Front Output
Right Front Output
Left Surround Output
Right Surround Output
–7–
Output Amplifier
The Output Amplifiers receive signals from the VCAs and the
internal Noise Generator. Each amplifier has a multiplexer switch
which will enable it to output a white noise waveform under
control of the Serial Bus. This simplifies balancing of the listening
system. The output amplifiers provide load drive capability with
typically 12 dBd of headroom. The overall gain from L and R
inputs to Circle Surround Outputs is unity; the Noise Generator
gives an output level of –10 dBd (100 mV rms).
Mode Control Logic
The various SSM2005 modes are shown in Figure 7. There are
modes for different types of source material, such as video sound
tracks or music. Other modes include 5.2.5/4.2.4, Sound Spread,
Phantom Center mode, autobalance, and Noise Generation. Each
mode’s status is stored in the Mode Control Logic, as determined
by the data sent via the serial bus. Refer to the Typical Outputs In
Various Modes section for a more detailed explanation of the vari-
ous modes.
Applying a logic low to the Reset input will override the Mode
Control Logic and put the SSM2005 into its Default mode. De-
fault modes for the device are video/5.2.5/center active/sound
spread on/autobalance on/noise off. See Figure 9 for logic timing
diagrams.
Autobalance Feature
The autobalance circuitry is activated when the SSM2005 is placed
into autobalance On Mode. In this mode, the device will adjust the
gains of its input amplifiers to balance the stereo inputs to equal
loudness. With a mono input signal, the autobalance circuitry will
typically balance L and R to within 0.5 dB. A 22 F external ca-
pacitor connected to Pin 25 sets a 5 second averaging period for
comparing the levels between the stereo inputs.
Using autobalance will slightly degrade the maximum channel
separation from the SSM2005. The autobalance mode can be
left off without fear of degrading the soundfield, unless the
stereo input signal is expected to be off balance by more than
to within 0.25 dB balance.
Noise Generator
When the noise mode is selected, the audio inputs are muted.
Noise will be available from the output of the channel determined
by the mode control logic. The noise generator uses a feedback
shift register that generates a pseudo-random digital output wave-
form with a repeat time of three seconds. This digital noise wave-
form is band-pass filtered externally to approximate white noise.
Power Supplies
The SSM2005 may be operated from regulated 5 V to 6 V
supplies that can supply 45 mA each. The recommended operating
voltage is
1.5 dB. Most CD, VCD and DVD player outputs are specified
6 V, which will give a typical headroom of 12 dBd.
C
dBd
ACOM
VCA
0 dBd = 300 mV rms
Quality Audio Ground
Voltage Controlled Amplifier
Center Output
SSM2005