SSM2005

–7–

REV. 0

Table I. Abbreviations and Notations Used in the Text

L

Left

R

Right

Surround Encoded Left Input

Surround Encoded Right Input

LF

Left Front Output

RF

Right Front Output

LS

Left Surround Output

RS

Right Surround Output

C

Center Output

dBd

0 dBd = 300 mV rms

ACOM

Quality Audio Ground

VCA

Voltage Controlled Amplifier

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.

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 µF 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

±1.5 dB. Most CD, VCD and DVD player outputs are specified

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

± 6 V, which will give a typical headroom of 12 dBd.