SSM2005

–13–

REV. 0

APPLICATION NOTES

Figure 8 shows a typical SSM2005 application schematic. The

stereo source signal is connected to the SSM2005 audio signal

inputs at Pins 7 and 10, as well as the Left/Right level detectors

at Pins 19, 20, 22, and 23. The input signal goes through the

autobalance circuitry and can be accessed at Pins 6 and 9.

A sum (L+R) and difference (L–R) signal is available at Pins 15

and 16, respectively. These signals are fed into the Front/Rear

level detector at Pins 27 and 28.

The L+R signal is also fed internally to the center channel VCA

to produce the center channel output at Pin 48. In addition, the

L–R signal is fed into two external filters, creating a low-band and

high-band signal with a crossover frequency of 2 kHz. The cross-

over filters are both 3

group delay to the surround channels. The L–R high-band signal

is connected to Pin 17, and the L–R low-band signal is connected

to Pin 18. These two pins provide the multiband steering to the

two surround outputs.

Low-pass filters are inserted between Pins 1 and 2, and Pins 3

and 4. These filters are used for the center-canceling circuitry,

which removes center channel information from the left front

and right front outputs. This circuitry is only active in Video

mode, providing maximum channel separation between the

center and front outputs. In an application that will only use

Music mode, these active filters can be removed and replaced

with a 1

µF capacitor between Pins 1 and 2, and Pins 3 and 4.

The capacitors and resistors connected to Pins 21, 24, 25, 26, 29,

30, 31 and 32 are used to create the time constants for the steer-

ing circuitry. The values shown in Figure 8 are strongly recom-

mended. Variation from these values will result in improper

operation of the Circle Surround decoder, and may result in the

assembled unit failing SRS Labs approval.

The noise generator output is at Pin 42, and the input to the noise

steering circuit is at Pin 43. The R-C network connected between

Pins 42 and 43 is used to remove dc voltage and high frequencies,

which could damage speakers. Other noise-shaping circuitry could

be used here to create noise patterns other than white, or to further

attenuate the noise output with a resistor divider.

Adding a Subwoofer Output

The SSM2005 provides an output of the summed Left In and

Right In signals. This L+R signal is used to drive the center chan-

nel output, and it can also be used to provide a subwoofer chan-

nel by connecting the output from Pin 15 to a low-pass filter. The

schematic for such a configuration is shown in Figure 9.

Here, Pin 15 is connected to a 2nd order Bessel low-pass filter.

The circuit uses the SSM2275, a low noise audio op amp that

can run from the same

±6 V that power the SSM2005. Using

the component values shown in Figure 9, the filter’s cutoff

frequency is 100 Hz.

48

47

46

45

44

7

10

39

SSM2005*

LEFT IN

RIGHT IN

10 F

10 F

CENTER

OUTPUTS

*ADDITIONAL PINS

OMITTED FOR CLARITY

SSM2275

2.05k

4.23k

1 F

SUBWOOFER

0.47 F

Figure 9. Adding a Subwoofer Output

Implementing a Stereo Bypass Mode

Figure 10 shows a schematic for implementing a clickless stereo

bypass around the SSM2005. The stereo bypass mode allows

the user to defeat the Circle Surround decoding and listen to

two-channel stereo from the left front and right front speakers.

The SSM2402 is a clickless dual audio single-pole single-throw

+0.8 V) the switch is open, and the five channel outputs are

high (above +2.0 V) the SSM2402 switch closes, connecting the

LF and RF outputs to Left In and Right In respectively. At the

same time, a data byte should be loaded into the SSM2005,

placing the Circle Surround decoder into Mute Mode. The data

byte required for Mute Mode can be found in Table III, and the

logic timing diagrams can be found in Figure 7.

48

47

46

45

44

7

10

39

14

13

12

11

10

9

8

1

2

3

4

5

6

7

SSM2402

+6V

6V

STEREO MODE

SURROUND MODE

SSM2005*

DATA IN

LEFT IN

RIGHT IN

10 F

10 F

CENTER

OUTPUTS

*ADDITIONAL PINS

OMITTED FOR CLARITY

x DENOTES A DON'T CARE BIT

0 0 0 x x x xx

Figure 10. Implementing a Clickless Stereo Bypass Mode