SSM2000

REV. 0

–13–

Mute (Pin 17)

Applying a +5 V voltage signal to this pin will mute the

SSM2000. Potentials applied to this pin should be reference to

GND. Standard TTL levels are recommended when using

MUTE. Activating MUTE causes maximum attenuation in the

VCA on both the L and R channels. If MUTE is being con-

trolled by noisy digital logic lines, then use adequate filtering to

avoid digital noise interfering with audio signals present in the

SSM2000.

DO NOT CONNECT/Detector Bandwidth Limit (Pin 18)

Pin 18 is connected internally to both the left and right channels

though 20 k

Ω resistors, as shown in Figure 29. This node al-

lows access to the input of the

×3 gain buffer, whose output is

SUM OUT (Pin 9). Do not connect this node to ground or an-

other potential. Connecting a 470 pF capacitor to this pin will

limit the range of the input signal that runs through the VCF

and VCA detectors to 35 kHz. This is an inexpensive way to

remove fixed-frequency tones of frequencies of greater than

35 kHz. Please refer the Precautions, Limitations, and Observa-

tions section under Pre-filtering Out-of-Audio Band Noise for

more information.

GND (Pin 19)

GND is normally connected to V– in single-supply applications

and ACOM in dual-supply applications. Please read the

MUTE and DEFEAT sections for more information.

V– (Pin 20)

This pin should be connected to the most negative power poten-

tial, typically chassis ground in single-supply applications and

the negative power rail in dual-supply applications.

R OUT and L OUT (Pin 23 and Pin 24)

The outputs of HUSH are buffered and can therefore drive

moderate loads. For loads (e.g., speakers or headphones) which

exceed 2 k

Ω or 300 pF additional buffering may be necessary.

OPTIONAL APPLICATION CIRCUITS

The SSM2000 has been designed to facilitate numerous addi-

tional functions by adding a modest amount of external cir-

cuitry. The direct access to the VCA gain control port plus

access to the amplitude and frequency detectors allow construc-

tion of valuable systems features such as:

• DC Volume Control—one dc level controls both audio chan-

nels without routing audio signals to the front panel.

• Automatic Volume Leveling—constrains long term average

volume levels to within a few dB, avoiding constant resetting

of volume as one tuner through various stations or switches

from radio to tape, etc.

• Compression—reduces dynamic range to improve audibility

of softer sounds in noisy environments such as when playing

CDs in cars.

• Volume vs. Road Speed—a speedometer signal can be used to

increase audio levels as road speed increases.

• Compression vs. Road Speed—see above.

Automatic Leveling

Automatic leveling is similar to compression, however instead of

attempting to attenuate all large amplitude audio signals, the

leveling circuit attenuates audio signal when the overall signal

amplitude has increased for a considerable amount of time,

>1 sec. Attenuating loud TV commercials whose average vol-

ume levels are significantly above the program material is an ap-

plication were this feature is particularly useful. The circuit

shown in Figure 30 is an example of HUSH automatic leveling.

Automatic leveling is a signal amplitude operation, therefore,

the input of the circuit is derived from Pin 12, VCA TC. This

input voltage is gained and level shifted. The volume control is

“counter intuitive,” because the higher the gain the lower the re-

sulting volume, as the VCA control port attenuates when posi-

tive voltage is applied. The level threshold potentiometer sets

the volume level, which is a gain range of 1.5 dB to –80 dB.

After the op amp, the signal passes through a positive peak

detector, and is averaged by a long time constant RC. The sig-

nal is now representative of the long term average amplitude of

the original audio that is applied to the L/R input of the

SSM2000. This signal is then passed to the VCA control port.

Audio Compression

Audio compression can be thought of as fast automatic leveling.

The purpose of audio compression is to attenuate larger ampli-

tude signals, such that signals above the adaptive threshold will

be scaled by 0.2 dB/dB, while the signals below the externally

set level threshold will remain essentially unchanged. A shorter

time constant and slight gain change are the only differences be-

tween the audio compression circuit and the automatic leveling

circuit. Figure 30 illustrates the audio compression circuit. The

OP275 is an excellent audio amplifier and is suggested for this

application when the power supply rails are greater than or

equal to

±5 V. For applications where the power supplies are

separated by less than 10 V, then a single-supply op amp like

the OP279 is recommended.

3k

Ω

100k

Ω

7.5k

Ω

27k

Ω

+5V

20k

Ω

+5V

–5V

5k

Ω POT

VOLUME CONTROL

10k

Ω POT

LEVEL THRESHOLD

1/2

OP275

0.1µF

OFF

LEVEL

COMPRESS

3.3µF

10M

Ω

12

SSM2000

7

6

+5V

–5V

19

5

20

SP3T

OFF

LEVEL

COMPRESS

Figure 30. An Automatic Leveling/Compression Circuit