SSM2000

REV. 0

–12–

VCF Capacitor Port (L Pins 3, 4—R Pins 21, 22)

The capacitors connected to these ports control the low-pass

filter range of the VCF. Choosing the recommended value,

1 nF, results in a VCF range where the lowest possible low-pass

corner frequency is about 660 Hz. There is a 20 dB/decade de-

crease in amplitude after the corner frequency regardless of the

capacitor value at this port. Raising the value to 10 nF lowers

the VCF range by a decade to 66 Hz. This will permanently at-

tenuate the high frequency audio signals regardless of the audio

frequency spectrum applied to the input of HUSH. In other

words, for VCF capacitor values higher than 1 nF, high fre-

quency loss will occur. On the other hand, lowering the value

of the VCF capacitor to 0.1 nF will result in a VCF range of

10 kHz to 100 kHz. Of course, most of this range is outside of

audible frequencies and is therefore not very useful.

V+ (Pin 5)

V+ is the supply pin. Please refer to the sections “Powering

the SSM2000—Dual Supply” and “Powering the SSM2000—

Single Supply” for more information.

ACOM (Pin 6)

ACOM is the internal ground reference for the audio circuitry.

In single supply applications this pin is normally connected to a

low impedance potential that is one half the positive supply.

“Powering the SSM2000—Dual Supply” and “Powering the

SSM2000—Single Supply” for more information.

VCA Control Port (Pin 7)

As shown in Figure 5, this port connects into the VCA circuitry.

The purpose of this port is to allow external control of the VCA

amplitude setting or volume in addition to the normal internal

VCA detector control. Every 22 mV applied to the port results

in a 1 dB decrease in amplitude, with 0 dB corresponding to

about 150 mV. Figure 10 illustrates the transfer characteristic

of the VCA control port.

VCF Level Control Input Port (Pin 8)

This pin is the input to the VCF control and noise threshold de-

tection circuitry. A three-pole filter is recommended between

the SUM OUT (Pin 9) and VCF IN (Pin 8).

SUM OUT (Pin 9)

SUM OUT is the buffered combination of the Left and Right

inputs. Figure 29 shows the internal configuration of SUM

OUT. It is this combined Left and Right audio signal that will

be used by the VCF and VCA detectors to determine audio am-

plitude and frequency content.

SSM2000

L IN

SUM OUT

3*(L+R)/2

470pF

R IN

20k

Ω

20k

Ω

1

2

TO L VCF

TO R VCF

18

X3

9

Figure 29. The Internal Circuit for SUM OUT

VCA Level Control Input Port (Pin 10)

This pin is the input to the VCA detector, which outputs a volt-

age that will control the VCA in the audio signal path. A single-

pole filter is recommended between the SUM OUT (Pin 9) and

VCA IN (Pin 10).

VCF Time Constant (Pin 11)

A 1

µF capacitor is normally connected to this point. The pur-

pose of which is to control the rate of change of the VCF’s low

pass corner frequency. Raising the value of this capacitor from

the recommended 1

µF will tend to increase “breathing” (noise

and high frequency signal that slowly fades in and out). Lower-

ing the value of this capacitor from the recommended 1

µF will

tend to increase “pumping” (noise that noticeably cuts in and

out). The final capacitor values are, of course, subjective and

should be chosen based on one’s own evaluation within the sys-

tem environment.

VCA Time Constant (Pin 12)

The function of this pin is similar to the VCF TC pin. Once

again, a 3.3

µF capacitor is placed on this pin, which controls

the rate of change of the VCA’s amplitude. Higher value ca-

pacitors will result in “breathing” and lower values will cause

“pumping.”

DO NOT CONNECT/Adaptive Noise Threshold Override

(Pin 13)

Connecting this pin to any low-impedance potential including

ground will disable the adaptive noise threshold. Pin 13’s inter-

nal connection is shown in Figure 24. It is possible to set up an

artificial noise threshold using this pin.

Default Noise Threshold (Pin 14)

This pin sets up the maximum noise threshold possible. As can

be seen in Figure 25a–c, the emitter voltage of Q1 can be no

higher than a diode drop above the potential set at Pin 14. A

maximum noise threshold level prevents HUSH from attempt-

ing to filter audio signal in the presence of very high noise. The

minimum noise threshold detection level has already been set

internally by Q3. By setting the lower limit, HUSH is prevented

from attenuating already very low levels of noise which would

reduce the responsiveness of HUSH.

Auto Threshold CAP (Pin 15)

The recommended 0.22

µF capacitor connected to this pin con-

trols the rate of change of the adaptive noise threshold level. As

shown in Figures 25a–c, if Q1 and Q2 are turned OFF, then a

35 nA constant current source begins to charge the Auto

Threshold CAP. This rising potential across the AT CAP rep-

resents the detected noise level. Because the capacitor’s charg-

ing current only 35 nA, a low-leakage ceramic or equivalent

capacitor is required to hold the charge.

DEFEAT (Pin 16)

Applying a +5 V volt signal to this pin will defeat the HUSH

noise reduction system. Potentials applied to this pin should be

reference to GND. Standard TTL levels are recommended

when using DEFEAT. Activating DEFEAT causes the VCF

detector and VCA detector to send their maximum control sig-

nal levels to the VCF and VCA respectively. If DEFEAT is be-

ing controlled by noisy digital logic lines, then use adequate

filtering to avoid digital noise interfering with audio signals

present in the SSM2000.