SSM2000 Analog, SSM2000 Datasheet - Page 12

• Manufacturer Part Number: SSM2000
• Description: HUSH Stereo Noise Reduction System with Adaptive Threshold
• Manufacturer: Analog

SSM2000
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.
Figure 29. The Internal Circuit for SUM OUT
SUM OUT
3*(L+R)/2
R IN
L IN
2
9
1
20k
20k
SSM2000
TO L VCF
TO R VCF
X3
18
470pF
–12–
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 F 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 F 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 F 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 F 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 F 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.
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