SSM2000_15 Datasheet, PDF (9/16 Pages) Analog Devices – HUSH Stereo Noise Reduction System with Adaptive Threshold

SSM2000
0
–10
VCA
–20
–30
–40
–50
VCF
–60
–70
1
10
100
1k
10k
100k
FREQUENCY – Hz
Figure 22. Preconditioning Filter Responses for the VCF
DET IN and VCA DET IN
Once the preconditioned VCF signal is applied to the input of
the VCF detector, the log(x) of the signal is taken. This log
function essentially removes the effect of large signal peaks and
smoothes out the response of the VCF control. After the log(x)
is taken the signal is rectified. The signal is rectified for the fol-
lowing two reasons: first, the following peak detector’s output
will have less ripple, because twice as many peaks are available;
and second, the most negative potential of the rectified signal is
equivalent to the noise floor. Following the rectifier, the signal
is positive peak detected, as shown in Figure 23.
RECTIFIED
AND LOG (x) OF
VCA DET IN
AND VCF DET IN
Q3
D1
TO AUTO
THRESHOLD
AND DIFFERENCE
AMPLIFIER
Q4
1.1µA
V_EE
V_EE
11
VCF DET CAP
1µF
Figure 23. VCA DET IN and VCF DET IN’s Positive Peak
Detector Circuitry
The attack time of the VCF control is set internally and cannot
be decreased, however the release time constant is directly pro-
portional to the value of the capacitor VCF DET CAP. Signals
above the potential at Pin 11 cause the emitter diode of Q3 to
turn on, thus rapidly charging the VCF DET CAP (Pin 11).
When the audio signal has dropped below the potential at VCF
DET CAP, the emitter diode of Q3 is turned off. During this
condition, an internal 1.1 µA current source sets the release time
by discharging the VCF DET CAP. The release time constant
of the VCF Detector is about 10 times faster than the release
time constant of the Auto Threshold Detector. This ratio
should not be decreased, because the output of the VCF Detec-
tor is negative peak detected to arrive at the Auto Threshold
level (for more information please refer to the Auto Noise
Threshold section). The output of the VCF detector (Pin 11) is
multiplied by 13 (A1 of Figure 24) and then the noise threshold
is subtracted (A2 of Figure 24) to arrive at the actual control
voltage for the L and R VCFs. D1 and Q4 are used to set limits
on the output of the detector circuitry to ensure that the detec-
tor remains responsive to pulsed high frequency audio.
VCA Detector
The VCF and VCA detectors are identical, therefore refer to the
previous section on the VCF detector for detailed information of
the internal operation of these detectors. The VCA detector is
used to detect the audio-band signal amplitude (20 Hz–20 kHz).
Usually the lower frequencies 50 Hz to 2 kHz contain the high-
est audio amplitudes. Therefore, the VCA’s preconditioning fil-
ter must allow low frequency signals to be presented to the VCA
detector. A single pole filter is used to accomplished this func-
tion. This filter is formed by a 2.2 µF capacitor and the 6 kΩ of
internal input impedance of Pin 10 as shown in the schematic,
Figure 21.
Adaptive Noise Threshold
The threshold level chosen for both the variable filter and down-
ward expander is of prime importance in differentiating between
signal and noise. In an automotive environment for example,
the audio sources are generally AM and FM radio, tape, and
CD. Setting the noise threshold at a value suitable to improve a
noisy FM station could easily wipe out most of the dynamic
range of a CD. FM station threshold setting is compounded by
the vast variations in signal strength in any given location, and
the fact that many FM receivers will revert to monaural opera-
tion with a greatly improved signal to noise ratio when signal
strengths become weak. It is also unreasonable to expect the
driver of an automobile to fiddle with panel controls in order to
improve the threshold tracking. The patented adaptive noise
threshold in the SSM2000 solves these problems, maintaining
the transparency of the noise reduction system under most op-
eration conditions while not introducing cumbersome end-user
controls.
SSM2000
VCF LOG (RECTIFIER)
8
AND FAST τ PEAK
DETECTOR
V_CC
35nA
20kΩ 20kΩ
20kΩ
A2
20kΩ
600Ω 7.2kΩ
20kΩ 20kΩ
V_{EE 20kΩ 20kΩ}
A1
V_EE
VCA LOG (RECTIFIER)
V_EE
AND FAST τ PEAK
DETECTOR + OP AMP
20kΩ
A3
20kΩ
VCF
CONTROL
TO VCA
CONTROL
11
VCF TC
1µF
13
14 15
10
12
DEF
ATCAP
VCA TC
0.22µF
3.3µF
REV. 0
Figure 24. Schematic Diagram of the Adaptive Noise Threshold and Related Circuitry
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