SSM2000

REV. 0

–9–

FREQUENCY – Hz

0

–10

–70

1

100k

10

100

1k

10k

–30

–40

–50

–60

–20

VCA

VCF

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.

1.1µA

D1

Q4

Q3

VCF DET CAP

1µF

TO AUTO

THRESHOLD

AND DIFFERENCE

AMPLIFIER

RECTIFIED

AND LOG (x) OF

VCA DET IN

AND VCF DET IN

11

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.

ATCAP

0.22µF

AND FAST

DETECTOR

VCF LOG (RECTIFIER)

A3

20k

Ω

20k

Ω

20k

Ω

20k

Ω

20k

Ω

20k

Ω

A2

20k

Ω

20k

Ω

20k

Ω

20k

Ω

A1

600

Ω

7.2k

Ω

DEF

35nA

SSM2000

VCF TC

1µF

VCA TC

3.3µF

VCF

CONTROL

TO VCA

CONTROL

AND FAST

DETECTOR + OP AMP

VCA LOG (RECTIFIER)

14

10

12

15

11

13

8

Figure 24. Schematic Diagram of the Adaptive Noise Threshold and Related Circuitry