SSM2000

REV. 0

–10–

Noise is most objectionable at high frequencies (3 kHz–8 kHz).

Therefore, only the VCF detector output signal is used to deter-

mine the adaptive noise threshold. Figures 25a–c, are a series of

circuits which illustrate how the noise threshold is derived. It is

important to remember that the signal that is applied to the

noise threshold detector circuitry has already been rectified and

averaged. Hence, the lowest potential over a set period of time

corresponds to the noise floor. Node A corresponds to the out-

put of the VCF Detector, and Node B is

proportional to the adaptive noise threshold.

Figure 25a illustrates the condition where the potential at Node

A is above the maximum possible potential for Node B. The

maximum noise threshold is set by the potential placed on Pin

14. If the potential at Node B rises to a diode drop above Pin

14, then Q1’s emitter-base diode turns on and clamps Node B.

This is represented by the current flow I2. However, if Node B

has not yet risen to the maximum noise threshold level, then

both Q1 and Q2 are OFF and the 35 nA current source is

charging C1 (A.T. CAP). The auto threshold capacitor should

be a ceramic or equivalent low leakage capacitor, because the

charging current could otherwise be of similar amplitude to the

capacitor leakage current.

A

20k

Ω 20kΩ

MAXIMUM

NOISE THRESHOLD

LEVEL

35nA

14

15

Q1

Q2

C1

0.22µF

MINIMUM

NOISE

THRESHOLD

LEVEL

B

I2

I1

t

Q1 CLAMPS NODE

B TO A DIODE DROP

ABOVE THE

POTENTIAL AT PIN 14

I1 IS

CHARGING

C1

B

VOLTS

A

MAX

MIN

Figure 25a. Condition Where the Actual Noise Threshold

Is Above the Maximum Noise Threshold Level Setting

(Pin 14)

Figure 25b illustrates the condition where the potential at Node

A is between the maximum and minimum potentials for Node

B. When Node A falls below Node B, then the emitter-base

diode of Q2 turns ON causing Node B to follow Node A. Cur-

rent I2 illustrates how the discharge current from C1 and the

35 nA current source are directed through Q2. Q2 shuts OFF

the moment that Node A rises above Node B. This forces the

35 nA current source to begin charging C1 at a constant rate set

by the value of C1 at Pin 15.

A

20k

Ω 20kΩ

MAXIMUM

NOISE THRESHOLD

LEVEL

35nA

14

15

Q1

Q2

C1

0.22µF

MINIMUM

NOISE

THRESHOLD

LEVEL

B

t

I2 FLOWS DISCHARGING

C1 CAUSING NODE B TO

VOLTAGE FOLLOW NODE A

I1 IS

CHARGING

C1

B

VOLTS

A

MAX

MIN

I2

I1

Figure 25b. Condition Where the Noise Level Is Between

the Maximum and Minimum Threshold Settings

Figure 25c illustrates the condition where the potential at Node

A is below the minimum potential for Node B. In this case the

internal minimum noise potential causes a diode to turn ON.

This clamps the Node A potential to the minimum noise thresh-

old level. I1 represents the current flow in this condition. In

addition, the 35 nA flows through Q2’s emitter-base diode as

shown by I2.

A

20k

Ω 20kΩ

35nA

14

15

Q1

Q2

C1

0.22µF

MINIMUM

NOISE

THRESHOLD

LEVEL

B

C

MAXIMUM

NOISE THRESHOLD

LEVEL

I2

I1

t

I 1 AND I 2 FLOW CLAMPING NODE

B TO THE MINIMUM NOISE

THRESHOLD LEVEL

B

VOLTS

A

MAX

MIN

C

Figure 25c. Condition Where the Noise Level Is Below the

Minimum Noise Threshold Level Setting

Simply subtracting the noise threshold from the average VCF

HF control signal plus noise threshold and the average VCA

control signal plus noise threshold will yield the final VCF and

VCA control signal. This operation is accomplished with two

internal difference amplifiers.

Figures 26a–b shows the response of the detector that controls

the VCF bandwidth and VCA gain respectively. Both L IN and

R IN pins receive a 10 kHz tone burst. The lower trace of Fig-

ure 26a shows the control voltage to the VCF (Pin 11) and the

lower trace of Figure 26b shows the control voltage to the VCA

(Pin 12). Note the quick rise and slow fall times. This allows

fast adaptation to changed input signal conditions, while avoid-

ing pumping effects and other sonic artifacts.

100

90

10

0%

100mV

100ms

500mV

Figure 26a. VCF Control Voltage for a Tone Burst

100

90

10

0%

500mV

100mV

100ms

Figure 26b. VCA Control Voltage for a Tone Burst