SSM2000

REV. 0

–15–

R

2

1k

Ω

2.2µF

SSM2000

8

R

1/2

OP275

R

R = 100k

Ω

C = 100pF

10k

Ω

2

3

1

C

2C

C

9

1k

Ω

VCF

DETECTOR

INPUT

SUM OUT

VCA

DETECTOR

INPUT

10

2.2nF

22nF

510

Ω

fc =

1

2

22nF

Figure 34. A Hi-Q Twin-T Notch Filter with the Center

Frequency at 15.625 kHz

2.2µF

SSM2000

8

1k

Ω

1/2

OP275

1k

Ω

2

3

1

C

0.033µF

0.022µF

9

1k

Ω

VCF

DETECTOR

INPUT

SUM OUT

VCA

DETECTOR

INPUT

10

2.2nF

22nF

510

Ω

0.056µF

1k

Ω

22nF

Figure 35. A Three-Pole Butterworth Filter

Channel-to-Channel Separation

The SSM2000 has a 60 dB of channel separation specification

at 1 kHz. As shown in Figure 36, the Left input to the R VCF

node and the Right input to the L VCF node are tied together

with 50 k

Ω of resistance, and tied to ground with 2.5 kΩ. If the

Left and Right ac coupling capacitors were not present this

would results in a dc separation of around –27 dB. However,

the inputs of the SSM2000 are usually ac coupled; therefore the

low frequency channel separation is a function of the value of

the ac coupling capacitors and the theoretical dc separation.

The higher the value of the ac coupling capacitor, the better the

channel separation will be. For example, using 10

µF ac cou-

pling capacitors will result in 32 dB of channel separation at

around 6 Hz–8 Hz, improving at a rate of 20 dB/decade, till it

reaches a maximum –60 dB. Therefore, to achieve 60 dB of

channel separation at 75 Hz and above, 47

µF capacitors are

recommended.

6.8µF

L

R

2.5k

Ω

470pF

TO L VCF

10k

Ω

6.8µF

20k

Ω

20k

Ω

TO R VCF

TO DETECTORS

10k

Ω

2.5k

Ω

Figure 36. The SSM2000’s Input Impedance Network

EVALUATING THE SSM2000

Because the SSM2000 is a dynamic system, it will respond to

test signals that are applied during the evaluation procedures.

Without thorough understanding of how the HUSH noise

reduction system works, attempts to apply standard test proce-

dures can produce misleading results. The following paragraphs

will present evaluation techniques to avoid many of these com-

mon evaluation pitfalls.

The SSM2000 has been designed as a stereo system. As a result

the input to the detector circuits is the sum of both the left and

right inputs. Monaural signals that used during evaluation

should be applied to both inputs, because the detectors’ input

requires both the left and right signal. Otherwise, the control

levels from the VCA detector, VCF detector, and the Adaptive

Noise Threshold will be about one-half the necessary amplitude

for proper evaluation results.

The SSM2000 has exceptional tolerance for varying line levels

with noise reducing capability between line levels of 100 mV

rms to 1 V rms. However, the IC has been optimized for an in-

dustry standard input line level of 300 mV rms. Audio line lev-

els not equal to 300 mV rms will exhibit slightly degraded

specifications (e.g., distortion). If the line levels are outside of

the 100 mV rms to 1 V rms range, then audio output may appear

to have no noise reduction or to be muted. The circuit shown

in Figure 32 will enable noise reduction for very low line level

audio.

THD measurement instruments typically sweep a sine wave

across the audio spectrum. The normal SSM2000 response to

this type of signal results in the automatic noise threshold detec-

tor slowly rising to its maximum value; the VCA detector at

maximum value, the VCF detector will initially be closed at low

frequencies and will gradually open as the frequency is in-

creased. When the testing signal reaches about 500 Hz, it will

begin to be attenuated by the “closed” VCF. This attenuation

will reach at maximum of about 1 dB at 660 Hz for recom-

mended line levels, at which point the VCF detector begins to

open up. However, all these conditions and characteristics are

not representative of a normal music signal. To properly mea-

sure THD, it is necessary to activate HUSH Defeat. THD

measurements in DEFEAT mode give accurate results because

it does not remove the VCA and VCF from the signal chain,

rather it sets there values to unity (VCA) and maximum fre-

quency (VCF).

The bandwidth of the SSM2000, as stated before, is limited to

35 kHz because of the VCF capacitors. This bandwidth limita-

tion must be taken into account when the slew rate of the

SSM2000 is measured. The simplest method to measure slew

rate is to remove the VCF capacitors from the circuit, and take

the slew rate measurement while in DEFEAT mode. If the

VCF capacitors are not removed, then the audio signal is limited

by the VCF’s bandwidth and not by the overall slew rate.

One standard of the SSM2000’s audio quality is the characteris-

tics of the attack and release times of the VCA and VCF. The

attack and release times of the VCA and VCF are controlled by

1.1

µA current sources which charge the VCA Time Constant

capacitor and VCF Time Constant capacitor. The attack and

released times are typically measured from the VCA’s and

VCF’s fully closed position (e.g., VCA—maximum attenuation,