SSM2000

REV. 0

–14–

Volume Control vs. Road Speed

In automotive applications the external volume control port

(Pin 7) can also be used to increase the volume as road noise

increases. The digital speedometer information available in

most vehicles may be filtered or D/A converted and sent to

the SSM2000 to control the volume. Figure 31 illustrates a

SSM2000 audio volume control system using digital speedom-

eter data.

SSM2000

7

6

+5V

–5V

19

5

20

VCA

CONTROL

PORT

D/A

CONVERTER

MPH

SPEEDOMETER

DATA

Figure 31. Volume Control vs. Road Speed Block Diagram

Using the SSM2000 in a Post-Volume Control Application

The SSM2000 is capable of operation for limited reasonable

changes in input levels, because of the internal adaptive noise

threshold detector. However, this detector is limited in range

for optimal performance and therefore external circuitry is re-

quired if the prevolume control line level is to be changed by

more than –20 dB. Figure 32 illustrates a circuit that alters the

range of the noise threshold.

18k

Ω

750k

Ω

120k

Ω

+5V

–5V

1/2

OP292

22µF

12

SSM2000

14

6

+5V

–5V

19

5

20

1.3k

Ω

1/2

OP292

20k

Ω

1.2k

Ω

3.3µF

10k

Ω

Figure 32. A Schematic to Use the SSM2000 in a Post-

Volume Control Application

PRECAUTIONS, LIMITATIONS, AND OBSERVATIONS

Operating Temperature

Under light loading conditions HUSH runs relatively cool, typi-

cally only 4

°C over ambient room temperature. If HUSH is

hot, then check to make sure that the capacitive and resistive

loading requirements are not being exceeded.

Operating Signal Amplitude and Frequency Range

The output signal range of the SSM2000 is set internally

(3.67 V p-p @ 0.1 THD) and will not increase even if the power

supply rails are raised. The frequency range is set by the input

ac coupling capacitors (< 10 Hz) and by the VCF which is inter-

nally limited to 35 kHz.

Excess External Noise

Even though the SSM2000 is a noise reduction IC, excessively

high noise floor amplitudes may result in no noise reduction.

Common causes of this condition are high audio volume or very

poor reception. In this case the VCF and the VCA will remain

at their maximum settings and the audio signal will simply pass

through HUSH.

Prefiltering Out-of-Audio Band Tones

The most common cause of SSM2000 operational error is due

to large amplitude fixed-frequency tones. When SSM2000

detector’s are exposed to large fixed-frequency tones, they rec-

ognizes these types of tones as signal and automatically adjust

the VCA and VCF to allow this frequency to pass through to the

output along with the underlying noise. The following is a list

of typical fixed-frequency tones which might cause this type

of HUSH behavior: switching power supply noise (20 kHz–

200 kHz), sampling frequency in digitized music (44 kHz), PC

monitor scan frequency (30 kHz), microprocessor operating fre-

quency or submultiple, and FM stereo subcarrier frequency

(19 kHz). Figure 33 is a scope photo of a spectrum analyzer

output showing the 19 kHz FM stereo subcarrier frequency.

So, if your application will encounter any of these types of noise

sources, then a preconditioning filter for the internal detector is

necessary.

20Hz

20kHz

Figure 33. Scope Photo of the 19 kHz FM Subcarrier

Frequency

Figures 34 and 35 illustrate two different approaches to filtering

these undesired tones. If the interfering tone frequency is less

than 20 kHz (e.g., TV-15.625 kHz), a Twin-T Hi-Q notch filter

as shown in Figure 34 should be used. The notch filter will at-

tenuate the tone while having a minimal effect on the audio

band signal. For interfering tones above 20 kHz, a general

3-pole low-pass filter is recommended, such as the Butterworth

filter shown in Figure 35. The input of the filter should always

be connected to the output, SUM OUT. Filtering the SUM

OUT signal will not affect audio quality because the SUM

OUT signal is not part of the main audio path. However,

overaggressive filtering may remove noise and signal that is

necessary to set the VCA control, VCF control, and noise

threshold levels. The OP292 or OP275 shown in the example

filter circuit are high performance, low cost amplifiers suitable

for this application.