SSM2120 Datasheet, PDF (9/12 Pages) Analog Devices – Dynamic Range Processors/Dual VCA

Figures 13a–c show the filter’s 3 dB frequency response with the
threshold potentiometer at V+, centered, and V–. Data was
taken by applying a 300 Hz signal to the wideband detector and
a 20 kHz signal to the high-frequency band detector simultane-
ously. These figures correspond to filter characteristics for
50 dB, 70 dB and 90 dB dynamic range program source
material, respectively. The system could thus treat signals from
anything ranging from 1/4" magnetic tape to high performance
compact disc players.
Note that in Figure 13a the control circuit is designed so that
the minimum cutoff frequency is about 1 kHz. This occurs as
the control circuit detects the noise floor of the source material.
Dynamic filtering limits the signal bandwidth to less than 1 kHz
unless enough highs are detected in the signal to cover the noise
floor in the mid- and high frequency range. In this case the filter
opens to pass more of the audio band as more highs are detected.
The filter’s bandwidth can extend to 50 kHz with a nominal
signal level at the input. At other signal levels with varying high
frequency content, the filter will close to the required band-
width. Here, noise outside the band is removed while the
perceived noise is masked by other signals within the band.
Even in this system, however, a certain amount of mid- and high
frequency components will be lost, especially during transients
at very low signal levels. This circuit does not address low
frequency noise such as “hum” and “rumble.”
SSM2120/SSM2122
20
50.6
10
50.6 50.6
0
50.6 50.6 50.6
–10
17.8 26 26 26
–20
6 8.3 11.7 11.7 11.7
–30
1.9 2.75 3.9 5.5 5.5 5.5
–40
1.0 1.0 1.2 1.7 2.4 2.4 2.4
–50
1.0 1.0 1.0 1.0 1.0 1.1 1.1 1.1
–50 –40 –30 –20 –10 0 10 20
WIDEBAND SIGNAL LEVEL – dB
a. V_THRESH at V+
20
50.6
10
50.6 50.6
0
50.6 50.6 50.6
–10
50.6 50.6 50.6 50.6
–20
48 49.2 49.2 49.2 49.2
–30
15.1 22 22 22 22 22
–40
4.9 7.1 10 10 10 10 10
–50
1.5 2.2 3.1 4.2 4.2 4.2 4.2 4.2
–50 –40 –30 –20 –10 0 10 20
WIDEBAND SIGNAL LEVEL – dB
b. V_THRESH Centered
20
50.6
10
50.6 50.6
0
50.6 50.6 50.6
–10
50.6 50.6 50.6 50.6
–20
50.6 50.6 50.6 50.6 50.6
–30
50.6 50.6 50.6 50.6 50.6 50.6
–40
40 41 41 41 41 41 41
–50
12.3 17.3 17.8 17.8 17.8 17.8 17.8 17.8
–50 –40 –30 –20 –10 0 10 20
WIDEBAND SIGNAL LEVEL – dB
c. V_THRESH at V–
Figure 13. 3 dB Filter Response
REV. C
–9–