SSM2000 Analog, SSM2000 Datasheet - Page 8

• Manufacturer Part Number: SSM2000
• Description: HUSH Stereo Noise Reduction System with Adaptive Threshold
• Manufacturer: Analog

SSM2000
Figure 19. A Frequency Amplitude Plot Illustrating the
Noise Reducing Effect of the VCF
The SSM2000 has been designed to minimize control feedthrough.
Control feedthrough may cause an audible output as the internal
control lines of the VCAs and VCFs are changed rapidly. This
feedthrough is the cause of many of the unpleasant artifacts
prevalent among noise reduction systems and is often due to
parasitic capacitance and mismatches within the IC. This speci-
fication is important for the VCA and VCF, but the VCF is the
most susceptible because it operates at constant dc gain. The
solution incorporated into the SSM2000 to reduce control feed-
through has been to convert from single-ended to full differen-
tial at the signal input and convert back again at the output
buffer. Because the audio path through the VCA and VCF are
fully differential, control feedthrough is determined purely by
mismatches with no systematic errors. The slight penalty to be
paid for a fully differential system, besides extra complexity, is
that the external VCF capacitors (1 nF recommended) now re-
quire two pins each instead of one. Careful attention to VCF
capacitor matching and layout symmetry reduces control
feedthrough to better than 40 dB through the signal path.
Downward Expander
After the audio signal passes through the VCF, it is then differ-
entially sent to the VCA. The VCA is characterized by a
downward expander transfer function as shown in Figure 20.
Attenuation begins at output levels below the internal threshold
at an effective rate of 2.2 dB/decade. Therefore, audible noise
is attenuated when source material is not present. Conversely,
when audio source levels are at or above the threshold, the VCA
is set to unity gain. Because of well documented psychoacoustic
effects, it is valid to assume that at high audio amplitudes audio
noise is being masked by the audio material. The control infor-
mation that is required for the downward expander to function
properly is provided by the internal VCA detector side-chain cir-
cuitry common to both the L and R channels.
Previous downward expander circuits have often used a higher
ratio than 2.2 dB/decade to provide impressive noise reduction
figures (50 dB or more) in the absence of signal information.
However, in most applications, such severe noise gating is to-
tally pointless, and actually exacerbates the effects of VCA con-
trol feedthrough, since control feedthrough is not attenuated as
the VCA shuts down. For this reason, the maximum VCA at-
tenuation in the SSM2000 is limited to 15 dB. In addition, the
“smooth” roll-off of the downward expander eliminates any
audible artifacts resulting from this type of noise reduction. In
contrast, many noise reduction system employ gates and com-
parators that result in dramatic shifts in attenuation. In these
types of noise reduction systems noise can be heard switching in
and out during periods of low volume audio near that attenua-
tion discontinuity. Telephone systems with noise gating are par-
ticularly prone to this type of behavior, because the noise level of
BYPASS FREQUENCY RESPONSE
MASKED NOISE
SIGNAL
AUDIO
FREQUENCY
0.5 dB
fc = 1kHz to 35kHz
FILTERED NOISE
6dB/OCTAVE
SLOPE
–8–
a poor quality connection is very near the gate’s switching level.
The Mute function (Pin 17) can override the VCA controls, as
shown in Figure 5. When mute is active (HIGH), it pulls the
VCA to maximum attenuation. An 85 dB professional quality
mute under worse case conditions can be expected over most of
the audible frequency range as shown in Figure 13. Mute over-
rides both the internal VCA control coming from the VCA De-
tector and the external VCA control port (Pin 7). The external
VCA control port is additive in nature to the internal VCA con-
trol signals; therefore, noise reduction and volume control may
occur simultaneously in the SSM2000. The VCA control port
allows the gain of the VCA to changed externally at approxi-
mately 22 mV/dB, where 150 mV is equal to 0 dB, as shown in
Figure 10a.
VCF Detector
Both the VCF and VCA detectors are amplitude detectors and
identical in every way. The applied input signals to the detec-
tors must be preconditioned for the detector circuitry to give
the information that is required by the L and R VCFs and
VCAs. Figure 21 is a schematic of the recommended precondi-
tioning filters for use with the VCA and VCF detectors.
Figure 21. The Recommended Signal Preconditioning
Filters for the VCA and VCF Detectors
The VCF detector is fed by 3 (L+R)/2 averaged input signal
processed by a three-pole high pass filter with a –15 dB point at
10 kHz. In Figure 22, the response of the VCF’s 3-pole filter is
shown. The VCF preconditioning filter performs two func-
tions: first, it eliminates the large amplitude, low frequency au-
dio which would otherwise mask the high frequency signals;
and second, it becomes increasingly sensitive throughout most
of the VCF’s frequency range of interest (660 Hz to 20 kHz),
compensating for the effect of most audio signals which typi-
cally decrease in amplitude as frequency increases.
Figure 20. Downward Expander Transfer function
–10
–20
–30
–40
–50
–60
–70
30
20
10
0
–70
SSM2000
–60
DETECTOR
DETECTOR
SUM OUT
INPUT
INPUT
–50
VCA
VCF
–40
10
8
9
–30
2.2nF
INPUT IN dB
FILTER IF NECESSARY
FIXED-FREQUENCY
–20
ADDITIONAL
1k
–10
22nF
2.2µF
0
510
10
22nF
20
30
REV. 0