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Document 600067 Rev 00
Page 9
point) from that of the detector, and k is the gain of
the sidechain.
Substituting the second equation into the first,
Out
In
k L M
Out
A
dB
dB
dB
dB
dB
=+
−
+
(.
.
)
,or
Out
In
k L M
k Out
A
dB
dB
dB
dB
dB
=+ ×
− ×
+
..
,or
OutdB
In
k L M
A
k
dB
dB
dB
=
+×
+
+
..
()
1
.
Similarly, for the expander shown in Figure 7, we
can state that:
Out
In
A
G
dB
dB
dB
dB
=−
+
Gk In
L M
dB
dB
dB
=−
(.
.
) . Thus:
Out
k In
k L M
A
dB
dB
dB
dB
=+
− ×
−
()
.
.
1
In both Figures 6 and 7, we show two ways to ad-
just the point at which input and output levels of the
compressor or expander become equal. These are
a) adjusting the 0 dB reference voltage level of the
RMS detector by setting its voltage-to current con-
verting resistor (R1 in Figure 2), and b) varying the
static signal-path gain offset (AdB). For the 4305, the
best practice is to achieve level match via a combina-
tion of setting the RMS detector's voltage-to-current
conversion, and offsetting the signal-path gain. Sig-
nal-path gain can be offset by altering the values of
the VCA's voltage-to-current and current-to-voltage
resistors. Both of these approaches have the advan-
tage of being temperature-independent.
Figure 8 shows the behavior of the compressor
of Figure 6 with varying sidechain gain k for a con-
stant AdB and LMdB. Note that as k increases, the
compression ratio increases, and the dynamic range
of output levels decreases. Low input signal levels
(including noise) are increased, and high input sig-
nal levels are decreased.
Figure 9 shows how the
same compressor behaves with varying LMdB, but
fixed AdB and k. Note that as LMdB is reduced, all
output levels decrease, and vice-versa.
-120
-100
-80
-60
-40
-20
0
20
dB In
-80
-60
-40
-20
0
L.M.= -10
L.M.= -20
L.M.= -30
L.M.= -40
k=1
Figure 9. Compressor input-output transfer
characteristics with varying LMdB
-120
-100
-80
-60
-40
-20
0
20
dB In
-120
-100
-80
-60
-40
-20
0
20
k=0
k=1
k=2
k=3
LM= -40
Figure 8. Compressor input-output transfer
characteristics with varying k.
IndB
RMS
G
-A
k
Out
In
L.M.
Σ
+
-
dB
dB
dB
dB
dB
dB
dB
dB
Figure 7. Feedforward expander with level match.
OutdB
RMS
G
A
-k
Out
In
L.M.
Σ
+
-
dB
dB
dB
dB
dB
Figure 6. Feedback compressor with level match
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