THAT4305 Datasheet, PDF (9/20 Pages) List of Unclassifed Manufacturers – Pre-trimmed Analog Engine® IC

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_dB = In_dB + k(L. M._dB −Out_dB) + A_dB , or
Out_dB = In_dB + k × L. M._dB −k × Out_dB + A_dB , or
Out_dB
=
In_dB + k×L.M._dB + A_dB
(1+ k)
.
In_dB
G_dB
A _dB
-k
Out _dB
RMS
Out _dB
Σ
+
-
L.M._dB
Figure 6. Feedback compressor with level match
Similarly, for the expander shown in Figure 7, we
can state that:
Out_dB = In_dB − A_dB + G_dB
G_dB = k(In_dB − L. M._dB ) . Thus:
Out_dB = (1 + k)In_dB − k × L. M._dB − A_dB
In_dB
RMS
In_dB
+
Σ
-
-A_dB
k
L.M._dB
G_dB
Out _dB
Figure 7. Feedforward expander with level match.
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 A_dB and LM_dB. 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 LM_dB, but
fixed A_dB and k. Note that as LM_dB is reduced, all
output levels decrease, and vice-versa.
20
LM= -40
0
-20
-40
-60
k=0
-80
k=1
k=2
-100
k=3
-120
-120 -100 -80 -60 -40 -20 0 20
dB In
Figure 8. Compressor input-output transfer
characteristics with varying k.
k=1
0
-20
-40
L.M.= -10
-60
L.M.= -20
L.M.= -30
L.M.= -40
-80
-120 -100 -80 -60 -40 -20 0 20
dB In
Figure 9. Compressor input-output transfer
characteristics with varying LM_dB
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