THAT4305 Datasheet, PDF (10/20 Pages) List of Unclassifed Manufacturers – Pre-trimmed Analog Engine® IC

Page 10
THAT4305 Pre-trimmed Low-power Analog Engine
140
100
60
20
-20
-60
-100
-140
-60
k=0
k=1
k=2
k=3
-40
-20
0
dB In
Figure 10. Expander input-output transfer
characteristics with varying k.
Figures 10 and 11 show how the input-output
transfer characteristics of the expander of Figure 7
change while varying k (Fig 10) and LM (Fig 11). In
each case, all other parameters remain fixed.
Compressors with Thresholds
The compressor of Figure 6 and expander of
Figure 7 form the primary basis for the linear
companding systems used in many audio applica-
tions. However, they are limited in application, since
the compression ratio is linear over the entire dy-
namic range of the applied signal. While this is fine
for companding systems, it is impractical for an ef-
fects compressor, not least due to the way very
low-level signals -- including noise -- are raised in
gain by the compressor's actions. A more practical
approach to effects compressors is shown in Fig-
ure 12, which offers control over the threshold of
compression and gain offset (or "makeup gain") in
addition to the ratio.
In Figure 12, we've extended this approach to
model a compressor with more of the features one
would expect in an effects compressor/limiter. This
model has a threshold adjustment (T_dB), a threshold
(set by the ideal diode), a means to vary the
sidechain gain (k), and a make-up gain adjustment
(A_dB). As with the previous equations, all these vari-
ables are expressed in decibels, in keeping with the
simple “log math” of AN101A.
As with all the previous figures,
Out_dB = In_dB + G_dB
For input signals above the level determined by
the threshold setting,
In_dB > T_dB .
40
20
0
-20
-40
-60
-80
-100
-120
-60
L.M.= -10
L.M.= -20
L.M.= -30
L.M.= -40
-50 -40 -30 -20 -10 0
dB In
Figure 11. Expander input-output transfer
characteristics with varying LM_dB
So, for signals above this level,
G_dB = −k(In_dB − L. M.−T. A.) + A_V .
Substituting yields
Out_dB = In_dB − k(In_dB − L. M.−T. A.) + A_V .
For input signals below the level set by the
threshold setting, the signal at the output of the
ideal diode threshold is 0 (dB), so
G_dB = A_dB , thus,
Out_dB = In_dB + A_dB .
In the circuit of Figure 12, static gain offset is ap-
plied via a dc voltage summed into the sidechain
and applied to the VCA control port. This illustrates
an alternative method of varying VCA gain (different
from offsetting the V-I and I-V converting resistors as
mentioned earlier). This is especially convenient
when the level match must be varied by the user, as
with a front-panel control. Note, however, that since
the VCA gain scale factor varies slightly with temper-
ature (+0.33%/ºC), the level match point will vary
In_dB
RMS
IndB
+
S
Ideal
Diode
-k
-
T_dB
G_dB
S
A _dB
Out _dB
Figure 12. Feedforward compressor with threshold (T), gain
(A), and ratio (k) adjustments
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