4305-DEMO THAT Corporation, 4305-DEMO Datasheet - Page 10

• Manufacturer Part Number: 4305-DEMO
• Description: Audio Modules & Development Tools Low-Cost Dynamics Processor Demo Board
• Manufacturer: THAT Corporation

Specifications of 4305-DEMO

• Description/function: Audio DSPs
• Operating Supply Voltage: 15 V
• Product: Audio Modules
• For Use With/related Products: THAT4305
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Page 10 of 20
Compressors with Thresholds
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
dynamic range of the applied signal. While this is
fine for companding systems, it is impractical for an
effects 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 Figure
12, which offers control over the threshold of
compression and gain offset (or "makeup gain") in
addition to the ratio.
model a compressor with more of the features one
would expect in an effects compressor/limiter. This
model has a threshold adjustment (T
(set by the ideal diode), a means to vary the
sidechain gain (k), and a make-up gain adjustment
(A
variables are expressed in decibels, in keeping with
the simple “log math” of AN101A.
the threshold setting,
threshold setting, the signal at the output of the ideal
diode threshold is 0 (dB), so
dB
). As with the previous equations, all these
The compressor of Figure 6 and expander of
In Figure 12, we've extended this approach to
As with all the previous figures,
Out
For input signals above the level determined by
In
So, for signals above this level,
G
Substituting yields
Out
For input signals below the level set by the
G
Out
dB
dB
dB
dB
dB
dB
= −k (In
= A
> T
= In
= In
= In
dB
dB
dB
dB
, thus,
dB
.
dB
+ G
− k (In
+ A
− L.M. − T.A.) + A
dB
dB
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
.
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
dB
− L.M. − T.A.) + A
dB
), a threshold
V
.
V
.
applied 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).
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
temperature (+0.33%/ºC), the level match point will
vary slightly with temperature unless the applied
voltage is appropriately compensated.
behavior yields the transfer function shown in
Figure 13. As predicted by the above equations, this
results in a 4:1 compression ratio above the thresh-
old of -20 dB (relative to the RMS detector's 0 dB
reference level. The output level increases by 10 dB
over a 40 dB change in input level.
In
Figure 13. Transfer function of feedforward compressor
Figure 12. Feedforward compressor with threshold (T),
dB
In the circuit of Figure 12, static gain offset is
If we let T
-100
-10
-20
-30
-40
-50
-60
-70
-80
-90
20
10
0
-100
In
+
-
RMS
T
dB
S
dB
gain (A), and ratio (k) adjustments
-80
THAT4305 Pre-trimmed Analog Engine®
Diode
dB
Ideal
=20, A
-60
This is especially convenient
-k
dB
=0 dB, and k=0.75, this
dB In
-40
G
A
dB
S
dB
-20
Out
In
0
Out
20
dB