that4315 THAT Corporation, that4315 Datasheet - Page 6

• Manufacturer Part Number: that4315
• Description: Pre-trimmed Low-voltage Low-power Analog Engine
• Manufacturer: THAT Corporation

Page 6
nal signal levels present at the input of the
compressor.
increases signal levels, bringing them up above the
noise floor of the noisy channel.
end, the expander reduces the signal back to it’s
original level, in the process attenuating the channel
noise.
sor decreases signal levels, reducing them to fit
within the headroom limits of the noisy channel.
The expander increases the signal back to its origi-
nal level. While the channel noise may be increased
by this action, in a well-designed compander, the
noise floor will be masked by the high-level signal.
suitable for a wide variety of dynamics processing
applications.
companding systems. For this datasheet, we show
the part in a simple 2:1 companding noise reduction
system that performs as well or better than any ana-
log companding solution on the market today. Many
other configurations of the 4315 are possible, but
are not shown here. THAT intends to publish addi-
tional circuits in forthcoming applications notes.
Please check with THAT’s applications engineering
department to see if your application has been cov-
ered yet, and for personalized assistance with spe-
cific designs.
The mathematics of log based
companding systems
Mathematics of Log Based Dynamics Processors, we
can state that for Figure 2,
In
During low-level audio passages, the compressor
During high-level audio passages, the compres-
The 4315 was designed to facilitate the design of
The 4315 provides the basic building blocks
Using the principles described in AN101A, The
Figure 2. Feedback compressor.
G
Chief
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
among
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
-k
A
these
At the receiving
L.M.
are
S
Copyright © 2009, THAT Corporation
+
-
wireless
Out
Applications
a wide variety of companding noise reduction sys-
tems. The RMS detector responds accurately over a
wide range of levels; the VCA responds accurately to
a wide range of gain commands; the detector output
and the VCA control input are fully configurable. All
these features mean that the 4315 will support a
wide range of compander designs (and more), in-
cluding simple 2:1 wide range (level-independent)
systems, level-dependent systems with thresholds
and varying compression slopes, systems including
noise gating and/or limiting, and systems with vary-
ing degrees of pre-emphasis and filtering in both the
signal and detector paths.
this can be accomplished by extensively conditioning
the control voltage sidechain rather than the audio
signal itself. The audio signal can pass through as
little as one VCA and one opamp, and still support
multiple ratios, thresholds, and time constants.
and A is any static gain or attenuation (in dB) ap-
plied to the signal. We can also state that:
referred to as level match point) of the detector, and
k is the gain of the sidechain. Substituting the sec-
ond equation into the first,
that:
In
Out
Where G is the VCA’s control port gain in dB,
G
Where L.M. is the zero dB reference point (often
Out
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Out
For the decoder shown in Figure 3, we can state
Out
G
L.M.
+
-
=
=
k L M
k In L M
S
=
=
=
=
=
(
( . .
In
In
In
In
In k
+ ×
-
+
+
+
+
Figure 3. Feedforward expander.
(
G
k L M Out
k
1
-A
A
-
. .), Thus:
+
( . .
L M
×
+
Out
k
+ , And that:
. .
)
L M k
G
A
. .
THAT4315 Pre-trimmed Low-voltage
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Low-power Analog Engine® IC
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Furthermore, much of
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