that4315s THAT Corporation, that4315s Datasheet - Page 6

• Manufacturer Part Number: that4315s
• Description: Low-power Analog Engine Ic
• 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.
a wide variety of companding noise reduction sys-
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
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
+
-
wireless
Out
Applications
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;
and the part contains enough opamps to provide
many options in signal conditioning. All these fea-
tures mean that the 4315 will support a wide range
of compander designs (and more), including simple
2:1
level-dependent systems with thresholds and varying
compression slopes, systems including noise gating
and/or limiting, and systems with varying degrees of
pre-emphasis and filtering in both the signal and de-
tector paths. Furthermore, much of this can be ac-
complished 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 ra-
tios, thresholds, and time constants.
can state that for Figure 2,
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
Out
Out
For the decoder shown in Figure 3, we can state
L.M.
+
-
wide
k L M
S
( . .
In
In
In
In k
Figure 3. Feedforward expander.
(
range
G
k L M Out
k
1
-A
( . .
L M
Out
k
. .
)
L M k
THAT4315S Pre-trimmed Low-voltage
A
. .
)
A
k
(level-independent)
Low-power Analog Engine® IC
)
Out
A
, Or
G
A
, Or
systems,
Out