that4320 THAT Corporation, that4320 Datasheet - Page 12

• Manufacturer Part Number: that4320
• Description: Pre-trimmed Low-voltage Low-power Analog Engine Dynamics Processor Ic
• Manufacturer: THAT Corporation

Page 12
pated dc leakage currents which can spoil the per-
formance of the 4320's VCA and RMS detector.
Noise Reduction (Compander) Configurations
systems, particularly within battery-operated de-
vices. In these applications, one 4320 is configured
for use as a compressor to condition audio signals
before feeding them into a noisy channel. A second
4320, configured as an expander, is located at the
receiver end of the noisy channel. The compressor
increases gain in the presence of low-level audio sig-
nals, and reduces its gain in the presence of
high-level audio signals. The expander works in op-
posite, complementary fashion to restore the origi-
nal signal levels present at the input of the
compressor.
increases signal levels, bringing them up above the
noise floor of the noisy channel. At the receiving 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
and operates from such a wide range of supply voltages
that it is suitable for a wide variety of dynamics pro-
cessing applications. Chief among these are wireless
companding systems. For this datasheet, we show the
part in a simple 2:1 companding noise reduction sys-
tem that performs as well or better than any analog
companding solution on the market today. Many other
configurations of the 4320 are possible, but are not
shown here. THAT intends to publish additional cir-
cuits in forthcoming applications notes. Please check
with THAT’s applications engineering department to
see if your application has been covered yet, and for
personalized assistance with specific designs.
The encoder
compressor. The encoder in a wireless companding
system is located in the transmitter and generally oper-
ates from a battery supply.
tions of the battery supply, the encoder VCA gain is off-
set by 6dB via the ratio of R
another 6 dB of static gain is injected at the control
A primary use of the 4320 is for noise reduction
During low-level audio passages, the compressor
During high-level audio passages, the compres-
The 4320 includes so many useful building blocks
Figure 12 shows a simple 2:1 encoder or feedback
To optimize signal levels within the voltage limita-
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
4
to R
1
. Additionally,
Copyright © 2007, THAT Corporation
Applications
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
in this action, a well-designed compander will mask
the noise floor with the signal itself.
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;
and the part contains enough opamps to provide
many options in signal conditioning. All these fea-
tures mean that the 4320 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.
port opamp, via V
quired to produce 6 dB of static gain. Since V
-72 mV, a gain of -1/2 will create the required 36 mV.
Because the PTAT generator voltage tracks in tempera-
ture with the VCA gain control constant, this gain will
be stable over temperature.)
pre-emphasis network at the input of the VCA (R3/C9)
that ultimately provides 20 dB of gain at 20 kHz. Its
lower corner frequency is at approximately 1.5 kHz
(f
clude a clipper somewhere in the signal path to prevent
overmodulation of the RF channel. The optional
anti-parallelled diodes D
function in this circuit, and should be placed ahead of
the 20 kHz Butterworth low-pass filter composed of
OA4 and its surrounding components. This placement
helps reduce “spectral splatter” that results from mo-
mentary clipping. What clipping takes place is limited
in duration to transients only, since the encoder will
eventually reduce its gain to below the clip point.
1
); the upper corner is near 15 kHz (f
The 4320 was designed to facilitate the design of
This
Companding noise reduction encoders often in-
The output of the low-pass filter is the output of the
wide
THAT4320 Pre-trimmed Low-voltage Low-power
encoder
range
Analog Engine® Dynamics Processor IC
PTAT
and R
(level-independent)
includes
1
and D
7
. (A 36 mV dc offset is re-
2
, can perform that
a
2
).
high-frequency
systems,
PTAT
~