Philips Semiconductors RF Communications Products
Product specification
Programmable analog compandor
NE/SA572
TEST CIRCUIT
1µF
22µF
2.2µF
V
1
(7,9)
6.8k
∆G
(5,11)
82k
1%
R
3
17.3k
+
100Ω
–15V
–
5Ω
= 10µF
BUFFER
1k
(4,12)
(8)
+
2.2µF
(2,14)
2.2k
(6,10)
+
270pF
NE5234
V
0
(1,15)
2.2µF
V
2
R
2
1%
3.3k
(3,13)
RECTIFIER
(16)
+
.1µF
22µF
+15V
AUDIO SIGNAL PROCESSING IC
COMBINES VCA AND FAST AT-
TACK/SLOW RECOVERY LEVEL
SENSOR
In high-performance audio gain control
applications, it is desirable to independently
control the attack and recovery time of the
gain control signal. This is true, for example,
in compandor applications for noise
reduction. In high end systems the input
signal is usually split into two or more
frequency bands to optimize the dynamic
behavior for each band. This reduces low
frequency distortion due to control signal
ripple, phase distortion, high frequency
channel overload and noise modulation.
Because of the expense in hardware, multiple
band signal processing up to now was limited
to professional audio applications.
With the introduction of the Signetics NE572
this high-performance noise reduction
concept becomes feasible for consumer hi fi
applications. The NE572 is a dual channel
gain control IC. Each channel has a
linearized, temperature-compensated gain
cell and an improved level sensor. In
conjunction with an external low noise op
amp for current-to-voltage conversion, the
VCA features low distortion, low noise and
wide dynamic range.
The novel level sensor which provides gain
control current for the VCA gives lower gain
control ripple and independent control of fast
attack, slow recovery dynamic response. An
attack capacitor C
A
with an internal 10k
resistor R
A
defines the attack time t
A
. The
recovery time t
R
of a tone burst is defined by
a recovery capacitor C
R
and an internal 10k
resistor R
R
. Typical attack time of 4ms for
the high-frequency spectrum and 40ms for
the low frequency band can be obtained with
0.1µF and 1.0µF attack capacitors,
respectively. Recovery time of 200ms can be
obtained with a 4.7µF recovery capacitor for
a 100Hz signal, the third harmonic distortion
is improved by more than 10dB over the
simple RC ripple filter with a single 1.0µF
attack and recovery capacitor, while the
attack time remains the same.
The NE572 is assembled in a standard
16-pin dual in-line plastic package and in
oversized SOL package. It operates over a
wide supply range from 6V to 22V. Supply
current is less than 6mA. The NE572 is
designed for consumer application over a
temperature range 0-70 The SA572 is
intended for applications from –40°C to
+85°C.
inherent low distortion, low noise and the
capability to linearize large signals, a wide
dynamic range can be obtained. The buffer
amplifiers are provided to permit control of
attack time and recovery time independent of
each other. Partitioned as shown in the block
diagram, the IC allows flexibility in the design
of system levels that optimize DC shift, ripple
distortion, tracking accuracy and noise floor
for a wide range of application requirements.
Gain Cell
Figure 1 shows the circuit configuration of the
gain cell. Bases of the differential pairs Q
1
-Q
2
and Q
3
-Q
4
are both tied to the output and
inputs of OPA A
1
. The negative feedback
through Q
1
holds the V
BE
of Q
1
-Q
2
and the
V
BE
of Q
3
-Q
4
equal. The following
relationship can be derived from the
transistor model equation in the forward
active region.
D
V
BE
Q
3
Q
4
+
D
BE
Q
1
Q
2
NE572 BASIC APPLICATIONS
Description
The NE572 consists of two linearized,
temperature-compensated gain cells (∆G),
each with a full-wave rectifier and a buffer
amplifier as shown in the block diagram. The
two channels share a 2.5V common bias
reference derived from the power supply but
otherwise operate independently. Because of
(V
BE
= V
T
I
IN
IC/IS)
October 7, 1987
4
PHILIPS [ NXP SEMICONDUCTORS ]
