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PDF LM1894 Data sheet ( Hoja de datos )
| Número de pieza | LM1894 | |
| Descripción | Dynamic Noise Reduction System DNR | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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December 1994
LM1894
Dynamic Noise Reduction System DNR®
General Description
The LM1894 is a stereo noise reduction circuit for use with
audio playback systems. The DNR system is
non-complementary, meaning it does not require encoded
source material. The system is compatible with virtually all
prerecorded tapes and FM broadcasts. Psychoacoustic
masking, and an adaptive bandwidth scheme allow the DNR
to achieve 10 dB of noise reduction. DNR can save circuit
board space and cost because of the few additional compo-
nents required.
Features
n Non-complementary noise reduction, “single ended”
n Low cost external components, no critical matching
n Compatible with all prerecorded tapes and FM
n 10 dB effective tape noise reduction CCIR/ARM
weighted
n Wide supply range, 4.5V to 18V
n 1 Vrms input overload
Applications
n Automotive radio/tape players
n Compact portable tape players
n Quality HI-FI tape systems
n VCR playback noise reduction
n Video disc playback noise reduction
Typical Application
*R1 + R2 = 1 kΩ total.
See Application Hints.
Order Number LM1894M or LM1894N
See NS Package Number M14A or N14A
FIGURE 1. Component Hook-Up for Stereo DNR System
DNR® is a registered trademark of National Semiconductor Corporation.
The DNR® system is licensed to National Semiconductor Corporation under U.S. patent 3,678,416 and 3,753,159.
Trademark and license agreement required for use of this product.
© 1999 National Semiconductor Corporation DS007918
DS007918-1
www.national.com
1 page  
External Component Guide
(Figure 1)
Component Value
Purpose
C1
0.1 µF–
May be part of power supply,
100 µF
or may be added to suppress
power supply oscillation.
C2, C13
1 µF
Blocks DC, pin 2 and pin 13
are at DC potential of VS/2.
C2, C13 form a low frequency
pole with 20k RIN.
C14
C3, C12
C4, C11
C5
25 µF–
100 µF
0.0033 µF
1 µF
0.1 µF
Improves power supply
rejection.
Forms integrator with internal
gm block and op amp. Sets
bandwidth conversion gain of
33 Hz/µA of gm current.
Output coupling capacitor.
Output is at DC potential of
VS/2.
Works with R1 and R2 to
attenuate low frequency
transients which could disturb
control path operation.
C6 0.001 µF Works with input resistance of
pin 6 to form part of control
path frequency weighting.
C8
L8, CL
C9
0.1 µF
4.7 mH,
0.015 µF
0.047 µF
Combined with L8 and CL
forms 19 kHz filter for FM
pilot. This is only required in
FM applications (Note 9).
Forms 19 kHz filter for FM
pilot. L8 is Toko coil
CAN-1A185HM (Notes 8, 9).
Works with input resistance of
pin 9 to form part of control
path frequency weighting.
C10
1 µF
Set attack and decay time of
peak detector.
R1, R2
1 kΩ
Sensitivity resistors set the
noise threshold. Reducing
attentuation causes larger
signals to be peak detected
and larger bandwidth in main
signal path. Total value of R1
+ R2 should equal 1 kΩ.
R8
100Ω
Forms RC roll-off with C8.
This is only required in FM
applications.
Note 8: Toko America Inc., 1250 Feehanville Drive, Mt. Prospect IL 60056
Note 9: When FM applications are not required, pin 8 and pin 9 hook-up as
follows:
DS007918-6
Circuit Operation
The LM1894 has two signal paths, a main signal path and a
bandwidth control path. The main path is an audio low pass
filter comprised of a gm block with a variable current, and an
op amp configured as an integrator. As seen in Figure 2, DC
feedback constrains the low frequency gain to AV = −1.
Above the cutoff frequency of the filter, the output decreases
at −6 dB/oct due to the action of the 0.0033 µF capacitor.
The purpose of the control paths is to generate a bandwidth
control signal which replicates the ear’s sensitivity to noise in
the presence of a tone. A single control path is used for both
channels to keep the stereo image from wandering. This is
done by adding the right and left channels together in the
summing amplifier of Figure 2. The R1, R2 resistor divider
adjusts the incoming noise level to open slightly the band-
width of the low pass filter. Control path gain is about 60 dB
and is set by the gain amplifier and peak detector gain. This
large gain is needed to ensure the low pass filter bandwidth
can be opened by very low noise floors. The capacitors be-
tween the summing amplifier output and the peak detector
input determine the frequency weighting as shown in the
typical performance curves. The 1 µF capacitor at pin 10, in
conjunction with internal resistors, sets the attack and decay
times. The voltage is converted into a proportional current
which is fed into the gm blocks. The bandwidth sensitivity to
gm current is 33 Hz/µA. In FM stereo applications at 19 kHz
pilot filter is inserted between pin 8 and pin 9 as shown in
Figure 1.
Figure 3 is an interesting curve and deserves some discus-
sion. Although the output of the DNR system is a linear func-
tion of input signal, the −3 dB bandwidth is not. This is due to
the non-linear nature of the control path. The DNR system
has a uniform frequency response, but looking at the −3 dB
bandwidth on a steady state basis with a single frequency in-
put can be misleading. It must be remembered that a single
input frequency can only give a single −3 dB bandwidth and
the roll-off from this point must be a smooth −6 dB/oct.
A more accurate evaluation of the frequency response can
be seen in Figure 4. In this case the main signal path is fre-
quency swept, while the control path has a constant fre-
quency applied. It can be seen that different control path fre-
quencies each give a distinctive gain roll-off.
Psychoacoustic Basics
The dynamic noise reduction system is a low pass filter that
has a variable bandwidth of 1 kHz to 30 kHz, dependent on
music spectrum. The DNR system operates on three prin-
ciples of psychoacoustics.
1. White noise can mask pure tones. The total noise energy
required to mask a pure tone must equal the energy of the
tone itself. Within certain limits, the wider the band of mask-
ing noise about the tone, the lower the noise amplitude need
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