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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, andan 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 components 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
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.
DS007918-1
*R1+R2=1kΩtotal.
See Application Hints.
Order Number LM1894M or LM1894N
See NS Package Number M14A or N14A
FIGURE 1. Component Hook-Up for Stereo DNR System
December 1994
LM1894 Dynamic Noise Reduction System DNR
© 1999 National Semiconductor Corporation DS007918 www.national.com
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage 20V
Input Voltage Range, V
pk
VS/2
Operating Temperature (Note 2) 0˚C to +70˚C
Storage Temperature −65˚C to +150˚C
Soldering Information
Dual-In-Line Package
Soldering (10 seconds) 260˚C
Small Outline Package
Vapor Phase (60 seconds) 215˚C
Infrared (15 seconds) 220˚C
See AN-450 “Surface Mounting Methods and Their Effect
on Product Reliability” for other methods of soldering
surface mount devices.
Note 1: “Absolute Maximum Ratings” indicate limits beyond which damage
to the device may occur. Operating Ratings indicate conditions for which the
device is functional, but do not guarantee specific performance limits.
Electrical Characteristics
V
S
=
8V, T
A
=
25˚C, V
IN
=
300 mV at 1 kHz, circuit shown in
Figure 1
unless otherwise specified
Parameter Conditions Min Typ Max Units
Operating Supply Range 4.5 8 18 V
Supply Current V
S
=
8V 17 30 mA
MAIN SIGNAL PATH
Voltage Gain DC Ground Pin 9, (Note 3) −0.9 −1 −1.1 V/V
DC Output Voltage 3.7 4.0 4.3 V
Channel Balance DC Ground Pin 9 −1.0 1.0 dB
Minimum Balance AC Ground Pin 9 with 0.1 µF 675 965 1400 Hz
Capacitor, (Note 3)
Maximum Bandwidth DC Ground Pin 9, (Note 3) 27 34 46 kHz
Effective Noise Reduction CCIR/ARM Weighted, (Note 4) −10 −14 dB
Total Harmonic Distortion DC Ground Pin 9 0.05 0.1
%
Input Headroom Maximum V
IN
for 3%THD 1.0 Vrms
AC Ground Pin 9
Output Headroom Maximum V
OUT
for 3%THD VS− 1.5 Vp-p
DC Ground Pin 9
Signal to Noise BW=20 Hz–20 kHz, re 300 mV
AC Ground Pin 9 79 dB
DC Ground Pin 9 77 dB
CCIR/ARM Weighted re 300 mV
(Note 5)
AC Ground Pin 9 82 88 dB
DC Ground Pin 9 70 76 dB
CCIR Peak, re 300 mV, (Note 6)
AC Ground Pin 9 77 dB
DC Ground Pin 9 64 dB
Input Impedance Pin 2 and Pin 13 14 20 26 kΩ
Channel Separation DC Ground Pin 9 −50 −70 dB
Power Supply Rejection C14=100 µF,
V
RIPPLE
=
500 mVrms, −40 −56 dB
f=1 kHz
Output DC Shift Reference DVM to Pin 14 and
Measuree Output DC Shift from 4.0 20 mV
Minimum to Maximum Bandwidth, (Note 7).
CONTROL SIGNAL PATH
Summing Amplifier Voltage Gain Both Channels Driven 0.9 1 1.1 V/V
Gain Amplifier Input Impedance
Voltage Gain
Pin6 243039kΩ
Pin 6 to Pin 8 21.5 24 26.5 V/V
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Electrical Characteristics (Continued)
V
S
=
8V, T
A
=
25˚C, V
IN
=
300 mV at 1 kHz, circuit shown in
Figure 1
unless otherwise specified
Parameter Conditions Min Typ Max Units
CONTROL SIGNAL PATH
Peak Detector Input Impedance Pin 9 560 700 840 Ω
Voltage Gain Pin 9 to Pin 10 30 33 36 V/V
Attack Time Measured to 90%of Final Value 300 500 700 µs
with 10 kHz Tone Burst
Decay Time Measured to 90%of Final Value 45 60 75 ms
with 10 kHz Tone Burst
DC Voltage Range Minimum Bandwidth to Maximum 1.1 3.8 V
Bandwidth
Note 2: For operation in ambient temperature above 25˚C,the device must be derated based on a 150˚C maximum junction temperature and a thermal resistance
of 1) 80˚C/W junction to ambient for the dual-in-line package, and 2) 105˚C/W junction to ambient for the small outline package.
Note 3: To force the DNR system intomaximum bandwidth, DC ground the input to the peak detector, pin9. A negative temperature coefficientof −0.5%/˚C on the
bandwidth, reduces the maximum bandwidth at increased ambienttemperatureorhigherpackage dissipation.ACground pin 9 or pin 6 to select minimum bandwidth.
To change minimum and maximum bandwidth, see Appliction Hints.
Note 4: The maximum noise reduction CCIR/ARM weighted is about14 dB. This isaccomplished by changing the bandwidth from maximum to minimum. In actual
operation, minimum bandwidth is not selected, a nominal minimum bandwidth of about 2 kHz gives −10 dB of noise reduction. See Application Hints.
Note 5: The CCIR/ARM weighted noise is measured with a 40 dB gain amplifier between the DNR system and the CCIR weighting filter; it is then input referred.
Note 6: Measured using the Rhode-Schwartz psophometer.
Note 7: Pin 10 is DC forced half way between the maximum bandwidth DC level and minimum bandwidth DC level. An AC 1 kHz signal is then applied to pin 10.
Its peak-to-peak amplitude is V
DC
(max BW) − VDC(min BW).
Typical Performance Characteristics
Supply Current vs
Supply Voltage
DS007918-13
Channel Separation
(Referred to the Output)
vs Frequency
DS007918-14
Power Supply Rejection
Ratio (Referred to the
Output) vs Frequency
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THD vs Frequency
DS007918-16
−3 dB Bandwidth
vs Frequency and
Control Signal
DS007918-17
Gain of Control Path
vs Frequency (with
10 kHz FM Pilot Filter)
DS007918-18
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Typical Performance Characteristics (Continued)
Main Signal Path
Bandwidth vs
Voltage Control
DS007918-3
Peak Detector Response
DS007918-4
Output Response
DS007918-5
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External Component Guide
(
Figure 1
)
Component Value Purpose
C1 0.1 µF–
100 µF
May be part of power supply,
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 V
S
/2.
C2, C13 form a low frequency
pole with 20k R
IN
.
C14 25 µF–
100 µF
Improves power supply
rejection.
C3, C12 0.0033 µF Forms integrator with internal
gm block and op amp. Sets
bandwidth conversion gain of
33 Hz/µA of gm current.
C4, C11 1 µF Output coupling capacitor.
Output is at DC potential of
V
S
/2.
C5 0.1 µF 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 0.1 µF Combined with L8 and C
L
forms 19 kHz filter for FM
pilot. This is only required in
FM applications (Note 9).
L8, C
L
4.7 mH,
0.015 µF
Forms 19 kHz filter for FM
pilot. L8 is Toko coil
CAN-1A185HM (Notes 8, 9).
C9 0.047 µF 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:
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 blockwith a variable current, and an
op ampconfigured as an integrator.As seen in
Figure 2
,DC
feedback constrains the low frequency gain to A
V
=
−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 theear’s sensitivity to noise in
the presenceof 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 bandwidth 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 between 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 discussion.Although theoutput ofthe DNRsystem isa linear function 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 asteady state basis with asingle frequency input 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 frequency swept, while the control path has a constant frequency applied. It can be seenthat differentcontrol pathfrequencies 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 principles 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 masking noise about the tone, the lower the noise amplitude need
DS007918-6
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Circuit Operation (Continued)
be. As long as the total energy of the noise is equal to or
greater than the energy of the tone, the tone will be inaudible. This principle may be turned around; when music is
present, it is capable of masking noise in the same bandwidth.
2. The ear cannot detect distortion for less than 1 ms. On a
transient basis, if distortion occurs in less than 1 ms, the ear
acts as an integrator and is unable to detect it. Because of
this, signals of sufficient energy to mask noise open bandwidth to 90%of the maximum value in less than 1 ms. Reducing the bandwidth to within 10%of its minimum value is
done in about 60 ms: long enough to allow the ambience of
the music to pass through, but not so long as to allow the
noise floor to become audible.
3. Reducing the audio bandwidth reduces the audibility of
noise. Audibility of noise is dependenton noise spectrum, or
how the noise energy is distributed with frequency. Depending on the tape and the recorder equalization, tape noise
spectrum may be slightly rolled off with frequency on a per
octave basis. Theear sensitivity onthe other handgreatly increases between 2 kHz and 10 kHz. Noise in this region is
extremely audible. The DNR system low pass filters this
noise. Low frequency music will not appreciably open the
DNR bandwidth, thus 2 kHz to 20 kHz noise is not heard.
Block Diagram
DS007918-7
FIGURE 2.
DS007918-8
FIGURE 3. Output vs Frequency
DS007918-9
FIGURE 4. −3 dB Bandwidth vs
Frequency and Control Signal
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Application Hints
The DNR system should always be placed before tone and
volume controls as shown in
Figure 1
. This is because any
adjustment of these controls would alter the noise floor seen
by the DNR control path. The sensitivity resistors R1 andR2
may need to be switched with the input selector, depending
on the noisefloors of differentsources, i.e., tape,FM, phono.
To determine the value of R1 and R2 in a tape system for instance; apply tape noise (no program material) and adjust
the ratio of R1 and R2 to open slightly the bandwidth of the
main signal path. This can easily be done by viewing the capacitor voltage of pin 10with an oscilloscope, or byusing the
circuit of
Figure 5
. This circuit gives an LED display of the
voltage on the peak detector capacitor. Adjust the values of
R1 and R2 (their sum is always1 kΩ) to light the LEDs of pin
1 and pin 18. The LED bar graph does not indicate signal
level, butrather instantaneous bandwidth of the two filters; it
should not be used as a signal-level indicator. For greater
flexibility in setting the bandwidth sensitivity, R1 and R2
could be replaced bya1kΩpotentiometer.
To change the minimum and maximum value of bandwidth,
the integrating capacitors, C3 and C12, can be scaled up or
down. Since the bandwidth is inversely proportional to the
capacitance, changing this0.0039 µFcapacitor to 0.0033 µF
will change the typical bandwidthfrom 965Hz–34 kHz to 1.1
kHz–40 kHz. With C3 and C12 set at 0.0033 µF, the maximum bandwidth is typically 34 kHz. A double pole double
throw switch can be used to completely bypass DNR.
The capacitor on pin 10 in conjunction with internal resistors
sets the attack and decay times. The attack time can be altered by changing the size of C10. Decay times can be decreased by paralleling a resistor with C10, and increased by
increasing the value of C10.
When measuring the amount of noise reduction of the DNR
system, the frequency response of the cassette should be
flat to 10 kHz. The CCIR weighting network has substantial
gain to 8kHz and anyadditional roll-off inthe cassette player
will reduce the benefits of DNR noise reduction. A typical
signal-to-noise measurement circuit is shown in
Figure 6
.
The DNR system should be switched from maximum bandwidth to nominal bandwidth with tape noise as a signal
source. The reduction in measured noise is the
signal-to-noise ratio improvement.
DS007918-10
FIGURE 5. Bar Graph Display of Peak Detector Voltage
DS007918-11
FIGURE 6. Technique for Measuring S/N Improvement of the DNR System
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Application Hints (Continued)
FOR FURTHER READING
Tape Noise Levels
1. “A Wide RangeDynamic Noise ReductionSystem”, Blackmer,
“dB” Magazine,
August-September 1972, Volume 6,#8.
2. “Dolby B-Type Noise Reduction System”, Berkowitz and
Gundry,
Sert Journal,
May-June 1974, Volume 8.
3. “Cassette vs Elcaset vs Open Reel”, Toole,
Audioscene
Canada,
April 1978.
4. “CCIR/ARM: A Practical Noise Measurement Method”,
Dolby, Robinson, Gundry,
JAES,
1978.
Noise Masking
1. “Masking and Discrimination”, Bos and De Boer,
JAES,
Volume 39,#4, 1966.
2. “The Masking of PureTones andSpeech by White Noise”,
Hawkins and Stevens,
JAES,
Volume 22,#1, 1950.
3. “Sound SystemEngineering”, Davis HowardW.Sams and
Co.
4. “High Quality Sound Reproduction”, Moir, Chapman Hall,
1960.
5. “Speech and Hearing in Communication”, Fletcher, Van
Nostrand, 1953.
Printed Circuit Layout
DNR Component Diagram
DS007918-12
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Physical Dimensions inches (millimeters) unless otherwise noted
SO Package (M)
Order Number LM1894M
NS Package Number M14A
Molded Dual-In-Line Package (N)
Order Number LM1894N
NS Package Number N14A
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Notes
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NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
National Semiconductor
Corporation
Americas
Tel: 1-800-272-9959
Fax: 1-800-737-7018
Email: support@nsc.com
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Tel: 81-3-5639-7560
Fax: 81-3-5639-7507
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LM1894 Dynamic Noise Reduction System DNR
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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