LM1894.PDF

December 1994

LM1894 Dynamic Noise Reduction System DNR É

General Description

The LM1894 is a stereo noise reduction circuit for use with

audio playback systems. The DNR system is non-comple-

mentary, meaning it does not require encoded source mate-

rial. 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 components re-

quired.

Y Compatible with all prerecorded tapes and FM

Y 10 dB effective tape noise reduction CCIR/ARM

weighted

Y Wide supply range, 4.5V to 18V

Y 1 Vrms input overload

Applications

Y Automotive radio/tape players

Y Compact portable tape players

Y Quality HI-FI tape systems

Y VCR playback noise reduction

Y Video disc playback noise reduction

Features

Y Non-complementary noise reduction, ``single ended''

Y Low cost external components, no critical matching

Typical Application

*R1 a R2 e 1k X total.

See Application Hints.

TL/H/7918±1

FIGURE 1. Component Hook-Up for Stereo DNR System

Order Number LM1894M or LM1894N

See NS Package Number M14A or N14A

DNR É is a registered trademark of National Semiconductor Corporation.

The DNR É 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.

C 1995 National Semiconductor Corporation

TL/H/7918

RRD-B30M115/Printed in U. S. A.

Absolute Maximum Ratings

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales

Office/Distributors for availability and specifications.

Supply Voltage

Soldering Information

Dual-In-Line Package

Soldering (10 seconds)

260 § C

20V

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 sur-

face mount devices.

Input Voltage Range, V pk

V S /2

Operating Temperature (Note 1)

0 § Cto a 70 § C

Storage Temperature

b 65 § Cto a 150 § C

Electrical Characteristics

V S e 8V, T A e 25 § C, V IN e 300 mV at 1 kHz, circuit shown inFigure1unless otherwise specified

Parameter

Conditions

Min

Typ

Max

Units

Operating Supply Range

4.5

Supply Current

V S e 8V

MAIN SIGNAL PATH

Voltage Gain

DC Ground Pin 9, Note 2

b 0.9

b 1

b 1.1

V/V

DC Output Voltage

3.7

4.0

4.3

Channel Balance

DC Ground Pin 9

b 1.0

1.0

Minimum Balance

AC Ground Pin 9 with 0.1 m F

675

965

1400

Capacitor, Note 2

Maximum Bandwidth

DC Ground Pin 9, Note 2

kHz

Effective Noise Reduction

CCIR/ARM Weighted, Note 3

b 10

b 14

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

V S b 1.5

Vp-p

DC Ground Pin 9

Signal to Noise

BW e 20 Hz±20 kHz, re 300 mV

AC Ground Pin 9

DC Ground Pin 9

CCIR/ARM Weighted re 300 mV

Note 4

AC Ground Pin 9

DC Ground Pin 9

CCIR Peak, re 300 mV, Note 5

AC Ground Pin 9

DC Ground Pin 9

Input Impedance

Pin 2 and Pin 13

k X

Channel Separation

DC Ground Pin 9

b 50

b 70

Power Supply Rejection

C14 e 100 m F,

V RIPPLE e 500 mVrms,

b 40

b 56

f e 1 kHz

Output DC Shift

Reference DVM to Pin 14 and

Measuree Output DC Shift from

4.0

Minimum to Maximum Band-

width, Note 6.

Electrical Characteristics

V S e 8V, T A e 25 § C, V IN e 300 mV at 1 kHz, circuit shown inFigure1unless otherwise specified (Continued)

Parameter

Conditions

Min

Typ Max

Units

CONTROL SIGNAL PATH

Summing Amplifier Voltage Gain

Both Channels Driven

0.9

1.1

V/V

Gain Amplifier Input Impedance

Pin 6

k X

Voltage Gain

Pin 6 to Pin 8

21.5

26.5

V/V

Peak Detector Input Impedance

Pin 9

560

700

840

Voltage Gain

Pin 9 to Pin 10

V/V

Attack Time

Measured to 90% of Final Value

300

500

700

m s

with 10 kHz Tone Burst

Decay Time

Measured to 90% of Final Value

with 10 kHz Tone Burst

DC Voltage Range

Minimum Bandwidth to Maximum

1.1

3.8

Bandwidth

Note 1: 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 2: To force the DNR system into maximum bandwidth, DC ground the input to the peak detector, pin 9. A negative temperature coefficient of b 0.5%/ § Con

the bandwidth, reduces the maximum bandwidth at increased ambient temperature or higher package dissipation. AC ground pin 9 or pin 6 to select minimum

bandwidth. To change minimum and maximum bandwidth, see Appliction Hints.

Note 3: The maximum noise reduction CCIR/ARM weighted is about 14 dB. This is accomplished 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 b 10 dB of noise reduction. See Application Hints.

Note 4: 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 5: Measured using the Rhode-Schwartz psophometer.

Note 6: 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) b V DC (min BW).

Typical Performance Characteristics

Channel Separation

Power Supply Rejection

Supply Current vs

(Referred to the Output)

Ratio (Referred to the

Supply Voltage

vs Frequency

Output) vs Frequency

b 3 dB Bandwidth

Gain of Control Path

vs Frequency and

vs Frequency (with

THD vs Frequency

Control Signal

10 kHz FM Pilot Filter)

TL/H/7918±2

Typical Performance Characteristics (Continued)

Main Signal Path

Bandwidth vs

Voltage Control

Peak Detector Response

TL/H/7918±3

TL/H/7918±4

Output Response

TL/H/7918±5

External Component Guide (Figure1)

Component

Value

Purpose

Component Value Purpose

C4, C11 1 m F Output coupling capacitor. Output

is at DC potential of V S /2.

0.1 m F±

May be part of power

100 m F

supply, or may be add-

ed to suppress power

supply oscillation.

0.1 m F Works with R1 and R2 to attenu-

ate low frequency transients

which could disturb control path

operation.

f 5 e

C2, C13

1 m F

Blocks DC, pin 2 and

pin 13 are at DC po-

tential of V S /2. C2,

C13 form a low fre-

quency pole with 20k

R IN .

f L e

2 q C5 (R1 a R2) e 1.6 kHz

0.001 m F Works with input resistance of pin

6 to form part of control path fre-

quency weighting.

f 6 e

2 q C2 R IN

2 q C6 R1 PIN 6 e 5.3 kHz

C14

25 m F±

Improves power sup-

ply rejection.

100 m F

0.1 m F Combined with L8 and C L forms

19 kHz filter for FM pilot. This is

only required in FM applications

(Note 1).

C3, C12

0.0033 m F

Forms integrator with

internal gm block and

op amp. Sets band-

width conversion gain

of 33 Hz/ m Aofgm

current.

External Component Guide (Figure 1)

(Continued)

Component Value

Purpose

peak detector input determine the frequency weighting as

shown in the typical performance curves. The 1 m F capaci-

tor 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/ m A. In FM

stereo applications at 19 kHz pilot filter is inserted between

pin 8 and pin 9 as shown in Figure1.

Figure3 is an interesting curve and deserves some discus-

sion. Although the output of the DNR system is a linear

function of input signal, the b 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 b 3 dB bandwidth on a steady state basis with a single

frequency input can be misleading. It must be remembered

that a single input frequency can only give a single b 3dB

bandwidth and the roll-off from this point must be a smooth

b 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 fre-

quency applied. It can be seen that different control path

frequencies 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 princi-

ples 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 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 inau-

dible. This principle may be turned around; when music is

present, it is capable of masking noise in the same band-

width.

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 band-

width to 90% of the maximum value in less than 1 ms. Re-

ducing 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 dependent on noise spectrum, or

how the noise energy is distributed with frequency. Depend-

ing on the tape and the recorder equalization, tape noise

spectrum may be slightly rolled off with frequency on a per

octave basis. The ear sensitivity on the other hand greatly

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.

L8, C L

4.7 mH, Forms 19 kHz filter for FM pi-

0.015 m F lot. L8 is Toko coil CAN-

1A185HM* (Note 1).

0.047 m F Works with input resistance

of pin 9 to form part of control

path frequency weighting.

f 9 e 1

2 q C9 R PIN 9 e 4.8 kHz

C10 1 m F Set attack and decay time of

peak detector.

R1, R2 1 k X Sensitivity resistors set the

noise threshold. Reducing at-

tentuation causes larger sig-

nals to be peak detected and

larger bandwidth in main sig-

nal path. Total value of R1 a

R2 should equal 1 k X .

R8 100 X Forms RC roll-off with C8.

This is only required in FM

applications.

* Toko America Inc., 1250 Feehanville Drive, Mt. Prospect IL 60056

Note 1: When FM applications are not required, pin 8 and pin 9 hook-up as

follows:

TL/H/7918±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 inFigure2,DC

feedback constrains the low frequency gain to A V eb 1.

Above the cutoff frequency of the filter, the output decreas-

es at b 6 dB/oct due to the action of the 0.0033 m F capaci-

tor.

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 Figure2. 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 ca-

pacitors between the summing amplifier output and the

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