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

C2, C13 form a low frequency

pole with 20k R

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

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 k

Ω

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 k

Ω.

R8

100

Ω

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 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 A

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

DS007918-6

www.national.com

5