Circuit Operation

The LM832 has two signal paths a main signal path and a

bandwidth control path The main path is an audio low pass

unity gain buffer As seen in

Figure 1 DC feedback con-

b

1 Above the cutoff

due to the action of the 0022 mF capacitor

The purpose of the control path 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 1 The R1 R2 resistor

divider adjusts the incoming noise level to slightly open the

bandwidth of the low pass filter Control path gain is about

60dB 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

peak detector input determine the frequency weighting as

shown in the typical performance curves The 1 mF capaci-

tor at pin 10 in conjunction with internal resistors sets the

attack and decay times The voltage is converted into a

stereo applications a 19 kHz pilot filter is inserted between

pin 8 and pin 9 as shown in

Figure 16

Normal methods of evaluating the frequency response of

the LM 832 can be misleading if the input signal is also

applied to the control path Since the control path includes a

frequency weighting network a constant amplitude but vary-

ing frequency input signal will change the audio signal path

bandwidth in a non-linear fashion Measurements of the au-

dio signal path frequency response will therefore be in error

since the bandwidth will be changing during the measure-

ment See

Figure 9 for an example of the misleading results

that can be obtained from this measurement approach Al-

though the frequency response is always flat below a single

high-frequency pole the lower curves do not resemble sin-

gle pole responses at all

A more accurate evaluation of the frequency response can

be seen in

Figure 8 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 Music and speech can mask noise In the absence of

source material background noise can be very audible

However when music or speech is present the human ear

is less able to distinguish the noisethe source material is

said to mask the noise The degree of masking is depen-

dent on the amplitude and spectral content (frequencies) of

the source material but in general multiple tones around 1

kHz are capable of providing excellent masking of noise

over a very wide frequency range

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 the

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 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

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

and R2 may need to be switched with the input selector

depending on the noise floors of different sources ie 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 slightly open the band-

width of the main signal path This can easily be done by

viewing the capacitor voltage of pin 10 with an oscilloscope

or by using the circuit of

Figure 12 This circuit gives an LED

display of the voltage on the peak detector capacitor Adjust

the values of R1 and R2 (their sum is always 1 kX) to light

the LEDs of pin 1 and pin 18 The LED bar graph does not

indicate signal level but rather instantaneous bandwidth of

the two filters it should not be used as a signal-level indica-

tor For greater flexibility in setting the bandwidth sensitivity

R1 and R2 could be replaced bya1kX potentiometer

To change the minimum and maximum value of bandwidth

the integrating capacitors C3 and C10 can be scaled up or

down Since the bandwidth is inversely proportional to the

capacitance changing this 0022 mF capacitor to 0015 mF

will change the typical bandwidth from 1 kHz – 30 kHz to 15

kHz – 44 kHz With C3 and C10 set at 0022 mF the maxi-

mum bandwidth is typically 30 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 C9 Decay times can be

decreased by paralleling a resistor with C9 and increased

by increasing the value of C9

When measuring the amount of noise reduction of DNR in a

cassette tape system the frequency response of the cas-

sette should be flat to 10 kHz The CCIR weighting network

has substantial gain to 8 kHz and any additional roll-off in

the cassette player will reduce the benefits of DNR noise

reduction A typical signal-to-noise measurement circuit is

shown in

Figure 13 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

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