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, i.e., tape, FM, phono.

To determine the value of R1 and R2 in a tape system for in-

stance; 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 ca-

pacitor voltage of pin 10 with an oscilloscope, or by using 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 always 1 k

Ω) 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 indicator. For greater

flexibility in setting the bandwidth sensitivity, R1 and R2

could be replaced bya1k

Ω potentiometer.

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 this 0.0039 µF capacitor to 0.0033 µF

will change the typical bandwidth from 965 Hz–34 kHz to 1.1

kHz–40 kHz. With C3 and C12 set at 0.0033 µF, the maxi-

mum 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 al-

tered by changing the size of C10. Decay times can be de-

creased 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 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 6.

The DNR system should be switched from maximum band-

width 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

www.national.com

7