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參數資料
| 型號： | LM1894MDC |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 消費家電 |
| 英文描述： | SPECIALTY CONSUMER CIRCUIT, UUC |
| 封裝： | DIE |
| 文件頁數： | 10/12頁 |
| 文件大小： | 345K |
| 代理商： | LM1894MDC |
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
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 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
) 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
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 F capacitor to 0.0033 F
will change the typical bandwidth from 965 Hz–34 kHz to 1.1
kHz–40 kHz. With C3 and C12 set at 0.0033 F, 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
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 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
LM1894
www.national.com
7
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