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| 型号： | LM1894MX/NOPB |
| 厂商： | National Semiconductor |
| 文件页数： | 18/19页 |
| 文件大小： | 0K |
| 描述： | IC NOISE REDUCT SYS DYNAM 14SOIC |
| 标准包装： | 2,500 |
| 类型： | 动态噪音抑制 |
| 应用： | 车载系统，播放器，电视 |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 14-SOIC（0.154"，3.90mm 宽） |
| 供应商设备封装： | 14-SOICN |
| 包装： | 带卷 (TR) |
| 配用： | LM1894MTBD-ND - BOARD EVALUATION LM1894 |
| 其它名称： | *LM1894MX *LM1894MX/NOPB LM1894MX |
SNAS551C – DECEMBER 1994 – REVISED APRIL 2013
the summing amplifier output and the peak detector input determine the frequency weighting as shown in the
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 12 is an interesting curve and deserves some discussion. Although the output of the DNR system is a
linear function 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 input 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 13. In this case the main signal
path is frequency swept, while the control path has a constant frequency 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 principles 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 masking 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 inaudible. This principle may be turned around; when music is present, it is capable of masking noise
in the same bandwidth.
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 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. Depending 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.
8
Copyright 1994–2013, Texas Instruments Incorporated
Product Folder Links: LM1894
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