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參數資料
| 型號： | LM1894M/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 消費家電 |
| 英文描述： | SPECIALTY CONSUMER CIRCUIT, PDSO14 |
| 封裝： | LEAD FREE, SOP-14 |
| 文件頁數： | 10/12頁 |
| 文件大小： | 340K |
| 代理商： | LM1894M/NOPB |
Circuit Operation (Continued)
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
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 inau-
dible. This principle may be turned around; when music is
present, it is capable of masking noise in the same band-
width.
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 band-
width 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.
Block Diagram
00791807
FIGURE 2.
LM1894
www.national.com
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