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. Re-

ducing 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 in-

creases 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

DS007918-7

FIGURE 2.

DS007918-8

FIGURE 3. Output vs Frequency

DS007918-9

FIGURE 4. −3 dB Bandwidth vs

Frequency and Control Signal

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