| The program material we are interested in listening to or recording we shall call the signal. Any sound that interferes with enjoyment or comprehension of the signal is called noise. If the signal dominates, excellent. But if noise dominates, the signal is useless. | SIGNAL…desired
NOISE…undesired
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| The relative strength of the signal compared to that of the noise is a very important factor in all communication systems. We even have a name for it: the signal-to-noise ratio. If the desired signal dominates, the signal-to-noise ratio is high, and all is well.
To get specific, let us designate the following musical selection as our signal: |
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| It will be convenient for us to use white noise to interfere with our signal. White noise sounds like this: |
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| Now, we shall mix the signal and the white noise at equal levels:
The signal is almost completely buried in noise, an obviously unworkable condition. In this case, we have a signal-to-noise ratio of one to one. This is commonly referred to as a signal-to-noise ratio of zero dB. That is, the signal level and the noise level are separated by zero dB. |
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| To improve the situation, the noise is reduced to a level 10 dB below that of the signal. The signal-to-noise ratio of the following selection is thus 10 dB: |
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| With a signal-to-noise ratio of 20 dB, our music signal really begins to emerge: |
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| A signal-to-noise ratio of 30 dB sounds like this: |
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| The following is a signal-to-noise ratio of 40 dB:
At a signal-to-noise ratio of 40 dB, it becomes more difficult to hear the noise. |
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| In the following selection, the white noise is first injected at a level 40 dB below the music and increased in 10 dB steps. During the first five seconds, however, only the noise of the reproducing system prevails. Can you hear it?: |
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| Noise interferes with speech as well as music. In a previous lesson, we saw how reverberation can reduce intelligibility of speech by masking the all-important consonants. Reverberation is a kind of noise, so we can expect white noise to affect the understandability of speech in a similar way. Let’s use the same six single-syllable test words we used before:
A signal-to-noise ratio of 30 dB is produced by injecting white noise: |
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| The speech is quite intelligible in spite of the random noise. Let’s repeat this with a signal-to-noise ratio of 20 dB: |
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| As the level of the consonants is about 20 dB below that of the rest of each word, a signal-to-noise ratio of 20 dB begins to take its toll in intelligibility. This is a signal-to-noise ratio of 15 dB: |
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| Next, a 10 dB signal-to-noise ratio really destroys intelligibility of the one-syllable words, because the noise covers the low-level consonants: |
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| We are naturally well acquainted with music and speech signals. The inevitable noise that often interferes with the desired signal is less familiar to us. For this reason, let’s focus our attention on some common sources of noise.
Every electrical circuit generates noise. In fact, electrons flowing in any circuit produce a characteristic noise which sounds like this: |
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| Electrical current flowing in every transistor, every piece of wire, generates a hissing sound like we have just heard. Fortunately, it is quite weak, but certain circuit faults can make it a problem.
A very common noise is hum originating in the alternating current power source. The following sample of 60-Hz hum will sound familiar to all: |
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| Many circuits create harmonics of 60 Hz and often what we call 60-Hz hum may actually be 120 Hz. This is a 60 Hz with harmonics: | |
| s is a term describing the case of an undesired signal interfering with the desired signal. In this case, someone else’s signal is noise to us! If the two signals are dissimilar, the interference is especially noticeable. In the following selection, it doesn’t take much music to be heard during the pauses of speech:
Radio frequency signals, such as those from nearby radio and television broadcasting stations, can easily penetrate audio circuits if there is improper shielding. Here again, the next person’s signal can be a noise problem to us: |
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| Although there are many other types of electrical noise, these are typical and illustrative. | |
| There are also noises which get into electrical circuits by way of some sort of mechanical action. |
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| Switching electrical circuits can produce clicks and pops if the switches are not carefully designed and built, and these noises can degrade the quality of a signal: |
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| The noise of heating, ventilating, and air conditioning equipment is often of high enough level to degrade a recording or interfere with listening. This is usually the noise of motors or fans:
In listening to live or reproduced music or speech, signal quality can be affected by environmental noise. The mere presence of people in an audience results in noises of breathing, movement, coughing, rustling paper, and so forth:
If the audience is paying rapt attention, this noise is low. If restive, the noise increases greatly.
Listening to interesting program material tends to make one oblivious to all but very high noise levels. However, the person evaluating such program material must be objective and alert, consciously focusing attention on possible flaws such as excessive background noise. |
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