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Analog & Digital Signals February 25, 2012

Posted by peterxu422 in Science, Technology.

For as long as I can remember, I have heard the terms Analog and Digital signals. In this advanced digital era, the terms have become more ubiquitous than ever. Though I was familiar with their names, I have never quite understood what they actually were or their differences until recently.

An analog signal is a continuous signal that varies with time. Continuous simply means that it is not interrupted. For example, a traditional clock with hands is an analog source. The hands keep moving without interruption and so it shows you EVERY moment of time during the day. A digital signal on the other hand only shows you finite quantities that change with time. A digital clock is, obviously, a digital source. Most only display up to the minutes. As a result, the time does not change continuously, but in steps of minutes. A digital clock that changes from 10:00 to 10:01 only shows you a change in the minute. But what about the seconds in between, or the milliseconds in between, or the nanoseconds in between? An analog clock on the other hand, technically shows you all of these and more, even though our human eyes are unable to detect them.

Another example of an analog signal would be that produced by a microphone. When you speak, your vocal chords cause the surrounding air molecules to vibrate which produce sound. If you hold a microphone close to you and speak into it, the vibration of the air molecules causes a metallic coil inside the microphone to vibrate accordingly. When the coil vibrates, it creates an electrical current that resembles the sound. The current is a continuous varying signal and so it is an analog signal. In technical terms, the current becomes an analog of the sound. This current is then fed to an amplifier and reconverted back into vibration which produces a louder sound.

What has puzzled me most about analog and digital signals is how they can be converted back and forth. Digital signals as we all know are composed of units of information called bits, which are represented by 0’s and 1’s. How then, I wondered, could a simple set of 0’s and 1’s represent something as complicated as a sound signal?

The way it works is as follows. Most analog signals can be converted to electrical currents. These electrical currents change with time and their “strength,” or amplitude, constantly changes as well. If you divide up the signal into sections, at certain points, the current will have a certain amplitude. At one instant it might be 7 Amps, then later on the signal could decrease and become 2 Amps. These values can be represented as binary numbers (number consisting only of 0’s and 1’s). If you do this for many amplitudes, you can essentially represent the entire analog signal in binary. This is the conversion from analog to digital.

In the end, the digital signal needs to be converted back to analog to get the original signal back (remember the microphone?). So you may ask what’s the point of this anyway? Well, the benefit of digital is that it is very useful for transporting data. Analog signals are constantly changing and controlling their amplitudes is difficult. Also, transporting them over long distances makes it more susceptible to signal degradation and quality loss. But with digital, you only need to control two values, 0 and 1, which is basically a no pulse or pulse of current. Since it uses just these two values and nothing in between, it makes it less likely for the signal to degrade. If a pulse of current is sent (1) and during its transport it slightly decreases and becomes say 0.80, on the receiving end it gets rounded to the closest value and becomes a 1 again, thus preserving the signal. This is the reason why digital signals are of such clear and high quality.

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   1. max - February 9, 2014

Hi! just to tell you that your explanation was very useful for me, because I’m an IT student and I’m new in the field. so thank you a lot and I wish to read again from you.

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