Introduction
Signals in the real world are analog: light, sound, you name it. So, real-world signals must be converted into digital, using a circuit called ADC (Analog-to-Digital Converter), before they can be manipulated by digital equipment. In this tutorial we will give an in-depth explanation about analog-to-digital conversion yet keeping a very easy to follow language.
When you scan a picture with a scanner what the scanner is doing is an analog-to-digital conversion: it is taking the analog information provided by the picture (light) and converting into digital.
When you record your voice or use a VoIP solution on your computer, you are using an analog-to-digital converter to convert your voice, which is analog, into digital information.
Digital information isn’t only restricted to computers. When you talk on the phone, for example, your voice is converted into digital (at the central office switch, if you use an analog line, or at you home, if you use a digital line like ISDN or DSL), since your voice is analog and the communication between the phone switches is done digitally.
When an audio CD is recorded at a studio, once again analog-to-digital is taking place, converting sounds into digital numbers that will be stored on the disc.
Whenever we need the analog signal back, the opposite conversion – digital-to-analog, which is done by a circuit called DAC, Digital-to-Analog Converter – is needed. When you play an audio CD, what the CD player is doing is reading digital information stored on the disc and converting it back to analog so you can hear the music. When you are talking on the phone, a digital-to-analog conversion is also taking place (at the central office switch, if you use an analog line, or at you home, if you use a digital line like ISDN or DSL), so you can hear what the other party is saying.
But, why digital? There are some basic reasons to use digital signals instead of analog, noise being the number one.
Since analog signals can assume any value, noise is interpreted as being part of the original signal. For example, when you listen to a LP record, you hear noise because the needle is analog and thus don’t know the difference from the music originally recorded from the noise inserted by dust or cracks.
Digital systems, on the other hand, can only understand two numbers, zero and one. Anything different from this is discarded. That’s why you won’t hear any unwanted noise when listening to an audio CD, even if you played it thousands of times before (actually depending on your sound system you can hear some noise when playing audio CDs, but this noise, called white noise, isn’t produced by the CD media, but by the CD player, amplifier or cables used, and is introduced in the audio path after the digital data found on the CD was already converted back to analog – as you see, the problem lies in the analog part).
Another advantage of digital system against analog is the data compression capability. Since the digital counterpart of an analog signal is just a bunch of numbers, these numbers can be compressed, just like you would compress a Word file using WinZip to shrink down the file size, for example. The compression can be done to save storage space or bandwidth. On all the examples given so far no compression is used. We will talk again about it when discussing surround sound.
How It Works: Sampling
For our explanations, consider the analog signal found in Figure 1. Let’s assume that it is an audio signal, since this the most popular applications for analog-to-digital and digital-to-analog conversions. The “y” axis represents voltage while the “x” axis represents time.
What the ADC circuit does is to take samples from the analog signal from time to time. Each sample will be converted into a number, based on its voltage level. In Figure 2 you see an example of some sampling points on our analog signal.
The frequency on which the sampling will occur is called sampling rate. If a sampling rate of 22,050 Hz is used, for example, this means that in one second 22,050 points will be sampled. Thus, the distance of each sampling point will be of 1 / 22,050 second (45.35 µs, in this case). If a sampling rate of 44,100 Hz is used, it means that 44,100 points will be captured per second. In this case the distance of each point will be of 1 / 44,100 second or 22.675 µs. And so on.
During the digital-to-analog conversion, the numbers will be converted again into voltages. If you think about it for a while, you will see that the waveform resulted from the digital-to-analog conversion won’t be perfect, as it won’t have all the points from the original analog signal, just some of them. In other words, the digital-to-analog converter will connect all the points captured by the analog-to-digital converter, any values that existed originally between these points will be suppressed.
You can see an example in Figure 3, where we show how the signal would be after being converted to digital and back to analog. As you can see, the original waveform is more “rounded”.
