A string tied to a doorknob would be an 'analog' system. Analog systems are what I call wave systems. They have a value that changes steadily over time and can have any one of an infinite set of values in a range. If you put a measuring stick or ruler at a specific point along the string, you can measure the string's value every so many seconds at that point. When you watch it move, you will see it moves constantly. It doesn't instantly jump up and down the ruler.

Our string example is not a digital system. A digital system would be to flick the light switch on and off. There's no 'in between' values, unlike our string. If the switch you are using is not a dimmer switch, then the light is either on, or off. In this case, the transmitter is the light bulb, the media is the air, and the receiver is your eye. This would be a digital system.

Unfortunately, the science guys have to go and confuse things. They talk about digital systems having a 'square wave' appearance, and they are correct. The light doesn't instantly turn on, but it does happen so fast that we humans don't normally see the light actually in the process of 'lighting up'. We see it either lit, or unlit. When a signaling system changes that fast between specific values, its usually called a 'digital' system, especially when a computer is hooked up at either end.

Another example of a digital system would be the digital voice telephone system. The phone system uses something called Pulse Code Modulation (PCM). PCM uses 256 discrete voltage states to represent specific values or symbols. The phone system uses these discrete symbols to represent various values of sound on the phone line. Because there are only 256 symbols, the phone system can't reproduce the sound exactly, but it can samle it 8,000 times a second. Some of the data is lost but because it is sampled so fast, we don't notice much of the lost sound. The same thing occurs with digital photography. The camera can only record a limited number of colors, not the billions of colors our eyes can percieve. This is why digital photos sometimes look washed out, flat or off-color compared to the real thing. This loss in conversion from analog to a digital format is referred to as digital noise.

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