Digital Signals

Introduction

As you saw in the Electricity 101 activity, we can make a HIGH or LOW signal by setting a voltage to 5V or 0V. The threshold for HIGH is arbitrary (but has good engineering reasons behind it). The measurement for 5V is relative to something we define as 0V, so we need an electrically neutral material to reference the voltage to, such as the ground.

You might see 0V marked as ref for reference, or GND for ground. The assumption is that because the ground is big, it will quickly dissipate any charge and return to roughly neutral. You can think of the Earth as a big, electrically neutral material—on average, of course, since it can have local charge buildup such as when lightning strikes.

If we create a way to indicate our signal, like attaching an LED to the circuit, we can see what's going on with the signal. If the circuit is complete, and the voltage is HIGH, the LED will be ON, and we usually write that with a 1 (because it looks like a closed circuit). If the circuit is not complete, the voltage will be LOW, the LED will be OFF, and we usually write that with a 0 (like an open circuit from a different direction).

A digital signal means that the signal can be represented entirely with a series of 1 or 0. For many devices, a simply 1 or 0 signal is enough to communicate a message. For example, on the bus, if you indicate that you would like the bus to stop, you simply press the "STOP" button, and a light turns on, letting the bus driver know that they should stop. Many other cases exist: a light to indicate that the door can open, a light to indicate that your gas is too low, a brake light that indicates that a car is slowing down or stopping, and many other examples.

Encoding

As the above examples show, the simple paradigm of ON or OFF can indicate many different meanings through context. We call the relationship between the pattern of 1 and 0 and a particular meaning the encoding. For 1-bit signals where a system can either indicate ON or OFF, you can only have two possible "patterns" of 1 or 0. For 2-bit encodings, you can have four possible patterns:

11
10
01
00

If you would like to send a message using these patterns as a signal, then you would have to decide, along with your interlocutor, what each pattern would indicate. For example, if we wanted to design a car braking system with two brake lights, we could say:

• 11 will mean stopping quickly
• 10 will mean stopping slowly
• 01 will mean accelerating
• 00 will mean accelerating quickly

The encoding is exactly the mapping written above. Notice, however, that the context is very important if you would like to be effective in a safety-critical environment. Consider the following questions:

• How big are the lights?
• What colour are the lights?
• Where are the lights placed?
• How long does each light stay on? You may need yet another encoding.
• What happens if only some people know about the encoding, but not all?