Representing Information with Voltage
On this page 6 headings
Start here Long ago, people stored information by carving on stone. Your body stores information in DNA. A computer has no stone and no DNA. It has only electricity. So how does a computer store a photo, a song or your name using just electricity?
A bit is the smallest piece of information: a 0 or a 1.
A computer needs billions of bits, so a good way to store one must be:
| Need | Why |
|---|---|
| Small and cheap | We need lots of them |
| Stable | It should stay the same and give the same answer every time |
| Easy and fast to use | We must read it, change it, combine it and send it quickly |
Electricity gives us four choices: voltage, current, phase and frequency. Computers mostly use voltage.
| Voltage: good points | Voltage: problems |
|---|---|
| Easy to make and to detect | Affected by the surroundings (noise) |
| Engineers know it well | Wires behave like tiny resistors and capacitors, which slow things down |
| Uses very little power when it’s not changing |
Take a black-and-white photo and look at it point by point:
| Voltage | Colour |
|---|---|
| 0 V | Black |
| 1 V | White |
| 0.67 V | 67% grey |
Scanning: go across the picture row by row (like reading a page) and turn each point into a voltage. You get a wavy voltage signal. A TV signal works in a similar way.
In theory, a voltage between 0 V and 1 V could be any value: 0.671, 0.6712… so one point could hold endless information.
In real life, no. Noise and imperfect parts mean we can only trust a few voltage levels. How many levels we can reliably tell apart decides how much information each point really holds.
Once information is a voltage, a circuit can change it. That’s computation.
| Block | What it does to the voltage V |
|---|---|
| COPY | Output = V (same picture) |
| INVERT | Output = 1 − V (black becomes white: a photo negative) |
These are ready-made blocks. If every block works reliably, we can join many of them to build a reliable big system.
But there’s a catch, and it leads to the most important idea of this week: the digital abstraction (next page).
In one line: Computers store information as voltages (for example, black, white), and circuits that change these voltages are doing computation. Exam words: bit, information, voltage, current, phase, frequency, noise, scan (raster order), signal waveform, COPY block, INVERT block, computation.