The big idea: An analogue signal can take any value between its limits and changes smoothly.
A digital signal can take only a fixed set of values — usually just two, on and off, 1 and 0.
Neither is better. They fail differently, and that is what decides which one a designer uses where.
One signal, four views: the analogue wave, the samples taken from it, the levels it is rounded to, and the digital result.
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| Analogue | Digital | |
|---|---|---|
| Detail | Infinitely fine — every value is available | Limited by the number of levels and the sample rate |
| Noise | Picked up at every connection, amplifier and metre of cable, and it can never be removed | Ignored, because a level near 1 is read as 1 — the noise is thrown away with the rounding |
| Copying | Every copy is worse than the one before | The thousandth copy is identical to the first |
| Processing | Needs physical components, one per operation | Software — a delay, an average or a filter costs nothing in parts |
| Where it wins | At the sensor and at the speaker, where the world itself is analogue | Everywhere in between — storage, transmission, computation |
Almost every real product is both: A phone call: the microphone is analogue, because air pressure is. The signal is converted to digital for processing and transmission, because that is what survives the journey.
Then it is converted back to analogue at the earpiece, because ears are.
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From an analogue signal to a digital one
Sample
Read the value at regular intervals. Everything happening BETWEEN two samples is not recorded, so a faster rate captures faster changes.
Quantise
Round each reading to the nearest available level — 256 of them for 8 bits, 65,536 for 16. A small error is added at every single reading.
Encode
Write each level as a binary number. What is left is a list of numbers with no further loss at all.
Accept the trade
Some detail is given up ONCE, in exchange for a signal that can be stored, copied, sent and processed without ever degrading again.
More bits and faster sampling reduce the loss, never remove it: That is the honest sentence, and it is worth a mark.
CD audio samples 44,100 times a second at 16 bits because that captures everything a human ear can hear — not because it captures everything.
How this is tested — distinguishing between analogue and digital systems. It comes up two ways:
Paper 1 — multiple choice
- Classify a named signal as analogue or digital.
- Identify what is lost when a signal is converted.
Paper 2 — analysing a product
- Explain why a product converts an analogue signal to digital.
- Evaluate an analogue and a digital version of the same product.
The trap: Claiming digital is simply better. It loses detail at the conversion; what it buys is that it never loses any more.
A school replaces its analogue intercom with a digital one that sends announcements over its computer network. Analyse the two systems against each other.
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