The big idea: A process block can be built two ways.
Signal conditioning (analogue) shapes the signal with physical components — amplify it, filter it, compare it.
Program control (digital) converts the signal to a number and lets software decide. Both meet the same requirement; what differs is what it costs to change your mind later.
The same requirement — switch a lamp on at dusk — built both ways, stage by stage.
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| Stage | What it does | Why it is needed |
|---|---|---|
| Amplify | Multiplies a small voltage by a chosen factor, usually with an op-amp | A microphone gives millivolts and a thermocouple microvolts; the next stage wants volts |
| Filter | Removes unwanted frequencies — mains hum, radio pickup, or fast noise on a slow signal | Real sensor signals arrive with interference on them, and it is easier to remove than to prevent |
| Compare | Tests the signal against a reference voltage and gives a high or low output | It turns a continuously varying signal into a decision, which is what the output stage needs |
| Switch | A transistor or relay driven by that decision | The comparator can supply a few milliamps; a lamp, motor or heater needs far more |
The behaviour lives in the component values: Two resistors set the gain. A resistor and a capacitor set the filter. A preset sets the threshold.
So changing the behaviour means changing components on a board — which is why an analogue process is cheap to build and expensive to alter.
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| Signal conditioning (analogue) | Program control (digital) | |
|---|---|---|
| Speed | Instantaneous — the components respond as fast as the signal | Limited by the sample rate and by how long the program takes |
| Cost of parts | Pennies, and no software to write | More per unit, and development time before anything works |
| Complex behaviour | Every extra behaviour is extra components | A delay, an average, a dead band or a menu costs nothing extra in parts |
| Changing it later | A new board | An edit and a reload |
| Memory and communication | None | Logs, displays, networks and updates all become possible |
Most real products use both: Analogue conditioning at the sensor, because the signal is tiny and noisy and has to be made readable before anything else can happen.
Digital control after it, because that is where complicated behaviour is cheap. Saying so is usually worth a mark on its own.
How this is tested — comparing analogue signal conditioning with digital program control. It comes up two ways:
Paper 1 — multiple choice
- Identify a stage of signal conditioning from a description.
- Choose the process type suited to a stated requirement.
Paper 2 — analysing a product
- Analyse analogue and digital processing for a named product.
- Justify the processing approach chosen in a product.
The trap: Treating digital as automatically more modern and therefore better. An analogue comparator responds in microseconds, costs pennies and cannot crash — which is why smoke alarms and protection circuits still use one.
A domestic smoke alarm uses analogue signal conditioning for its sensing and a small microcontroller only for its test button, low-battery chirp and hush function. Justify this division.
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