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A communication system runs from an input transducer through a transmitter, a transmission medium and a receiver to an output transducer. 15 questions answered.
Introduction to Communication Systems is part of Data communication systems (A-level only) in A Level AQA Physics.
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A communication system runs from an input transducer through a transmitter, a transmission medium and a receiver to an output transducer. 15 questions answered.
Use this lesson to stay inside Data communication systems (A-level only) while connecting the detail back to the wider Electronics (A-level only) topic.
Read the core explanations from Introduction to Communication Systems before testing yourself from memory.
Every phone call, radio broadcast, and internet video relies on the same underlying idea: information must travel from one place to another without being destroyed in the process. The question is — how do engineers guarantee that what arrives at the other end faithfully represents what was sent?
The answer is a communication system — a chain of carefully designed stages, each with a specific job. Understanding what each stage does, and why it is necessary, is the core of this topic for AQA A Level Physics.
This lesson focuses on real-time communication systems, where information is transmitted and received essentially simultaneously — think live telephone conversations or radio broadcasts, as opposed to recorded messages stored and played back later.
Picture the journey of your voice during a phone call. You speak, your phone processes the sound, sends it across a network, and the person at the other end hears you — all within a fraction of a second. That journey maps directly onto the block diagram you need to know.
The standard AQA block diagram for a real-time communication system contains the following stages in order: input transducer → signal processor (transmitter side) → transmission medium → signal processor (receiver side) → output transducer. Noise enters the system at the transmission medium stage, which is why the receiver-side processing is essential.
Each block has one clearly defined purpose. The AQA specification requires you to state that purpose accurately — not describe the internal electronics of each stage. Keep that distinction in mind throughout this lesson.
Now that the overall picture is clear, look at each stage in turn. The logic is cumulative — each stage solves a problem created by the previous one.
The electrical signal produced by the input transducer is typically very small and in a form that cannot travel long distances efficiently. This is where the transmitter-side signal processor takes over.
Modulation deserves particular attention here. A voice signal occupies frequencies roughly between 300 Hz and 3400 Hz — far too low to radiate efficiently as a radio wave. By modulating a high-frequency carrier wave with the voice signal, the transmitter shifts the information to a frequency range that propagates well through the chosen medium.
Once the signal leaves the transmitter, it enters the transmission medium. This is the stage where the signal is most vulnerable, because noise — unwanted random electrical or electromagnetic disturbances — is added to the signal during transit.
Attenuation and noise are the two enemies of any communication system. Attenuation reduces signal amplitude with distance; noise adds random fluctuations on top of the signal. Both degrade the signal-to-noise ratio, making it harder to recover the original information accurately.
At the receiving end, the signal processor on the receiver side must undo the damage done during transmission and reverse the encoding applied by the transmitter.
Notice the symmetry: the receiver-side processor mirrors the transmitter-side processor. Where the transmitter modulates, the receiver demodulates. Where the transmitter amplifies before sending, the receiver amplifies after receiving. This paired structure is fundamental to how communication systems are designed.
The output transducer completes the chain. The system has taken a physical signal, converted it to electrical form, processed and transmitted it, recovered it, and converted it back — all in real time.
Noise is unavoidable in any real communication system. It enters primarily at the transmission medium, but it can also be introduced by electronic components within the signal processors themselves. The key measure of how well a system handles noise is the signal-to-noise ratio (SNR).
A high SNR means the wanted signal is much stronger than the noise — the system is performing well. A low SNR means noise is comparable in strength to the signal, making accurate recovery difficult or impossible. Engineers design every stage of the system with the goal of maintaining a high SNR throughout.
This is precisely why the receiver-side signal processor must amplify the signal before attempting demodulation. If the signal arrives too weak relative to the noise, no amount of subsequent processing can cleanly separate them — the noise has already corrupted the information.
The block diagram structure applies to both analogue and digital communication systems, but the way each system handles noise is fundamentally different — and this difference explains why digital systems have largely replaced analogue ones in modern telecommunications.
In an analogue system, the signal is a continuously varying voltage that directly represents the original information. When noise is added during transmission, it distorts the signal continuously. When the receiver amplifies the signal to restore its strength, it amplifies the noise along with it — the two cannot be separated.
In a digital system, the information is encoded as a series of discrete voltage levels — typically representing binary 1s and 0s. When noise is added, the receiver only needs to decide whether each received pulse represents a 1 or a 0. Provided the noise is not so large that it flips a 1 to a 0 (or vice versa), the original signal can be perfectly reconstructed. This process is called regeneration.
This advantage of digital systems over analogue ones is a key reason why the transmitter-side signal processor in modern systems often includes an analogue-to-digital converter (ADC), and the receiver-side processor includes a digital-to-analogue converter (DAC) — though for AQA, you only need to know the purpose of each stage, not the detailed circuitry.
Theory is clearest when anchored to a concrete scenario. Work through the following example, which traces a voice signal through every stage of a real-time communication system — the kind of question an AQA examiner might set as a 'describe' or 'explain' task.
Worked example: Tracing a Voice Signal Through a Telephone Communication System
Describe the purpose of each stage as a voice signal travels from a speaker's mouth to a listener's ear via a telephone system.
Answer: Microphone (input transducer) → Signal processor: amplify, filter, modulate (transmitter side) → Transmission medium: carries signal, introduces attenuation and noise → Signal processor: amplify, filter, demodulate (receiver side) → Loudspeaker (output transducer)
AQA questions on communication systems most commonly use the command words 'describe', 'state', or 'explain'. Each demands a different response, and confusing them is one of the most reliable ways to lose marks.
A 'state' question wants a brief, precise answer with no justification. A 'describe' question wants a sequential account of what happens. An 'explain' question requires you to give the reason — the scientific principle behind what happens. Practise distinguishing these before your exam.
For extended-response questions asking you to evaluate the advantages of digital over analogue communication, structure your answer as: state the problem with analogue (noise cannot be separated from the signal during amplification) → state the digital solution (regeneration reconstructs a clean signal) → link to the consequence (digital systems can transmit over greater distances with higher fidelity).
Every stage in the block diagram — from input transducer to output transducer — exists to solve a specific problem in the journey of information from source to destination, and being able to state that purpose precisely and in the correct AQA terminology is the foundation of full marks on this topic.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Signal-to-noise ratio (SNR) is a measure of how strong the wanted signal is compared with the noise. A high SNR means the system is performing well; a low SNR means noise is comparable in strength to the signal, making accurate recovery difficult. Attenuation and noise in the transmission medium both reduce it.
Demodulation is extracting the original information signal from the carrier wave, carried out by the receiver-side signal processor. It reverses the modulation done by the transmitter, alongside amplification to restore signal strength and filtering to remove noise.
A real-time communication system has five stages in order: an input transducer, a transmitter-side signal processor, a transmission medium, a receiver-side signal processor and an output transducer. Noise enters mainly at the transmission medium, which is why receiver-side processing is essential.
Examples of transmission media are copper wire for electrical signals, optical fibre for light pulses, and free space for electromagnetic waves. Every medium introduces attenuation, a reduction in signal strength, and noise, which together degrade the signal-to-noise ratio.
The transmitter-side signal processor prepares the electrical signal for transmission by amplifying it, filtering out unwanted frequencies and modulating it onto a carrier wave suited to the transmission medium. A voice signal of roughly 300 Hz to 3400 Hz is far too low to radiate efficiently as a radio wave on its own.
The receiver-side signal processor recovers the original information by amplifying the signal to restore strength lost to attenuation, filtering out noise and demodulating it to extract the information from the carrier wave. It mirrors the transmitter: where the transmitter modulates, the receiver demodulates.
An input transducer converts a non-electrical signal, such as sound, into an electrical signal, as a microphone does. An output transducer converts the recovered electrical signal back into a useful physical form, as a loudspeaker does, turning a varying current into pressure variations in air.
A real-time communication system transmits and receives information essentially simultaneously, as in a live telephone conversation or radio broadcast, rather than storing a recorded message to play back later. It runs from input transducer through signal processing and a transmission medium to an output transducer.
A receiver must amplify the signal before demodulating it to restore the strength lost to attenuation while keeping the signal-to-noise ratio high. If the signal arrives too weak relative to the noise, no amount of later processing can cleanly separate them, because the noise has already corrupted the information.
Noise in a communication system is unwanted random electrical or electromagnetic disturbance added to the signal. It enters mainly at the transmission medium, but electronic components within the signal processors can also add it, and it degrades the signal-to-noise ratio.
The transmitter and receiver signal processors mirror each other: where the transmitter modulates, the receiver demodulates, and where the transmitter amplifies before sending, the receiver amplifies after receiving. This paired structure is fundamental to how communication systems are designed.
In a telephone system, the microphone is the input transducer, turning sound into a voltage; a signal processor amplifies, filters and modulates it; copper wire, optical fibre or a radio link carries it, adding attenuation and noise; a receiver processor amplifies, filters and demodulates it; and the loudspeaker turns it back into sound.
Digital communication systems have largely replaced analogue ones because analogue noise cannot be separated from the signal during amplification, while a digital receiver only decides whether each pulse is a 1 or a 0. Regeneration then reconstructs a clean signal, so digital systems can transmit over greater distances with higher fidelity.
A microphone is an input transducer because it converts a non-electrical signal, pressure variations in air from sound waves, into a varying voltage the rest of the system can process. The signal it produces is typically very small, so the transmitter-side signal processor then takes over.
Modern communication systems often include an analogue-to-digital converter (ADC) in the transmitter-side signal processor and a digital-to-analogue converter (DAC) in the receiver-side processor. Converting to digital allows regeneration of a clean signal at the receiver, the key advantage of digital over analogue transmission.