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Free A-Level Computer Science guide
A sampled recording stores measurements of a waveform. Vary the time spacing and amplitude precision separately, then account for every channel when calculating storage.
Content owner: Michael Print · Written for A-Level learners · Checked against official specifications
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Change sampling rate or bit depth separately. Compare the continuous signal with discrete amplitude codes, then calculate an uncompressed PCM payload.
Sampling measures amplitude at discrete times; quantisation assigns each measurement to an available code. Sample rate gives measurements per second, while bit depth determines how many amplitude codes are available.
Uncompressed PCM payload bits = sample rate × duration × bits per sample × channels. MIDI records performance instructions rather than the microphone waveform. MIDI and Nyquist terminology are explicit AQA additions to the shared sampled-sound foundation.
AQA 7517 · 4.5.6.1–4.5.6.3; 4.5.6.7–4.5.6.8
Useful foundations
By the end, you should be able to
AQA: sampled sound, ADC/DAC, sampling-frequency reasoning, PCM storage and MIDI events are included. File calculations explicitly distinguish bit depth, channel count and overhead.
Shared foundations and optional extensions are labelled separately; this page does not claim complete coverage of the qualification.
An analogue sensor or microphone supplies a continuously varying electrical quantity. An ADC samples it and quantises the amplitude. With b bits there are 2^b possible codes; quantisation generally introduces a difference between the input and represented level.
A DAC converts codes to analogue output levels; reconstruction filtering helps produce the output waveform. Conversion does not recreate information already lost during capture. Greater bit depth reduces amplitude step size over the same range, rather than placing samples closer in time.
Sampling rate is measured in samples per second. For an ideally band-limited signal, a rate above twice its highest relevant frequency avoids ambiguity between components. Practical systems use an anti-alias filter and allow a transition margin.
At or below the threshold, different continuous signals can yield misleading samples. At exactly twice the frequency, phase can make every sampled point a zero crossing. Increasing bit depth cannot repair a waveform already aliased by insufficient sampling rate.
An 8000 Hz mono recording lasting two seconds contains 16,000 samples. Eight bits per sample gives 128,000 bits, or 16,000 bytes, before headers. Stereo stores two channels and doubles this payload at unchanged duration, rate and depth.
Higher rate, depth, duration or channel count each increases the uncompressed size proportionally. Compression and headers need separate information. A 44,100 Hz sample rate is not 44,100 bytes per second unless the other factors make that so.
MIDI events can identify note on/off, pitch, velocity, timing and instrument selection. A synthesiser or sound bank generates the waveform. Changing the instrument can change the sound while leaving the note timings and pitches intact.
A short event sequence may be compact for a musical performance, but it cannot directly store a spoken interview or arbitrary environmental sound as note instructions. A sampled recording captures a waveform; its size follows different factors from an event list.
| Time in beats | Instruction |
|---|---|
| 0 | select piano |
| 0 | note on: pitch 60, velocity 80 |
| 1 | note off: pitch 60 |
| 1 | note on: pitch 64, velocity 60 |
| 2 | note off: pitch 64 |
Worked example
4 original questions total 12 marks. Attempt each before opening the independently written indicative marking guidance.
Calculate the uncompressed payload bytes for five seconds at 8000 Hz, 16 bits/sample, mono. No header is included.
Indicative answer
Keeping the duration fixed, double sample rate and change mono to stereo. What happens to payload size? Explain the two factors separately.
Indicative answer
AQA extension: a signal includes frequencies up to 6 kHz. Explain why a practical sample rate comfortably above 12 kHz is chosen and why extra bit depth does not repair aliasing.
Indicative answer
AQA extension: give two useful MIDI event properties and explain why MIDI cannot replace a sampled recording of a speaker's voice.
Indicative answer
This guide addresses AQA 7517 4.5.6.1–4.5.6.3; 4.5.6.7–4.5.6.8. Check your examination year and the complete specification for the assessment scope.
These are independently written explanations and practice questions. CompSciTutoring.co.uk is not affiliated with or endorsed by an examination board. The marking guidance is indicative; always check the syllabus for your examination year.
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