GCSE · Computer Science · AQA · Spec 8525

Analogue to digital sound

Inside your phone, your favourite song is a very long list of numbers. How does a smooth sound become numbers, and what gets lost on the way?

Computer Science · Sound

Record a sound, one snapshot at a time

16 audio samples at 3 bits, 8 levels, 8 hertz, 1 channels

Sample 1 of 16
Levels8Bit depth3-bitSample rate8 Hz
Drag across the bars to set their heights (silence is level 0, the loudest is level 7)

Imagine a sound that swells from silence to its loudest and then fades away again. A computer can't keep a smooth wave, so it takes snapshots instead. Each bar below is one sample: the height of the wave (its amplitude) measured at a single instant. Paint the 16 bars so they follow the swell: drag across the strip, or use the arrow keys with the Previous and Next buttons. Watch what happens as you try to place a bar between two levels. This strip stores each sample in 3 bits, so there are 8 levels (0 to 7), and level 5, for instance, is stored as the binary number 101. The 16 samples cover 2 seconds, which is a sampling rate of 8 Hz (8 samples every second). Real recordings take far more samples than this toy strip; it is kept small so you can see every single one.

Watch out: Nothing is recorded in the gaps between the bars. Whatever the wave did between two samples is simply not in the file.

Computer Science · Sound

Put the conversion in order

Put these in the order they happen when a sound ends up as a file on a computer

1 · First stage4 · Last stage

  1. Each measurement is stored as a binary number

  2. The sound is a smooth, continuously varying (analogue) wave

  3. The file holds those binary numbers one after another, in order

  4. The wave's amplitude is measured at fixed, regular intervals

Computer Science · Sound

How many levels do bits buy you?

A sample stored in 3 bits can take 8 different levels (0 to 7, like the strip at the top). How many different levels can a sample stored in 8 bits take?

Your estimate

512 levels

0 levels1024 levels

Computer Science · Sound

Is digital the same as perfect?

Maya records her friend playing the guitar on her phone and says: "It's digital, so it must be exactly the same as the real thing."

Which of these is closest to what you think about Maya's recording?
How sure are you?

Computer Science · Sound

Two dials, same pull

For each change to the recording settings, tick every effect it has (assume nothing else is changed).

Raise the sampling rate
Raise the sample resolution
Lower the sampling rate
Lower the sample resolution

Computer Science · Sound

Explain the trade-off in your own words

Explain why raising the sampling rate or the sample resolution gives a closer copy of a sound, but also makes the recording need more storage. [4 marks]

0 words · your answer stays on this page and is not sent anywhere.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How a smooth sound wave turns into numbers, and what you gain and lose when you measure it more often or more finely

What you need to know

  • Sound is an analogue wave: smooth and continuous. A computer stores only binary, so the wave has to be converted.
  • Sampling means measuring the wave's amplitude at regular, fixed intervals. Each measurement is a sample, stored as a binary number.
  • The sampling rate is the number of samples per second. The sample resolution is the number of bits per sample, and n bits give 2^n levels.
  • Raising the rate or the resolution gives a closer copy, and costs more bits. Only the samples are stored, never the wave between them.

The big picture

A sound is an analogue wave: smooth and continuous. To store it, a computer measures the wave's amplitude at regular intervals. Each measurement is a sample, and each sample is stored as a binary number. The sampling rate (samples per second) and the sample resolution (bits per sample) decide how closely that list follows the wave. Raising either gives a closer copy but needs more bits.

Key points

1A digital sound is a list of measured heights, not the wave itself.
2A higher sampling rate means less of the wave slips through the gaps.
3A higher sample resolution means each height can be stored more precisely.
4Each improvement costs more bits, so the recording needs more storage.
5However good it gets, a digital copy is an approximation.

Worked example

Problem

A sound is sampled at five instants. On a scale whose levels are the whole numbers 0 to 7, the amplitude readings are 1.2, 3.8, 6.1, 4.4 and 2.0. Store each sample using 3 bits.

⚠ Watch out

Don't mix up the two settings. The sampling rate is how many measurements are taken every second. The sample resolution is how many bits each measurement gets. Neither one is loudness, and neither makes the recording a perfect copy.

🧠

Memory hook

Rate runs across, resolution runs up. Picture the staircase from the top of the page: a higher sampling rate gives you more steps across, and a higher resolution gives you shorter steps up. Both make the staircase hug the wave more closely.

✓

Check yourself

Picture the strip at the top of the page. Suppose you kept the 3 bits per sample but only had 8 bars instead of 16. What would get worse, and what would stay the same?

Flashcards

(13)
What is an analogue sound wave?
The real sound: a smooth, continuous wave that can take any value, before any conversion.
Why must a sound be converted before a computer can store it?
A computer stores only binary (1s and 0s), and a smooth wave is not in that form.
What is a sample?
One measurement of the wave's amplitude at a single instant, stored as a binary number.
What is the sampling rate?
How many samples are taken every second.
What is the sample resolution?
How many bits are used to store each sample.
How many levels do n bits give?
2^n levels. Each extra bit doubles the number: 3 bits give 8 levels, 8 bits give 256.
What is left out of a digital recording?
The wave in between the samples. Only the measured points are kept.
What does raising the sampling rate do?
More samples are taken every second, so less of the wave is missed and the copy is closer.
What does raising the sample resolution do?
More bits per sample means more levels, so each measurement can be nearer the true height.
Why do both improvements need more storage?
More samples, or more bits for each one, means more bits to store in total.
Is a digital recording an exact copy of the sound?
No. It is an approximation built from samples, and how close it gets depends on the rate and the resolution.
What happens to a reading that falls between two levels?
It is stored as the nearest available level, so a small amount of detail is lost.
What does a digital sound file actually hold?
A sequence of binary numbers, one for each sample, in the order they were measured.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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