Each sample of a sound wave is stored in binary using a fixed number of bits.
Explain one limitation of representing an analogue sound wave in binary when only a small number of bits is available for each sample.
Mark scheme
Award up to two marks for a linked explanation such as:
- Only a limited number of different amplitude values can be stored (1) so each sample is rounded to the nearest available value and the digital wave does not match the analogue wave exactly (1)
- The wave is continuous but only a limited set of values can be represented (1) so amplitude detail/accuracy is lost (1)
- Quiet and loud parts of the sound may be stored as the same value (1) because there are too few values to tell them apart (1)
- The range of amplitudes that can be represented is limited (1) so the loudest/quietest parts of the sound cannot be stored accurately (1)
For both marks, the expansion must follow/associate with the statement.
Do not award marks for answers that say the sound cannot be stored or heard at all.
Do not award answers about the number of samples taken/sample rate, as the number of bits limits the amplitude values, not how often the wave is sampled.
Do not accept 'lower quality'/'less accurate'/'less detail' on its own without a link to the amplitude values that can be stored/the number of bit combinations.
Do not accept 'the file size is smaller' without a link to the accuracy or the range of values that can be stored.
Example answers and how they were marked
Only a small number of amplitude values can be stored, so each sample is rounded to the nearest value and does not match the original wave exactly.
Why: Point (limited number of amplitude values) (1) with linked reason (samples are rounded and the wave is not exact) (1).
Only a small number of amplitude values can be stored.
Why: Creditworthy point (1) but no linked limitation such as rounding or lost accuracy, so only one mark.
The sound cannot be stored at all because there are not enough bits to hold every sample.
Why: Misconception: the sound can still be stored, just less accurately, so no mark is awarded.