A sound wave is sampled and the amplitude of each sample is stored using a small number of bits.
Give two limitations of storing the sound in binary when only a small number of bits is available for each sample.
Mark scheme
Award one mark for any of the following up to a maximum of two marks:
- Only a limited/fixed number of amplitude values can be stored (1)
- The amplitude of each sample is rounded/approximated to the nearest value that can be stored (1)
- The recording is less accurate than the original analogue sound/does not match the analogue wave exactly (1)
- Sounds louder than the highest amplitude that can be stored are clipped/distorted (1)
- Sounds quieter than the lowest amplitude that can be stored cannot be represented/are lost (1)
Allow quantisation error/noise for the amplitude being rounded to the nearest value that can be stored, but do not award a second mark for the same idea
Allow a stated consequence as a separate mark only when it is a different point from the list, e.g. fewer amplitude values can be stored (1) so very quiet sounds are lost (1)
Do not accept 'lower quality'/'poorer sound' on its own; the answer must say the recording is less accurate than the original/does not match the analogue wave
Do not accept sample rate/sample interval/number of channels (stereo/mono), as these are not limited by the number of bits available for each sample
Do not accept remedies such as 'a larger bit depth would make it more accurate', as these do not describe a limitation
Do not accept answers about file size, compression or playback method
Do not accept a statement that repeats the stem, e.g. 'the sound is stored in binary using a fixed number of bits'
Example answers and how they were marked
The amplitude of each sample has to be rounded to the nearest value that can be stored, and very quiet sounds cannot be represented.
Why: Amplitude rounded to the nearest stored value (1) and quiet sounds cannot be represented (1): two different points from the list.
The recording is less accurate than the original sound.
Why: The recording is less accurate than the original analogue sound (1); a second, different limitation is not given.
The sample rate is too low, so parts of the wave are missed.
Why: Sample rate is not limited by the number of bits per sample, so it is not accepted.