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AQA GCSE CS · 3.3.6

Representing Sound

Section A: 10 multiple choice questions, 10 marks. Section B: 4 written questions, 15 marks. Total 25 marks.

Name

Class

Date

Mark  / 25

Section A

Circle the letter of the correct answer. One mark each.

  1. 1What does sample rate measure?1
    1. AThe number of bits stored per sample
    2. BThe number of samples taken each second
    3. CThe loudness of the recording
    4. DThe length of the file in seconds
  2. 2What does increasing the sample resolution do?1
    1. AAdds a second audio channel
    2. BTakes more measurements each second
    3. CGives more possible levels for each sample, so heights are recorded more precisely
    4. DReduces the file size
  3. 3A 2 minute mono recording is made at 44100 Hz with 16 bit resolution. What is the size in bits?1
    1. A705600
    2. B1411200
    3. C42336000
    4. D84672000
  4. 4How many different levels can a 16 bit sample resolution represent?1
    1. A16
    2. B256
    3. C32768
    4. D65536
  5. 5What effect does recording in stereo rather than mono have on file size?1
    1. AIt doubles it, because two channels are stored
    2. BIt has no effect
    3. CIt halves it
    4. DIt quadruples it
  6. 6Why can a digital recording never be identical to the original analogue sound?1
    1. AMicrophones are not accurate enough
    2. BThe file is always compressed
    3. CThe wave is measured only at intervals and each measurement is rounded to an available level
    4. DBinary cannot represent sound
  7. 7A recording is made at 8000 Hz. What is the likely consequence?1
    1. AIt is suitable for speech but higher frequencies will be lost
    2. BEach sample will be stored more precisely
    3. CThe file will be larger than a 44100 Hz recording
    4. DThe recording will be in stereo
  8. 8Which change would improve the quality of a recording without affecting how precisely each individual height is stored?1
    1. ARecording in mono
    2. BIncreasing the sample rate
    3. CReducing the duration
    4. DIncreasing the sample resolution
  9. 9A 30 second stereo recording at 44100 Hz and 16 bits is how large in megabytes, using 1 kB as 1000 bytes?1
    1. A0.529 MB
    2. B10.584 MB
    3. C5.292 MB
    4. D2.646 MB
  10. 10Which unit is a sample rate measured in?1
    1. AHertz
    2. BDecibels
    3. CBits
    4. DBytes per second

Section B

Answer in the space provided. The number of marks tells you how many separate points to make.

  1. 1StateState what is meant by sample resolution.1
  2. 2CalculateA recording lasts 4 minutes. It is sampled at 44100 Hz with a sample resolution of 16 bits, in mono. Calculate the file size in megabytes, using 1 kilobyte as 1000 bytes. Show your working.4
  3. 3ExplainExplain the difference between sample rate and sample resolution, and the effect each has on the quality of a recording.4
  4. 4DiscussA music streaming service wants to reduce the amount of data it sends to users on slow connections. Discuss two changes it could make to the audio and the effect each would have.6

Mark scheme

Section A

1B2C3D4D5A6C7A8B9C10A

Section B

1 State what is meant by sample resolution. 1

One mark for each of the following, to a maximum of 1.

  • The number of bits used to store each sample

Model answer. Sample resolution is the number of bits used to store each individual sample taken from the sound wave.

2 A recording lasts 4 minutes. It is sampled at 44100 Hz with a sample resolution of 16 bits, in mono. Calculate the file size in megabytes, using 1 kilobyte as 1000 bytes. Show your working. 4

One mark for each of the following, to a maximum of 4.

  • Converts 4 minutes to 240 seconds
  • Multiplies sample rate by resolution by duration
  • Divides by 8 to get bytes
  • Gives approximately 21.168 MB

Model answer. Four minutes is 4 times 60, which is 240 seconds. The size in bits is 44100 times 16 times 240, which is 169344000 bits. Dividing by 8 gives 21168000 bytes, dividing by 1000 gives 21168 kilobytes, and dividing by 1000 again gives 21.168 megabytes.

3 Explain the difference between sample rate and sample resolution, and the effect each has on the quality of a recording. 4

One mark for each of the following, to a maximum of 4.

  • Sample rate is how many samples are taken each second
  • A higher sample rate captures rapid changes and higher frequencies
  • Sample resolution is how many bits are used for each sample
  • A higher resolution gives more levels, so each height is recorded more accurately

Model answer. Sample rate is the number of measurements taken from the wave each second, measured in hertz. Raising it means the measurements are closer together in time, so rapid changes in the wave are captured and higher frequencies survive into the recording. Sample resolution is the number of bits used to store each individual measurement. Raising it increases the number of levels a sample can take, so each measured height is rounded by a smaller amount and the recorded amplitude is closer to the true one. The two controls therefore improve different things: the rate improves detail across time and the resolution improves detail in loudness, and both increase the file size in direct proportion.

4 A music streaming service wants to reduce the amount of data it sends to users on slow connections. Discuss two changes it could make to the audio and the effect each would have. 6

One mark for each of the following, to a maximum of 6.

  • It could reduce the sample rate
  • This reduces the file size in proportion but loses higher frequencies
  • It could reduce the sample resolution
  • This gives fewer levels, so loudness is recorded less accurately and the sound is noticeably worse
  • It could send mono rather than stereo, halving the data
  • Reaches a reasoned conclusion about the trade off for this situation

Model answer. The first change is to reduce the sample rate, for example from 44100 Hz to 22050 Hz. That halves the number of samples and therefore halves the data sent, but it also halves the highest frequency that can be reconstructed, so cymbals and consonants sound dull and the recording loses its brightness. The second change is to reduce the sample resolution, for example from 16 bits to 8. That also halves the data, but it cuts the number of available levels from 65536 to 256, so every measured height is rounded much more coarsely, which is heard as a hiss behind quiet passages. A third option is to send mono rather than stereo, which halves the data again without touching the accuracy of any individual sample, at the cost of losing the sense of position in the mix. For a user on a slow connection the sensible order is to drop to mono first, because the loss is the least noticeable on phone speakers or a single earpiece, then reduce the sample rate, and only reduce the resolution as a last resort since that is where the audible damage is worst. Better still is to offer several quality levels and let the player choose automatically, so the user gets the best the connection can carry.