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Edexcel GCSE CS · 2.2

Representing Text, Images and Sound

Section A: 10 multiple choice questions, 10 marks. Section B: 4 written questions, 13 marks. Total 23 marks.

Name

Class

Date

Mark  / 23

Section A

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

  1. 1How many characters can a 7 bit character set represent?1
    1. A64
    2. B127
    3. C128
    4. D256
  2. 2If 'A' is 65 and 'a' is 97, what is the code for 'c'?1
    1. A67
    2. B98
    3. C99
    4. D100
  3. 3What does colour depth control?1
    1. AThe physical size of the image
    2. BThe file format
    3. CHow many pixels the image has
    4. DHow many different colours each pixel can be
  4. 4An image is 500 by 400 pixels at 8 bit colour depth. What is its size in bits?1
    1. A160000
    2. B200000
    3. C1600000
    4. D3200000
  5. 5Using Edexcel's units, how many bytes are in 2 kibibytes?1
    1. A1024
    2. B2000
    3. C2048
    4. D16000
  6. 6What does a higher sample rate improve?1
    1. AThe number of channels recorded
    2. BHow closely the recording follows changes in the wave over time
    3. CThe metadata stored with the file
    4. DHow precisely each individual height is stored
  7. 7A 30 second mono recording at 22050 Hz with 8 bit depth is how many bits?1
    1. A661500
    2. B2646000
    3. C5292000
    4. D10584000
  8. 8Why must an image file store its width as metadata?1
    1. AWithout it, software cannot know where each row of pixels ends
    2. BIt reduces the file size
    3. CIt stores the average colour
    4. DIt is needed for compression
  9. 9What is the main disadvantage of Unicode compared with ASCII?1
    1. AIt cannot be used for web pages
    2. BCharacters outside the basic set need more bits, so files can be larger
    3. CIt is no longer supported
    4. DIt cannot represent English
  10. 10Doubling both the width and height of an image has what effect on file size?1
    1. AIt becomes four times larger
    2. BIt doubles
    3. CIt becomes eight times larger
    4. DIt stays the same

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 the term metadata in relation to an image file.2
  2. 2CalculateAn image is 800 pixels wide, 600 pixels high and has a colour depth of 16 bits. Calculate its size in kibibytes. Show your working.4
  3. 3ExplainExplain the difference between sample rate and bit depth when recording sound.4
  4. 4ExplainExplain why a text file containing 2000 characters is much smaller than a screenshot of the same text.3

Mark scheme

Section A

1C2C3D4C5C6B7C8A9B10A

Section B

1 State what is meant by the term metadata in relation to an image file. 2

One mark for each of the following, to a maximum of 2.

  • Data about the file rather than the picture content itself
  • Gives an example such as width, height, colour depth or date

Model answer. Metadata is data stored in a file that describes the file itself rather than forming part of the picture, such as its width and height in pixels, its colour depth, its format and the date it was created.

2 An image is 800 pixels wide, 600 pixels high and has a colour depth of 16 bits. Calculate its size in kibibytes. Show your working. 4

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

  • Multiplies width by height
  • Multiplies by the colour depth to get bits
  • Divides by 8 to get bytes
  • Divides by 1024 to give approximately 937.5 KiB

Model answer. The number of pixels is 800 multiplied by 600, which is 480000. At 16 bits each that is 480000 multiplied by 16, giving 7680000 bits. Dividing by 8 gives 960000 bytes. Dividing by 1024 gives 937.5 kibibytes.

3 Explain the difference between sample rate and bit depth when recording sound. 4

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

  • Sample rate is the number of samples taken each second
  • A higher sample rate captures faster changes and higher frequencies
  • Bit depth is the number of bits used to store each sample
  • A higher bit depth gives more possible levels, so each height is recorded more accurately

Model answer. Sample rate is how many measurements of the wave are taken each second, measured in hertz. Raising it places the measurements closer together in time, so faster changes in the wave are captured and higher frequencies survive into the recording. Bit depth is how many bits are 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 loudness is closer to the true value. The rate therefore controls detail across time and the bit depth controls detail in amplitude, and both increase the file size in direct proportion.

4 Explain why a text file containing 2000 characters is much smaller than a screenshot of the same text. 3

One mark for each of the following, to a maximum of 3.

  • A text file stores one code per character
  • An image stores a colour value for every pixel
  • The image includes all the blank space, and there are far more pixels than characters

Model answer. A text file stores one character code per character, so 2000 characters in extended ASCII takes 2000 bytes. A screenshot is a bitmap, which stores a colour value for every single pixel in the picture. A modest screenshot of 1000 by 800 pixels contains 800000 pixels, and at 24 bit colour that is 2.4 megabytes before any compression. Crucially the image also has to store the blank background between and around the letters, because a bitmap has no concept of a character at all: it records only colours at positions.