Stretch tasks Representing Text, Images and Sound

Round 1: Quiz Blitz

1. 128 2 to the power 7 = 128. Extended ASCII uses 8 bits for 256.
2. 68 Codes are in order: A 65, B 66, C 67, D 68.
3. 32 97 - 65 = 32.
4. ASCII has no codes for most of the world's writing systems Unicode has room for over a million characters.
5. True So every ASCII file is already valid Unicode.
6. 500 500 x 8 = 4000 bits, which is 500 bytes.
7. Bits used per pixel It decides how many colours each pixel can be.
8. 16 2 to the power 4 = 16.
9. 40000 100 x 50 x 8 = 40 000 bits.
10. Without it, software cannot tell where each row ends The pixels are stored as one long sequence.
11. Takes more samples per second, so rapid changes survive The staircase follows the wave more closely.
12. A higher sample rate; A greater bit depth Size = sample rate x bit depth x seconds x channels.

Round 2: Quick Fire

1. • '7' is stored as its character code, 55
   • It must be converted to a number first
   Model answer

   The character '7' is stored as its ASCII code, 55, not the number 7. It must be converted to an integer first, for example by subtracting 48 from its code.

2. • It covers every writing system and emoji
   • It needs more bits per character, so files can be larger
   Model answer

   Unicode has room for over a million characters, so it covers every writing system, such as Arabic and Chinese, and emoji. However, characters outside the basic set need more bits, so the same text can take up more storage.

3. • Each pixel can be more colours, so it looks more realistic
   • Each pixel needs more bits, so the file is larger
   Model answer

   Each pixel can be one of more colours, so the image looks more realistic, with smoother shading. But every pixel needs more bits, so the file is larger.

4. • Raise the sample rate, so more samples are taken per second
   • Raise the bit depth to 16 bits, so each sample is rounded less
   Model answer

   Increase the sample rate, so more samples are taken per second and rapid changes in the wave are captured. Increase the bit depth to 16 bits, so each sample has 65536 levels and is rounded less.

Round 3: Big Think

1. • Size = width x height x colour depth
   • 786 432 pixels
   • 18 874 368 bits
   • Divide by 8 for bytes
   • 2 359 296 bytes
   • Divide by 1024 for each larger unit
   • 2304 KiB
   • 2.25 MiB
   Model answer

   Size in bits = width x height x colour depth. Pixels = 1024 x 768 = 786 432. Bits = 786 432 x 24 = 18 874 368 bits. Divide by 8 to get 2 359 296 bytes. Divide by 1024 to get 2304 KiB, and divide by 1024 again to get 2.25 MiB.

2. • Sampling measures the wave at regular intervals
   • A higher sample rate captures rapid changes and high frequencies
   • A greater bit depth gives more levels, so less rounding
   • Stereo stores two channels, doubling the data
   • Size = sample rate x bit depth x seconds x channels
   • Better quality always means a larger file
   • Compression makes it download or stream faster
   • Lossy compression suits it, because listeners will not notice
   Model answer

   Sampling measures the height of the sound wave at regular intervals. A higher sample rate takes more samples per second, so rapid changes and high frequencies survive. A greater bit depth gives more levels, 65536 at 16 bits, so each sample is rounded less and the recording is more faithful. Stereo stores two channels, which doubles the data. The size in bits is sample rate x bit depth x seconds x channels, so every improvement in quality makes the file larger. Before going online, the file would be compressed so it downloads faster and uses less bandwidth when streaming. Lossy compression such as MP3 suits a podcast, because listeners cannot hear the sounds it removes.

This or That

1. Unicode ASCII only covers English letters, digits, punctuation and control characters.
2. 7 bit, 128 codes Extended ASCII is the 8 bit version with 256 codes.
3. 99 Codes run in order: a 97, b 98, c 99. 67 is the code for 'C'.
4. Higher Resolution is the number of pixels, so more pixels means more detail and more data.
5. 2 2 to the power 1 = 2, such as black and white.
6. Seconds The sample rate is per second, so convert minutes to seconds first.
7. It doubles Two channels are stored instead of one.
8. 1024 Edexcel uses binary prefixes: a kibibyte is 1024 bytes.

Memory Match

• A 6 Character set: A code for every character
• B 5 Pixel: The smallest dot in a bitmap
• C 1 Colour depth: Bits per pixel
• D 2 Metadata: Data about the file, such as its width
• E 3 Sample rate: Samples taken each second
• F 4 Bit depth: Bits stored for each sample

Sort It

1. Text Unicode covers every writing system Unicode is a character set for text.
2. Sound The wave is measured at regular intervals That is sampling sound.
3. Images Colour depth: bits per pixel Colour depth belongs to images.
4. Images Metadata says where each row ends Without the width, a bitmap cannot be rebuilt.
5. Sound Size = rate x depth x seconds x channels That is the sound file size formula.
6. Sound Sample rate in hertz Samples per second belong to sound.
7. Images Size = width x height x colour depth That is the image file size formula.
8. Text 'A' is 65 and 'a' is 97 Character codes represent text.
9. Text Size = characters x bits per character That is the text file size formula.
10. Sound Stereo stores two channels Channels belong to sound.
11. Images Resolution: width by height Resolution counts an image's pixels.
12. Text A 7 bit code for each character That is ASCII, a character set for text.