Stretch tasks Representing text, images and sound

Round 1: Quiz Blitz

1. 69 Letters are in order: A 65, B 66, C 67, D 68, E 69.
2. 102 f is five letters after a, so 97 + 5 = 102.
3. False The character 5 has its own code. In ASCII it is 53.
4. Each character uses more bits ASCII uses 7 bits per character. Unicode uses 16 or more, to fit over a million characters.
5. 16 n bits give 2 to the power n colours: 2 x 2 x 2 x 2 = 16.
6. 5,000 bytes 8 x 100 x 50 = 40,000 bits. Divide by 8 for bytes: 5,000.
7. Increasing the resolution; Increasing the colour depth More pixels, or more bits per pixel, means more numbers to store.
8. The number of samples taken each second Sample rate is measured in hertz: samples per second.
9. 80000 1,000 x 10 x 8 = 80,000 bits.
10. 10 x 10 pixels, 1-bit colour → 10 x 10 pixels, 8-bit colour → 100 x 100 pixels, 1-bit colour → 100 x 100 pixels, 24-bit colour 100, 800, 10,000 and 240,000 bits. Multiply width, height and colour depth each time.
11. True Twice as many samples each second means twice as many numbers to store.
12. So the computer knows its width and height to arrange the pixels Without the dimensions, a long list of pixel colours could not be put back into the right grid.

Round 2: Quick Fire

1. • ASCII has only 128 characters, enough for English only
   • Unicode covers every major writing system and emoji
   Model answer

   ASCII only has 128 characters, which is enough for English but not for other languages. Unicode has over a million characters, covering every major writing system and emoji.

2. • 4 x 200 x 100 = 80,000 bits
   • 80,000 / 8 = 10,000 bytes
   Model answer

   4 x 200 x 100 = 80,000 bits. 80,000 / 8 = 10,000 bytes.

3. • The quality is better: closer to the original sound
   • The file is bigger
   Model answer

   More samples each second capture the sound wave more accurately, so the quality is better and closer to the original. But there are more numbers to store, so the file is bigger.

4. • 16,777,216, about 16.7 million
   • Because n bits give 2 to the power n colours, and each extra bit doubles them
   Model answer

   Each extra bit doubles the number of colours, so n bits give 2 to the power n colours. 2 to the power 24 is 16,777,216, about 16.7 million colours.

Round 3: Big Think

1. • The sound wave is sampled: measured at regular intervals
   • Each measurement is stored as a binary number
   • The sample rate is how many samples are taken each second
   • A higher bit depth gives more possible values, so each sample is more precise
   • But the file is larger
   Model answer

   Sound is a continuous wave, so it is sampled: its height is measured at regular intervals, and each measurement is stored as a binary number. The sample rate is how many samples are taken each second, measured in hertz. The bit depth is how many bits store each sample. A higher bit depth gives more possible values for each sample, so each measurement can be more precise and closer to the real wave, which improves the quality, but the file is larger.

2. • A lower resolution means fewer pixels, a smaller file and faster loading
   • Too low a resolution looks blocky or pixelated
   • A lower colour depth means fewer bits per pixel, so a smaller file
   • Too few colours makes photos look unrealistic, with banding
   • File size is colour depth x width x height
   • Photos need no more pixels than the screen will display
   • Compression could also reduce the size
   • A reasoned conclusion
   Model answer

   File size is colour depth x width x height, so reducing either helps. A lower resolution means fewer pixels, a smaller file and faster loading, and there is no point storing more pixels than the screen will display. But too low a resolution makes photos look blocky and pixelated, especially when enlarged. A lower colour depth means fewer bits per pixel and a smaller file, but photos need many shades, so too few colours makes them look unrealistic, with visible banding. Compression could also reduce the size. Overall, the best approach is to resize each photo to the size it is actually shown at and keep a high colour depth, because photos suffer far more from too few colours than from a sensible resolution.

This or That

1. Unicode ASCII has only 128 characters. Unicode has over a million.
2. 128 7 bits give 2 to the power 7 different codes, which is 128.
3. The number of pixels, width by height Resolution is the pixel count. The bits per pixel are the colour depth.
4. 4 n bits give 2 to the power n colours, so 2 bits give 4.
5. The bits used for each sample Bit depth is the bits per measurement. The samples per second is the sample rate.
6. A higher sample rate More samples per second follow the wave more closely, at the cost of a bigger file.
7. Better quality means bigger files Better quality always needs more numbers, and more numbers always make a bigger file.
8. 100 colour depth x width x height = 1 x 10 x 10 = 100 bits.

Memory Match

• A 4 ASCII: 7 bits per character, 128 characters
• B 1 Unicode: Over a million characters, including emoji
• C 6 Pixel: One dot of colour in an image
• D 5 Colour depth: The bits used for each pixel
• E 3 Sample rate: Measurements taken per second, in hertz
• F 2 Metadata: Data about the file, such as its dimensions