Subtopic Notes

1.2 Text, sound and images

1. Data representation

Text

A character set generally includes upper & lower case letters, number digits, punctuation marks and other characters
Character sets use different binary representations for each character via character encoding
Value of A = 65, a = 97, 0 = 48
Character Encoding Standards:

ASCII¹	Extended ASCII	Unicode
Only English alphabets can be represented	ASCII’s extension - Also includes most European languages’ alphabets	Superset for ASCII & extended ASCII - recognized by various global languages
Each character encoding takes up 7 bits, hence 128 possible characters	ASCII extended to 8 bits, hence 256 possible characters.	Greater range of characters, as it uses 2 or 4 bytes per character.
Takes less space		2 or 4 times more space needed per character

Analogue data is continuous electrical signals whereas digital data is discrete electrical signals.
Sound signals are vibrations through a medium. Hence are analogue in nature as there can be infinite detail for sound.
Analogue signals converted (encoded) to digital signals by sampling:
- Sound wave’s amplitude sampled at set time intervals
- These samples are encoded as a binary number sequence
- This provides a digital representation of the sound wave
Sampling Rate
- Number of samples taken in a second
- Unit: Hertz (Hz)
- ↑ Sampling Rate = ↑ Accuracy = ↑ File size.
Sampling Resolution
- Number of bits per sample
- ↑ Sampling Resolution = ↑ Accuracy = ↑ File size
Bit Rate
- Number of bits for storing 1 second of sound
- Bit Rate = Sampling Rate x Sampling Resolution
File Size = Bitrate x Length of Sound
For Stereo sound (sound with two channels) file, multiply results by two
File Formats:
- Musical Instrument Digital Format - MIDI (.mid): Stores musical performance data, such as notes, timings, and control signals, without including actual audio, allowing instruments or software to reproduce the composition
- WAV (.wav), FLAC (.flac) - Uncompressed, High-quality
- MP3 (.mp3) - Compressed format with lossy audio quality

Pixel: Smallest picture element whose color can be accurately represented by binary
Data for a bitmapped image is encoded by assigning a solid color to each pixel, i.e., through bit patterns.
Bit patterns are generated by considering each grid row as a series of binary color codes corresponding to each pixel’s color.
These bit patterns are ‘mapped’ onto the main memory.
File Header: Stores the metadata of bitmap file; image size, color depth
Image Resolution
- Number of pixels in an image
- Image Resolution = width × height; E.g. 1920 × 1080 pixels
- Image resolution ↑ = sharper/more detailed image = ↑ File Size
Screen Resolution
- Number of pixels which can be viewed horizontally & vertically on the device’s screen
- Number of pixels = width × height; E.g. 1680 × 1080 pixels
Color depth: Number of bits used to represent each color
- An image with n bits has 2n colors per pixel
- E.g. 16-color bitmap has 4 bits per pixel ∵ 24 = 16
- Color Depth ↑ = Color Quality ↑ = File Size ↑
File Size = Number of Pixels (Image Resolution) × Colour depth
Formats:
- JPEG (.jpg or .jpeg) - Compressed format with lossy quality
- PNG (.png) - Lossless format supporting transparency
- GIF (.gif), BMP (.bmp), PSD (.psd), HEIF (.heic or .heif), ICO (.ico)
Applications: Scanned images and general computer usage