1.3 Data Storage Flashcards
Lossless vs Lossy
Lossless compression 1. No loss of data quality 2. Lesser reduction in size of data file Lossy compression 1. Data quality is reduced 2. Bigger reduction in size of data file - image might become more blurred
Bitmap images
Bitmap images are made of millions of pixels and the colour of each pixel is stored using a number code
*bitmap files store the position and colour of every pixel of the image
Why is a bitmap file good for storing photographs and other realistic images?
Bitmap files can store all the details and varied colours of an image
*bitmap files can be very large
What are ‘high resolution’ and ‘low resolution’ images?
High resolution: Images made of lots of tiny pixels, sharp and detailed
Low resolution: Images made of fewer, larger pixels, take up less space
Vector images
Vector images are made of shapes constructed from lines. The computer stores mathematical formulas that tell it how to draw the shapes and lines
- Vector files are smaller than bitmap files
- Vector graphics are good for images made of simple lines and shapes (eg. cartoons, diagrams) but not good for storing photographs.
Why does pixelation not affect vector images?
Vector images are drawn to the correct size using the stored mathematical formulas
Colour depth
The number of colours that can be used in an image
Types of colour format
Monochrome: each pixel uses 1 bit (either B/W)
16-colour: each pixel uses 4 bits
256-colour: each pixel uses 8 bits / 1 byte
Audio formats - WAV
- Stores music very accurately and with high quality (lossless)
- However, WAV files are very large, so they take up a lot of space, and take a long time to copy
- Used by radio broadcasters; unsuitable for use on websites or personal music players
Audio formats - MP3
- Compressed file format (lossy)
- Digital recording of sound
- Has worse sound quality than WAV
- But uses less storage space
- Widely used for websites; suitable for Internet download, and storage on portable devices
Audio formats - MIDI (Musical Instrument Digital Interface)
- Non-audio recording
- Stores instructions to send to an audio synthesiser, to generate the notes from the different digital musical instruments required (works like a piece of sheet music)
- Uses very little storage
eg. used to store ringtones on mobile phones
Video formats - MP4 / MPEG-4
MP4 is a ‘container’ format, which means that an MP4 file can hold other digital formats (video, audio, image, text).
- Can be used by a wide range of devices
- But MP4 files can only contain pre-recorded content, cannot be used to live-stream broadcasts
Bit rate
The number of bits used to store one second of sound / video
What does bit rate depend on?
- Sample rate (number of times the sound changes per second)
eg. 44100 samples per second - Number of channels (mono, stereo, quad etc.)
eg. 2 (2-track stereo) - Bit depth (range of sound frequencies used)
eg. 16 bits per sample
Ways to compress an image file
- Use fewer, larger pixels to make the image (reduce image resolution)
- Use fewer bytes to store the colour code (reduce colour depth)
- Store colour only once, with a number stating how often that colour is repeated
5 methods of error detection and correction
- Parity checks
- Check digits
- Checksums
- Automatic Repeat reQuests (ARQ)
- Echo check
Explain how MP3 files retain most of the original music quality.
- removes sounds human ears cannot hear very well
- if two sounds played at same time, softer sound is removed
- uses perceptual music shaping (=the process of removing inaudible sounds in order to make a file size smaller)
Describe how the size of the text file can be reduced.
- the file can be compressed
- the compression that is used is lossless (not lossy)
- use of a compression ALGORITHM
- REPEATED words are identified and are indexed/put into a table
- repeated words are replaced with their index and their positions are stored in the table
- the number of times the repeated word appears is stored
- save file as a pdf/convert to pdf
