Computer Science topic test - images, sound, compression

Pixel

'picture element' - a tiny square in a large grid of squares on a computer screen

Bitmap

a method for storing images using pixels - it is a map of where the bits of information are stored (stored as a sequence of numbers for each pixel's colour)

Resolution

how tightly packed the pixels are; affects image quality - a higher resolution means the image has more pixels closer together, which results in a larger file size (pixels per inch)

Colour depth

the number of bits used for each pixel to define a colour - the larger colour depth, the larger the file size

Metadata

additional data about the file itself e.g. file type, data created, author, height/width, resolution, colour depth

Image size =

width x height (number of pixels)

Width (in pixels) =

width (inches) x resolution (pixels per inch)

File size (image)=

width (pixels) x height (pixels) x colour depth (bits) + metadata

Amplitude

the value of the wave (y-axis) at a given point in time (x-axis)

Bit depth

the number of bits needed to represent the range of values on the y-axis

X-axis

time values

Y-axis

binary values

Sampling

taking a snapshot of the analogue signal at one single point in time

Quantisation

mapping that snapshot to a binary value

Sample interval

the time between samples (seconds)

Sample rate

the number of sound samples taken per second (Hz)

Bit rate =

bit depth x sample rate x number of channels

File size (sound, simple) =

Bit rate x duration

File size (sound, full) =

bit depth x sample rate x number of channels x duration

CD quality

sample rate 44.1kHz, bit depth 16, 2 channels

ADC

Analogue-to-Digital conversion

DAC

Digital-to-Analogue conversion

Compression

algorithms which reduce file sizes

Decompression

the process by which compressed data is restored to its original format

Lossy compression

permanently loses some data, mostly used with images, audio and video as it is harder for humans to notice the loss in quality, can result in digital artefacts in images/video

Lossless compression

no data is lost and the original file can be restored, used with computer programs and with text files where data loss would be unacceptable

RLE (Run-Length Encoding)

lossless - used to reduce repeating strings in lossless compression, splits the repeating string into two bytes where the first is the number of occurrences, and the second is the item of information

Dictionaries

lossless - creates a dictionary where a part of a string is assigned a key, and the text can then be made up of those keys, taking up overall less storage space if those parts of strings are repeated within the text