ACARA Digital Technologies Years 7–8 — Representing Text, Images and Sound Hangman

The 48 key terms

1digital data

Computers store text, pictures and sound as numbers. Data is converted into number codes so a digital system can store it and work with it.

2binary

Using only two values, like 0 and 1 or on and off. Computers store all data using just these two states.

3bit

A bit is a single binary digit that is either 0 or 1, like a switch that is off (0) or on (1). A group of 8 bits is called a byte.

4transistor

A tiny electronic switch that is either off (0) or on (1), so it holds exactly one bit. Digital hardware joins millions of them to work with data.

5encode / decode

To encode is to turn information into a code. To decode is to turn the code back into the information, using a key. They are opposites.

10Add the place values that have a 1

To read a binary number, add up the place values (1, 2, 4, 8...) that have a 1 under them and ignore the 0s. For example, 1011 is 8 + 2 + 1 = 11.

12leading zeros

Extra 0s added at the left of a binary number to fill a fixed number of bits. They do not change its value: 101 and 00000101 both mean 5.

14character code

A number the computer uses to stand for one letter or symbol. It stores the number, then turns it back into the character on screen. In one A=1 code, A is 1 and B is 2.

15character set

The agreed list of characters a system can use, with a unique binary code for each one, so every device reads the same code as the same character.

16ASCII

A standard table that gives every character a number, such as A = 65 and a = 97, so computers can store text as binary. Lowercase codes are 32 more than capitals.

18ASCII decimal code

Look up each character in the ASCII table and write its decimal number in order. Capitals and lowercase differ, so Hi is 72 105 (H is 72, i is 105).

208-bit ASCII encoding

Look up each letter's ASCII code and write it as an 8-bit binary group, then join the groups in order. Example: A = 65 = 01000001.

22Unicode

A system that gives every character a unique number, including letters from many languages and emoji, so it covers far more than the small ASCII table.

23Unicode vs ASCII

Unicode supports far more characters than 8-bit ASCII but uses 16 or 32 bits per character, so the same text takes up more storage space.

24pixel

A pixel is one tiny square in a grid that makes up a digital image. Each pixel is stored as a number, such as 0 for white or 1 for black.

25row by row

Read a pixel grid in order: start at the top row, go left to right across it, then move down to the next row until the grid ends.

261 = black, 0 = white

To encode a black-and-white picture, write 1 for each black pixel and 0 for each white pixel. Work row by row, one bit per pixel.

27decode

To turn a code back into a picture: fill the grid row by row, colouring a square for each 1 and leaving it white for each 0. Then check the picture makes sense.

281 bit = 2 colours

One bit per pixel can only be 0 or 1, so each pixel can only be one of two colours, like black and white. A photo needs many more colours, so it needs more bits.

302^bits

Each extra bit doubles the colours a pixel can show: 1 bit gives 2, 2 bits give 4 and 3 bits give 8. So the number of colours is 2 times itself, once for each bit.

31raster image

A picture made of a grid of tiny pixels, each storing its colour as numbers. Also called a bitmap. Photos are raster images and look blocky (pixelated) when zoomed in.

32RGB

A colour model where any screen colour is made by mixing red, green and blue light, written as three numbers (R, G, B). Each pixel on a screen uses its own three numbers.

33rgb(0-255)

Each RGB number runs from 0 (none of that colour) to 255 (full strength). It fits in one byte, which holds 256 values.

34rgb(255, 0, 0)

Pure red: red at full 255, green and blue at 0. Green is (0, 255, 0), blue is (0, 0, 255), and mixing red and green light makes yellow (255, 255, 0).

35rgb(r, g, b)

Black is (0, 0, 0), white is (255, 255, 255). Any grey has equal red, green and blue values, like (128, 128, 128). Smaller equal numbers are darker; unequal numbers give a tint.

36eyedropper

A tool that picks one pixel in an image and shows the red, green and blue numbers (0 to 255) stored for it, such as rgb(255, 0, 0) for pure red.

37colour depth

The number of bits used to store the colour of each pixel. More bits per pixel means more possible colours, such as 1 bit for 2 colours or 24 bits for true colour.

38colours = 2^n

A colour depth of n bits gives 2^n different colours, so each extra bit doubles the number. For example, 2 bits give 4 colours and 8 bits give 256 colours.

3924-bit colour

Each pixel uses 8 bits for red, 8 for green and 8 for blue. That is 3 bytes (24 bits) per pixel, giving 256 x 256 x 256 = about 16.7 million possible colours.

41resolution

The number of pixels in an image, given as width x height (like 1920 x 1080). More pixels give more detail but make a bigger file.

42pixels = width * height

Total pixels = pixels per row times number of rows. For 1920 x 1080 that is 2,073,600 pixels. This is the first step when working out image file size.

43colour code table

A table giving each colour a binary code. Write each pixel's code row by row to encode an image; read the codes back to redraw it. 4 colours need 2 bits.

44vector image

An image stored as shapes and paths (lines, curves) described by numbers and drawn when shown, not as a grid of pixels. It can scale up without pixelation. SVG is a common format.

45raster vs vector

Raster images store a colour number for every pixel and blur when enlarged; vector images store shape instructions and stay sharp at any size.

46analogue to digital

A sound wave is analogue: its amplitude changes smoothly. A computer stores only separate binary numbers, so the wave a microphone captures must be converted into discrete digital values.

47waveform

A graph of a sound. Time runs left to right and volume goes up and down. Louder sounds make taller waves and silence is a flat line.

48Sampling

Measuring a sound wave thousands of times per second and storing each measurement as a binary number.

49sample

One measurement of a sound wave's volume at a single moment in time, stored as one whole number. Many samples taken at equal time steps are dots on the waveform.

50sample sequence

An ordered list of whole numbers, each measuring a sound's volume at a regular time. Plot them in order and join the dots to redraw an approximation of the wave.

51sample rate

The number of sound measurements taken each second, in hertz (Hz). CDs use 44,100 Hz. A higher rate gives better quality but makes a bigger file.

521 kHz = 1000 Hz

Sample rate is measured in hertz (Hz), the number of samples per second. 1 kHz is 1000 Hz, so 44.1 kHz is 44100 Hz. Frequency uses 1000, not 1024.

53sample resolution

The number of bits used to store each sample of a sound. More bits give more possible volume levels and a more accurate recording. 16 bits per sample is common.

54levels = 2^n

A sample resolution of n bits gives 2^n volume levels, so each extra bit doubles the levels. 8 bits give 256 levels and 16 bits give 65,536. Fewer levels means more rounding.

55bit rate

The number of bits used to store each second of sound: sample rate x sample resolution. A higher bit rate means better accuracy but more storage per second.

56Zoom in to samples

Zooming far into a waveform in an audio editor like Audacity shows each sample as a dot. A higher project rate gives more dots every second, so the wave is drawn more smoothly.

57trade-off

Balancing quality against file size: pick the lowest sample rate, resolution or colour depth that still looks or sounds good enough, so the file stays small enough to store or send.

58encoding

Encoding turns text, images and sound into whole numbers (character codes, RGB pixel values, sound samples), then stores each number as binary 0s and 1s.

59type -> numbers -> binary

Name the data type, say which numbers represent it, then show how they become binary. Give a short example for each, e.g. A is 65 because of ASCII, and 65 is 01000001.