Images, Sound & Compression - 1.2.3

Question 1

What is a bitmap image?

Answer 1

Made of tiny dots called pixels
Colour of each pixel is represented by a binary code
Number of colours available is related to the number of bits the binary code uses
E.g. a black-white image need 1 bit, 0 for white, 1 for black
2 bit images can be made up of 4 colours

Question 2

What is the colour depth?

Answer 2

Number of bits used for each pixel
You can work out number of colours given the colour depth by formula:
2^n

Question 3

What is the usual colour depth of an image?

Answer 3

24 bit colour depth (16 million different colours) more than a human can see (10 million)

Question 4

What is the image resolution? Formula?

Answer 4

Number of pixels in the image
width x height
Higher resolution (more pixels) = better quality of picture

Question 5

What is the formula for file size?

Answer 5

width x height x colour depth = size in bits

Question 6

What two factors affect file size?

Answer 6

Large width, height (resolution)
Larger colour depth
Better quality but a larger file size

Question 7

What is metadata? Examples?

Answer 7

The data stored in an image file which helps the computer recreate the image from the binary data in each pixel
Includes file format, height, width, colour depth, resolution, time, date
Around 10% of the file size

Question 8

What is a pixel (2)?

Answer 8

Short for picture element
A single point in an image

Question 9

What is a sample?

Answer 9

A very short recording made of a sound wave at a certain point in time

Question 10

What is the sample rate?

Answer 10

Number of samples recorded per second
High sample rate = closer to the original the final recording sounds
Measured in hertz

Question 11

What is the bit-depth?

Answer 11

Number of bits available for each sample
Higher bit-depth = quieter sounds can be picked up too, closer to the quality of original

Question 12

What is the bit-rate?

Answer 12

How many bits of data are processed every second
Higher bit rate = better sound quality

Question 13

Formula for bit-rate?

Answer 13

Sample rate x bit depth

Question 14

Formula for file size (in bits):

Answer 14

Sample rate (Hz) x bit depth x duration (secs)

Question 15

Why do we use compression?

Answer 15

To make file sizes smaller while keeping it as true to the original as possible
To take up less storage
Streaming and downloading files is quicker as they take less bandwidth (amount of data that can be transferred in a given time)
Web pages load quicker
Send the same content of a file for a smaller size

Question 16

What are the two types of compression?

Answer 16

Lossy compression - permanently removes data from a file to limit the number of bits the file needs
Lossless compression - makes the file temporarily smaller, removes data to store the file then restores to its original state when reopened

Question 17

Pros (4) & cons (3) of lossy compression:

Answer 17

Greatly reduced file size - more files can be stored
Less bandwidth - can be downloaded/streamed quicker
Commonly used - lots of softwares can read lossy files
Normally unnoticeable

Loses data - can’t be turned back to the original
Can’t be used on text or software files - needs all data back
Worse quality than original

Question 18

Pros (3) & cons (1) to lossless compression:

Answer 18

Data is removed temporarily - no reduction in quality
Can be decompressed back to original
Can be used on text and software files

Only a slight reduction in file size - still take up a bit of space

Question 19

Examples of lossy compression file types (3):

Answer 19

MP3 (audio)
AAC (audio)
JPEG (image)

Question 20

Examples of lossless compression file types (3):

Answer 20

FLAC (audio)
TIFF (image)
PNG (image)