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AS comp sci > 1.2/1.3 Multimedia and Compression > Flashcards

1.2/1.3 Multimedia and Compression Flashcards

1
Q

How are bitmap images created?

A

They are created using a mix of 3 colours, each set to a different brightness, 0 for black and 256 for 100% brightness

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2
Q

How many bits does one coloured pixel require in a bitmap image?

A

24 (1 for b/w)

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3
Q

Define lossless compression

A

An algorithm that decreases file size without losing any of the details

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4
Q

Define lossy compression

A

Complex algorithms used to reduce file size, whilst minimising the noticeable reduction in file quality

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5
Q

How do bitmap images represent image data?

A

They use a grid of pixels combines to create a whole picture

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6
Q

How do vector files work?

A

They use mathematic formulas to store the details of components of an object (e.g shape, size, colour) that combine together to form the final image

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7
Q

What are the advantages of using vector graphics? (4)

A
  • can be resized without distortion (pixelation)
  • smaller file size than bitmapped images
  • simpler to create and edit
  • stay of good quality regardless of the resolution they are displayed on
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8
Q

What are the disadvantages of using vector graphics? (2)

A
  • More difficult to learn how to use vector drawing software
  • Not appropriate for storing digital photos as the data is too complex
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9
Q

Why are videos compressed?

A

Storing such a large number of frames can take up a lot of storage

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10
Q

What do you need to consider before choosing a video resolution? (3)

A
  • Higher resolutions require more storage capacity and a greater transmission bandwidth
  • Higher resolutions use more battery power in your video camera
  • Most playback devices don’t support 8k
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11
Q

Name the 2 ways videos can be compressed.

A
  • Spatial redundancy
  • Temporal redundancy
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12
Q

What is spatial redundancy?

A

When there is a close correlation between pixels and neighbouring pixels, not all the data needs to be kept so some of it can be discarded or recorded at a lower bit depth.

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13
Q

What is temporal redundancy?

A

Most images stay pretty much the same from frame to frame with only certain areas changing significantly. This means that entire frames or large parts of each frame can be calculated and applied to the content of the following frame.

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14
Q

Define bit rate

A

Refers to the number of bits required for each second of a video

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15
Q

Define sound

A

Sound is an analogue value, which needs to be digitalised to store it in a computer

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16
Q

How is sound recorded with a microphone? (3)

A
  • the microphone picks up sound waves and converts them into analogue electrical signals
  • these signals are then processed by an ADC which converts the electrical signals into digital values that can be stored in a computer
  • this is called sampling and involves taking measurements of the level of the analogue signal, called samples, at regular time intervals
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17
Q

Define sample rating.

A

How often the ADC should sample the sound wave

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18
Q

How does the frequency of the sample rating change the sound?

A

High frequency - close to the original sound
Low frequency - different to the original sound

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19
Q

What is the frequency used to sample a sound?

A

The number of samples taken per second (Hz)
- one hertz = one sample taken per second

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20
Q

What effect does a high sample rating have? (2)

A
  • better quality of the audio recording
  • bigger file size
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21
Q

Define sample resolution

A

The number of bits used to represent each sample

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22
Q

What is the effect of having a low sample resolution? (3)

A
  • fewer bits per sample
  • limited binary patterns for samples, so analogue signal cannot be represented accurately in digital form
  • smaller file size
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23
Q

What is the difference between mono and stereo sounds?

A

Monophonic sounds only have one track of sound and stereophonic sounds contain 2 different tracks

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24
Q

Define monophonic sound

A

Only has one track of sound

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25
Define stereophonic sound
Contains 2 different tracks of sound
26
What are the effects of having 2 tracks in stereo sound? (2)
- adds impression of positioning and direction to the recording - needing 2 tracks doubles the storage
27
What is the formula for finding the storage requirement of an audio file? (bits)
Storage requirement (bits) = sampling rate x seconds x sample resolution
28
What is the formula for finding the stereo storage requirements of an audio file? (bits)
Stereo storage requirements (bits) = sampling rate x seconds x sample resolution x 2
29
Define compression technique
An algorithm or a sequence of steps, that is followed to reduce file size
30
Define resolution
The number of pixels that make up a digital image usually expressed as width x height (the clarity of an image)
31
What is it meant by "Compression can change the data in a file but not the fundamental properties"
A bitmap image that is compressed will still have the same resolution and a video will be the same time
32
Define compression ratio
A measure of the relative reduction in the size of a file as a result of applying a compression algorithm
33
What is the formula to find the compression ratio?
Uncompressed file size -------------------------------- Compresses file size
34
Why do images need to be compressed? (2)
- so they don't take a long time to be uploaded - so they don't use up as much storage
35
What is JPEG format? (2)
- highly compressed - around 10% of the original size
36
What does WAV stand for and is it compressed or uncompressed?
- Waveform audio file format - Uncompressed
37
Is MP3 compressed or uncompressed?
Compressed
38
Why do people compress audio? (3)
- streaming - downloading - not much difference heard
39
Define analogue
A signal that varies continuously over a period of time. Sound waves are a typical example of analogue waveforms.
40
What happens to the accuracy of representation if the sample rate or the sample resolution is increased?
If sample rate increased = accuracy increased If sample resolution increased = accuracy increased
41
Name and state used for 4 lossy compression formats
MP3 - Audio files: typically to download music from the internet JPEG - Photographs MPEG-1 - Video files (low resolution) MPEG-2 - Video files (high resolution)
42
What files are not suitable for lossy compression and why?
If files contain words or numbers because their contents need to be exactly the same when decompressed and cannot be approximated
43
What is the risk of lossy compression?
Performing repeated rounds of lossy compression can cause severe loss of data that the file output becomes completely unrecognisble from the original
44
Name and state the uses of 3 lossless compression formats
PNG - Images that must retain the highest level of quality GIF - Images and short animations ZIP - To compress one or more files together
45
Define pixel
The smallest element of a bitmapped graphic
46
Define bitmapped image
A type of digital image that is stored as pixels where each pixel is represented by single colour codes
47
Define bit pattern
A unique string of binary digits used in a representation
48
Define colour depth
The number of bits needed to represent a colour
49
What is the formula for finding the size of a bitmapped image file? (bits)
Width (pix) x Height (pix) x colour depth
50
Define metadata
Data that provides information about other data
51
Why is the actual size of a bitmapped image always greater than its sample file size calculation?
In addition to the pixel data, the file must store additional information so that the image can be reproduced accurately
52
Name some examples of metadata
- Image dimensions - File format - Date and time of creation - Geographical location of creation - Details about the device used to create the image - Camera settings
53
Define file header
The area at the beginning of a file where the metadata is found
54
Define screen/display resolution
The number of pixels that a screen is set to display
55
What happens when an image with a low resolution is displayed on a screen with a high resolution?
The image looks pixelated
56
What are the file formats for bitmapped graphics (full names) and their extensions? (4)
- Bitmap - .bmp - PNG (Portable Network Graphic) - .png - JPEG (Joint Photographic Experts Group) - .jpg/.jpeg - GIF (Graphics Interchange Format) - .gif
57
Define vector graphics
Images that are made up of lists of objects and their properties
58
What is the file format for vector graphics?
SVG (Scalable Vector Graphics)
59
Define drawing list
Properties that determine the dimensions, appearance and position of a vector graphic
60
What do the XY coordinates in a rectangle vector graphic represent?
The position of the top left-hand corner of the rectangle
61
Define drawing object
The shape of the object
62
What is the advantage of using bitmapped graphics?
Can show almost any level of complexity and detail
63
Define RLE
Run length encoding is a lossless compression technique, which finds runs of repeated binary patterns and replacing them with a single instance of the pattern and a number
64
Define dictionary-based compression
A lossless compression technique that relies on finding patterns in data
65
How does dictionary-based compression work?
Instead of each character in the text being stored using a separate code, each unique word is allocated a binary code and stored using those to save space
66
Define character set
A standardised collection of characters and the bit-patterns used to represent them
67
How does progressive scan work?
Draws all the lines in a single frame of video sequentially so that each frame contains a complete image that fits the screen
68
How does interlaced scan work?
Only every other line of an image is displayed in one frame
69
Why not display one whole image per frame for interlaced scan? (2)
- Due to the bandwidth issues of sending videos over the airwaves - When television receives signals, interlaced frames need to be combined together and then de-interlaced before displaying
70
How is video content distributed through broadcasting? (3)
- At first, video was broadcast using radio waves and everyone received the same video (limited tv channels) - Then, cable and satellite tv was developed so more channels could be broadcast - Standard tv broadcast switched from analogue to digital, allowing more channels with high resolution and better quality (some redesigned sets)
71
What are the 3 cd's in order?
VHS DVD Blu Ray
72
How is streaming used to broadcast video content?
- More convenient than buying - Videos are encoded at multiple frame rates and resolutions so that the quality and reliability of playback can be maximised for each device - causing a lot of businesses to go bust
73
What is transcoding?
When uploading a video onto a streaming site, it is converted from a single source resolution into multiple resolutions for optimum playback on different devices and internet speeds
74
How do companies limit or stop piracy? (5)
- Restricting media to only playback on certain devices and regions - Restricting downloaded media to only be played when the device is connected to the internet to validate the license - Restricting media to only play using certain softwares and streaming plugins - Time limiting content so that it will only play back within a certain time frame - Installing softwares that stops DVDs from being illegally copied

AS comp sci (2 decks)

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