Video + Compression

Question 1

State what is meant by frame rate?

Answer 1

The number of frames displayed per second. Higher frame rates = higher quality = larger file size

Question 2

How are individual video frames displayed?

Answer 2

Frames are created line by line.

Question 3

What is an issue with video frame rates?

Answer 3

The resolution needs to be defined (the number of lines per frame and the number of pixels per line).

The needs to be compatibility between the resolution of the stored image and the resolution of the display screen.

The screen needs to be refreshed at least 50 times per second otherwise we would see a flicker

Question 4

What are the positives and negatives of interlaced video technology?

Answer 4

Pros: Refreshes faster, better visual smoothness and saves bandwidth
Cons: Becoming outdated and interlaced screen will
not show fast moving objects clearly

Question 5

What are the positives and negatives of progressive video technology?

Answer 5

Pros: Crisp, detailed frames and is new and popular
Con: Rough frame transition

Question 6

State the two types of compression

Answer 6

Lossless and lossy compression

Question 7

What is the objective of compression?

Answer 7

To reduce the size of a file which would reduce memory storage requirements and improve transmission rates

Question 8

What is meant by lossless compression

Answer 8

Coding techniques used that allow subsequent decoding to recreate exactly the original file

Question 9

What is meant by lossy compression?

Answer 9

Coding techniques that cause some information to be lost so that the exact original file cannot be recovered in subsequent decoding

Question 10

When should you use lossless compression?

Answer 10

When compressing text - as you may lose important information.

Question 11

Give examples of lossless compression

Answer 11

Huffman Coding - it looks for the most common characters used. It then gives these common characters a shorter code. The original stream of bytes become a bit stream.

Run Length Encoding - Great for bitmap files. Converts sequences of the same bit pattern into a code that defines the bit pattern and the number of times it is repeated.

Question 12

When can lossy compression be used?

Answer 12

For sound files or image files that can have some of the detailed coding removed or modified where the human eye/ear will hardly notice.

Question 13

Give an example of lossy compression

Answer 13

reducing the colour depth of a bitmap image

Question 14

State what is meant by tackling video spatial redundancy

Answer 14

Spatial Redundancy is the redundancy of information which exists within the same frame - similar to run length encoding. It is a type of intra-frame compression

Question 15

State what is meant by inter-frame compression

Answer 15

Type of video compression and decreases file size
o It removes neighboring frames which are similar
o Some change in image data is redundant
o How redundant the change in image between frames is determines the amount of compression possible

Question 16

State what is meant by tackling temporal redundancy in videos

Answer 16

Changing the frame by frame coding to one which mainly records differences between adjacent frames - such as FRAME TRANSITIONS. It is a type of interframe compression

Question 17

State what is meant by a video/multimedia container

Answer 17

A video has images and sound - but they do not go to the same part of a receiving/displaying system.
Audio and visual needs to go to different parts of the system but be synchronised.
A multimedia container is used to package the audio and image. It is developed by different organisations and companies. The multimedia container contains many audio/video streams as well as streams for subtitles and chapter headings.

Question 18

Give examples of multimedia containers

Answer 18

MP4; AVI; MOV; OGG; RM

Question 19

How can we solve the issue with video frame rates?

Answer 19

• Traditionally we could use interlaced encoding. TV broadcasting used it. The image for each frame is split into 2 halves - one containing the odd numbered lines and the other the even lines. The first half is displayed completely, then the other half is used. This creates a refresh rate and is halving the transmission requirements (one half at a time).
• Progressive encoding - a whole frame is displayed each time. There has been an improvement in transmission bandwidth which suits progressive encoding.