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Summary AQA A-Level Computer Science Revision Notes
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Summarised revision notes of the specification topics, helping students achieve top grades in exams.
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CompressionWhat are the benefits of compression?
The amount of space required to store informations is reduced, meaning that
more files can be stored on a computer system or on external storage
The amount if data required to transmit data is reduced, meaning that less
bandwidth is used and this will reduce congestion and increase transmission
speed
Using compressed images on webpages reduces load times
Due to its reduced size, compressing video and audio streams improves
playback with less buffering/stuttering
Multiple files can be compressed into a single archive, simplifying their storage
and exchange
Lossy and Lossless Compression
What is lossy compression?
Unnecessary detail is permanently removed from the original data
This results in a permanent loss of quality but also the greatest reduction in file
size
Ideal for compressing images and audio, especially for transmission over the
Internet
Used for MP3, JPG and MP4
What is lossless compression?
Uses algorithms to efficiently represent repeating patterns within the original
data
No original data is lost, making it possible to perfectly restore the original data
Less reduction in file size than lossy compression but the only option where
data cannot be lost
Compression 1
, Used for compressing text, program files and other binary data
Used for TXT, EXE and HTML
Compression in Images
What types of compression works on images?
Images can be compressed using both lossy and lossless compression methods;
the method chosen depends on the type and purpose of the image being
compressed
What do JPEG images use?
The JPEG image format compresses files through the use of lossy compression by
permanently removing detail by ‘averaging’ similar colours across adjacent pixels,
JPEG is suitable for photographs but not images that contain ‘crisp’ details such as
text as the ‘averaging’ results in a ‘smudgy’ appearance on sharp edges.
What do PNG images use?
The PNG format, however, uses lossless compression by efficiently encoding
repeating patterns in the image to compress it; this results in higher quality images,
particularly for those that include sharp edges such as logos, icons and text, but
also results in larger files.
Run Length Encoding
What is it?
RLE is a simple form of lossless compression that represents repeating patterns of
data more efficiently
Compressing Audio
Why do we usually compress audio?
Digital audio files are relatively large, in order to store thousands on a computer or
phone or to stream audio over a 4G connection, compression is used
What format do we usually use?
MP3 is a very common audio compression format as it reduces the size of an audio
file without reducing the quality significantly by removing detail from an audio
Compression 2
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