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0478 UNIT 1: 1.3 - FILE SIZES - CALCULATIONS AND COMPRESSION

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1.3 - FILE SIZES - CALCULATIONS AND COMPRESSION

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0478 UNIT 1: 1.3 - FILE SIZES - CALCULATIONS AND COMPRESSION
 

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0478 UNIT 1: 1.3 - FILE SIZES - CALCULATIONS AND COMPRESSIONVersion en ligne

1.3 - FILE SIZES - CALCULATIONS AND COMPRESSION

par Jonathan Dunsby
1

The file of an is calculated as : image ( in ) × depth ( in )

2

The size of a sound file is calculated as : rate ( in ) × sample resolution ( in ) × of sample ( in ) For a sound file , you would then the result by .

3

Sound and files can be very . It is therefore necessary to ( or compress ) the of a .

4

Reasons to reduce the size of a file are :

To storage on devices such as the disk drive / solid drive

5

Reasons to reduce the size of a file are :

To the taken to a music or fil e

6

Reasons to reduce the size of a file are :

To reduce the time to , download or a file across a

7

Reasons to reduce the size of a file are :

The / upload process uses up network ? this is the maximum of transfer of data across a , measured in per second . This occurs whenever a is downloaded , for example , from a server . files contain bits of data than files and therefore use bandwidth , which results in a data rate .

8

Reasons to reduce the size of a file are :

Reduced file size also reduces . For example , when using storage , the cost is based on the of the files . Also an internet service provider ( ) may a user based on the of data .

9

Lossy file compression
With this technique , the file compression algorithm eliminates data from the file . This means the original file cannot be once it has been . Lossy file compression results in some loss of when compared to the file . The used in the lossy technique have to decide which of the file need to be retained and which parts can be .

10

When applying a lossy file algorithm to :

» an image , it may the resolution and / or the / colour .
» a sound file , it may reduce the rate and / or the .

Lossy files are than lossless files which is of great when considering storage and data transfer rate requirements .

Common lossy file compression algorithms are : » MPEG - 3 ( ) and MPEG - 4 ( MP4 ) » .

11

While MP3 music files can never match the sound found on a DVD or CD , the quality is satisfactory for most general purposes .
But how can the music file be by 90% while still retaining most of the music quality ? Essentially the algorithm sounds that the human ear can ? t properly . For example : » removal of outside the human ear range
» if two sounds are played at the time , only the louder one can be heard by the ear , so the sound is . This is called perceptual music shaping .

12

MP4 files are slightly different to MP3 files . This format allows the storage of files rather than just sound ? music , , photos and can all be stored in the MP4 format . As with MP3 , this is a file compression format , but it still retains an acceptable quality of sound and video .

13

JPEG :
As with MP3 , once the image is subjected to the JPEG compression , a new file is formed and the
original file can no longer be .
The JPEG file process is based on two key concepts :
» human eyes don ? t detect differences in shades quite as well as they detect differences in image brightness ( the eye is less sensitive to colour
variations than it is to variations in )
» by separating pixel colour from brightness , images can be split into 8 × 8 pixel blocks , for example , which then allows certain ? information ? to be from the image without causing any real deterioration in quality .

14

Lossless file compression
With this technique , all the from the uncompressed file can be . This is particularly for files where any loss of data would be ( e . g . when transferring a large and complex spreadsheet or when a large computer application ) .
Lossless file compression is so that none of the original from the file is .

15

Run - length ( RLE ) can be used for lossless of a number of
different file formats :
» it is a form of lossless / file compression
» it reduces the size of a string of adjacent , data ( e . g . repeated colours in an image )
» a string is encoded into two values :
? the first value represents the of identical data items ( e . g . characters ) in the run
? the second value represents the of the data item ( such as ASCII codeif it is a keyboard character )
» RLE is only where there is a run of repeated units / .

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