Standard File Formats

Transcription

1 Standard File Formats Introduction:... 2 Text: TXT and RTF... 4 Grapics: BMP, GIF, JPG and PNG... 5 Audio: WAV and MP Video: AVI and MPG Page 1

2 Introduction You can store many different types of information on your computer: text, graphics, audio video etc. The computer needs to know what application to use when you click on a file to open it, and this is done by adding a dot and a three letter extension to the filename. This extension tells the operating system what kind of file it is, and the operating system uses this information to select the icon for the file, and to use the correct application to open it. Usually extensions are hidden for the user, but you can see the extensions on your file names by right clicking on the file and choosing properties. Open up Notepad and type in a sentence. Save the file with the name Text1.gif Including the quotes Open the folder where you saved your file. What icon does the file have? What happens when you try and open the file? Try saving the file again using the following names: Text1.xls Text1.doc And check to see what icon the files now have Finally save the file as Text1.txt and the file will be saved with the normal notepad icon. Page 2

3 A standard is an agreement that has been made between organisations or between countries about how they should do something, to make it easier to share information or to use equipment. For instance in the UK we have a standard mains plug which everyone uses.. There is no standard worldwide telephone connection standard, so if you want to use a dial-up modem abroad, this is what you need: It makes sense to have a standard file format for types of information on computers because nowadays most computers are connected together by the Internet, and people need to be able to view and share files across the world no matter what type of computer they are using or what software they are using to read the file. You are going to investigate the standard file formats for: Text Graphics Audio Video Page 3

4 Text: TXT and RTF The simplest file format for text is ASCII code. This is an 8 bit code which can store each character on the keyboard using only 8 bits for each one. Each character takes up 1 byte (a bit is a single on or off,8bits = 1 byte) The file extension for an ASCII code document is TXT Start up Notepad and type in a short sentence. Don t press enter at the end of the line. Count the characters in the sentence, including spaces and the full stop. Save the file. Right click on the file and choose Properties to look at the file size. What do you notice? Word processors store a lot more information than just the text in a document. They store details of font and font size, margin widths, page orientation and much more. Microsoft Word stores files in DOC format. Copy and paste the text from your notepad file into a word document and save it. Right click on the file and choose Properties to look at the file size. What do you notice? Right click on the word document and from the open with menu, choose notepad. Can you find the text from the notepad file? What other information can you se? There are many different word processors and they all store their information in different file formats. Because of this a common file format was designed which all word processors can read. It is the Rich Text Format or RTF. Save your word file as an RTF document using the Save as Type option. Right click on the file and choose Properties to look at the file size. What do you notice? Right click on the RTF document and from the open with menu, choose notepad. Can you find the text from the notepad file? Page 4

5 Graphics: BMP, GIF, JPG and PNG There are a number of different graphics file formats. The ones you are going to investigate are all Bitmap graphics files. A bitmap is where the graphic image is stored as a series of 1s and 0s corresponding to the pixels in the image. The following bits in a computer's memory are the pixels of a 10 x 10 grid, with 0 representing black and 1 representing white Draw out a 10 x 10 grid, and work out what the computer graphic is. How much space in the computer s memory would this graphic take up? (remember 1 byte = 8 bits) A computer has a screen which displays a picture 1600 pixels wide and 1200 pixels deep. How much memory would it need in kilobytes (1 kb = 1024 bytes) Compression: Graphics files tend to be very large, especially if they are stored as bitmaps. This means that they take a long time to download if they are on a web page.. For this reason most graphics file formats are compressed. A simple compression method is to look for a pattern which is repeated and then just store how many times it appears rather than a copy each time. For example a large blue rectangle could be stored as the code for a single blue pixel and then the number which tells the graphics program how many there are. GIF and PNG are two bitmap file compression formats. Both use lossless compression which means that no information is lost when they are uncompressed again. Create a graphic using Microsoft Paint which is a single colour rectangle and save it as a BMP file. Now save it again in the GIF format and again in the PNG format. Right click on the files and choose Properties to look at the file sizes. What do you notice? Page 5

6 Colour Depth: The colour depth of a graphic is a measure of how many possible colours can be represented. The larger the number of bits used to store each pixel, the larger the number of colours can be represented. 1 bit per pixel = 2 colours (black and white) 2 bits per pixel = 4 colours 4 bits per pixel = 8 colours 5 bits per pixel = 16 colours etc GIF uses 8 bits per pixel which gives a possible 256 colours PNG uses 24 bits per pixel which gives a possible 16 million colours ( ) Compare the sizes of your GIF and PNG files again and see if the PNG file is larger even though it is storing the same information. Why is the PNG file larger? If you increase the colour depth of a graphic then you increase the amount of storage space it will need. Resolution: The resolution of a graphic image is the number of pixels per square inch. The more pixels per square inch, the higher the resolution and the higher quality the picture. The higher the quality the picture, the more storage space it will take up because more pixels will have to be stored. If you increase the resolution of a graphic then you increase the amount of storage space it will need. Find out what resolution and colour depth your computer display is using and work out how much storage space it would need if you were to save it as an uncompressed bitmap. Page 6

7 Lossy compression: Photographic images tend to be very large indeed. The file format used by digital cameras is JPG which is a format which allows you to choose how good a quality image you want. This example from Wikipedia shows the effect of different quality settings. A photo of a cat with the compression rate decreasing, and hence quality increasing, from left to right The JPG file format is lossy, which means that some of the information on the original picture will be lost when it is uncompressed again. Watch the video on JPG iteration in the shared documents folder to see what happens if you repeatedly compress and decompress a JPG file. Open the single colour rectangle which you saved as a BMP file, and this time re-save it as a JPG file Right click on the files and choose Properties to look at the file sizes. What do you notice when you compare it with the other formats? Open a photographic image in Microsoft Paint and try saving it in the 4 different graphic file formats: BMP, GIF, PNG and JPG Compare the file sizes. What do you notice? When to use GIF PNG or JPG? The GIF and PNG formats are best for computer generated images. The JPG format is best for photographic images. because they are all compressed formats, they are used on web pages so that they load quickly. GIF uses less colours than PNG, but can be used to make animated images. JPG formats use lossy compression, but have a much larger range of colours. Page 7

8 Audio: WAV and MP3 Audio files are very large an audio CD stores around 800mB of data which can hold around 90 minutes of music. In order to save sound to a computer it needs to be converted to a digital signal. This is done by taking a sample every fraction of a second and storing this as a digital value. The number of samples taken every second is called the sample rate is measured in Hertz and the sample size is measured in bits. The higher the sample rate and the bigger the sample, then the higher the quality of the recording. A WAV file is an example of an audio file recorded in this way. Page 8

9 Make a recording using Audacity Edit the file so that it is exactly 5 seconds long. Highlight then delete Export the file as a WAV file Change the settings for the Sample rate and the Sample size Now make another recording with the new settings and export the file as a WAV file. What difference does the change in the recording settings make to the file size? Page 9

10 The MP3 file format is a standard compressed format for audio files and is used by many personal stereos. The MP3 file format uses lossy compression. This means that some information is lost when it is uncompressed. Open one of your WAV files in Audacity and export it as an MP3 file What extra information can you add to the MP3 file? What difference does the compression make to the file size? Find out about the different bit rates (bits per second) that are used for MP3 files and what effect they have on the quality Page 10

11 Video: AVI and MPG Like audio, video takes up a large amount of storage space, and if you want to transmit video over the Internet you need to compress it. The AVI file format is the one which is created by digital video cameras. AVI files can be uncompressed, or compressed using a variety of compression systems called codecs The MPG file format is a compressed format for video. It uses a number of compression techniques including only transmitting data which changes from one frame to another and removing other information which is not perceived by the human eye. There are many other video file formats. Another common one is the MOV file format which is used by QuickTime. Investigate a number of video file formats and find out what kind of compression they use and what they are used for. Page 11