Image and Sound Representation Component 1.4 Image and Sound Representation

Assessment Outcomes Explain the digital storage of graphics Explain the digital storage and sampling of sound Describe the use of metadata in files

Starter: Pre-Reading Questions What is a bitmap graphic? What is sound resolution? What is a vector graphic? What is the sound sample frequency? What is metadata? What are the benefits of sampling sound digitally? What is analogue sound? What is digital sound? What are the drawbacks of digital sound sampling? How are bitmap graphics stored digitally? What are vectors used for? How are vector graphics stored? How is analogue sound stored?

Image Representation Images on a computer system are made up of thousands of small coloured dots, known as pixels (short for picture elements). Bitmap images are stored as an array of pixels. A black and white bitmap image will store a 1 for a black pixel and 0 for a white pixel.

Vector graphics Images can be stored in different ways in computers. When you create a drawing in power point this is a vector graphic. Each line, circle or square is made up of simple properties such as position height and width.

Bitmap graphics When you take a picture with a digital camera it is not made up of lines. To store this type of image on a computer the image is broken down into very small elements called pixels. Each pixel is set to one colour.

Resolution A Bitmap image is made up of a grid of pixels. An image could be 600 pixels wide by 400 pixels high. The resolution of this image would be 600X400. If the resolution is increased the quality, not the physical size, is increased and it takes up more of memory to store it.

Making an image file For a picture of a flower four colours are used. These numbers are represented as binary. The number of bits we use dictates how many colours we could use. If we wanted to store more than 4 colours we would have to use more than 2 bits.

Making an image file We take the information from the image file and build a image data file We read left to right. Top to bottom.

Meta Data For the computer to interpret an image file and rebuild the picture it must know some other things about this data file. For example… The resolution is 8X11 The colour depth is 2 bits per pixel What colour is each pixel Without metadata, a computer system may render an image incorrectly on screen, such as displaying all pixels in one row. Other data may also be stored in the metadata of an image file, such as the date the image was made, the geographical location of a photograph.

Making an image file with metadata We put the information about the image file (height, width, bit depth) before the information to make it We take the information from the image file and build a image data file We read left to right. Top to bottom. 1011 1000 0010 10 10 10 10 10 10 10 10 10 00 10 00 10 00 10 10 10 10 00 00 10 10 10 10 00 00 01 00 00 10 10 10 10 00 00 00 10 10 10 10 00 10 00 11 00 10 10 10 10 10 10 11 10 10 10 10 10 10 10 10 11 10 10 10 11 11 11 10 11 11 10 10 10 11 11 11 11 10 10 10 10 10 10 11 10 10 10

Graded exercises

Sound Representation As we have already established, a computer system is only able to store and process binary digits, as it is a digital device. Since this is the case, how can sound be stored as it is an analogue signal not digital? If an analogue signal, such as sound, is sent to a computer system, it has to be converted into a digital signal before it can be processed.

Analogue Sound waves are analogue, which means that they continuously change. Computers cannot store analogue signals. They must be converted in order for the computer to use it.

Digital Anything stored on a computer must be stored as a series of binary numbers or digital. Digital signals are exact and square.

Analogue-to-Digital Converter (ADC) The device that converts the analogue wave to digital is know as the analogue-to-digital converter or ADC. You might know this as a sound card.

Storing sound waves Storing analogue sound waves as numbers is very easy. We match the interval with the resolution

Resolution This is the number of bits used to store each sample. The more accuracy you have the larger amount of numbers you will need.

Sample Frequency The number of samples taken in a given time period. The more samples taken the better the quality of feedback

Sampling Sound is converted into a digital signal by a process called sampling. Sampling is where hardware, such as a microphone, measures the level of sound many times per second and records this as binary digits. The number of times that the sound level is sampled per second is called the sampling frequency. The higher the sampling frequency, the better the quality of the sound recorded. A typical sampling frequency is 44,000 times per second, also known as 44 kHz. This is the sampling frequency used on most audio CDs.

Sampling Sound sampled at 44 kHz in stereo will produce a large amount of data and as such, this data may need to be compressed. When sound files are compressed, data is removed to reduce the size. This reduction in size means that

Graded Exercises

Past Exam Questions

Crib Sheet – What I must remember about _______ Starting Point: Read the learning outcomes from the exam board Record the key facts that you need to remember about Image and Sound representation Think about what you have learnt today, what questions you have been asked, definitions of words, or anything else you think is important!

Homework Question Sheets: Q25 – Images and Pixels Q26 – Image Size Q27 – Digitising Sound Exam Questions: Complete your Year 11 target grade Pre-Reading: 50-56 Notes on Data Types and Data Structures