3


3.4 Extending skills


determines how many pixels are created, that is to say the resolution of the image. The camera on the first iPhone had around 1.5 megapixels – 320 pixels x 480 pixels, compared to between 10 and 20 megapixels on phones today. So, once the camera has captured the images they are stored as files … in the file format that is most suitable, depending on how it will be used … The next area we want to look at is how this data can


be converted back into images we can see and sounds we can hear … Currently, the most common way to view visual information is on a screen. So, when we look at a screen, the first thing we can see is how big the screen is. The physical size of a screen is usually measured diagonally, from one corner to another, and it is usually measured in millimetres or centimetres. Bigger screens are good, but they don’t always show an image more clearly. Image clarity depends on the image resolution of the screen … measured in ppm, the number of pixels per millimetre. For example, a screen might be described as having a resolution of 2532 x 1170 (an iPhone 13 screen) or 2340 x 1080 (a Samsung Galaxy S22), but to compare the resolution properly we need to be able to compare the physical size of the screens as well. So, now we know that a screen has a physical size


and it also has a resolution and these affect the quality of the images. However, there is also another factor that affects quality: colour depth. Colour depth refers to the number of colours that can be represented by each pixel. Remember, colours are created on a screen by combining the three primary colours: red, green and blue. The lowest level of colour depth (apart from black and white) is 8-bit colour, which represents 256 different colours. The simplest colour information is whether the


pixel is black or white and this can be represented as 1 bit – with 0 as black and 1 as white. At the next level, the colour information is stored in 8 bits (one byte), which can provide 256 different colours. At 16 bits (two bytes), we can have over 65,000 different colours and at 32 bits (three bytes), we can have over 16 million different colours on a pixel. This produces a very realistic image and it is often referred to as ‘true colour’. OK, so one last point to consider is the difference


between bitmaps and vectors. With bitmaps, there is specific information about the location and colour of the image on the screen. If the bitmap is low resolution, it will appear blurred on a large screen. In contrast, vector images always appear sharp because the information is stored in a different way. Vector files contain information on how to construct the image so it can adapt to whatever screen size it is shown on and that makes them very good for web graphics, not only because they are always sharp, but also because of their smaller file size. I should say that, of course, images are also displayed


in print formats and … 24


Part 3


So, the other thing I want to talk about is transforming sound information into data. To begin, it is useful to remember that sound is a result of changing air pressure on our eardrums. Voices, music and other sounds cause the air pressure to change at different speeds. When it changes or vibrates quickly, the sound has a higher frequency or pitch. When it vibrates more slowly, it has


64


a lower frequency or pitch. If we draw a graph of the changes in vibration, it forms a line which rises and falls, more often for higher frequencies and less often for lower frequencies. We call this type of graph ‘analogue’ because it is analogous – that is, it follows as closely as possible the information that it represents. So, all microphones can convert the vibrations in


the air into electrical signals – analogue signals – which represent that sound. However, to use the signal in a computer, the analogue information needs to be converted into digital data. This is done by sampling: that is, taking samples which represent the analogue data at specific points in time … So, for digital sound data, thousands of parts are sampled per second and these give us the sample rate which is measured in hertz. We can have different sample rates for different purposes. For music, a common sample rate is 44.1 kilohertz (kHz). This means that 44,100 samples are taken every second. Some of the original sound information is lost, but there is still enough information for listeners to hear the music clearly. For voice, a standard sample rate is 8 kHz, that is, around 8,000 samples per second. This is about five times lower than the sample rate for music, but – because a voice signal is less complex than a music signal – enough of the information remains to allow us to understand what the person is saying and to recognize the voice … So, the next area related to quality is how the digital


data in a sound file is transformed back into information – the sound you hear in your ears … from a speaker or in your headphones … To create the sound waves you hear, we need to convert the digital data back into analogue signals. When we looked at how digital sound was encoded, we saw that we needed to take samples from the analogue signal to create a digital representation of the sound wave … so now we need to take these samples and put them together to recreate as much of the original analogue signal as we can … The original sample rate will affect the quality of the output – we would expect to see higher sound quality with a higher sample rate … Another factor that affects sound quality is


something called bit depth … the greater the bit depth, the smaller the changes in the sound which can be represented in the data. A bit depth of 8 would allow each sample to represent one of 256 different variations in sound. A bit depth of 16 can represent over 65,000 variations and a bit depth of 24 bits would allow over 16 million variations. So, the greater the bit depth, the easier it is to recreate the original signal. I should also mention something called DAC – digital to analogue convertors – which are often used to increase sound quality. These can take the original digital data and increase the number of samples … they do this by looking at two samples, guessing what values should go between them … and creating new values for these samples … so the digital signal which is used to create the analogue signal is much smoother …


D 25


Set for pairwork, and play Part 4. Elicit key points as a whole-class activity.


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