5.3 Digital camera
The last computer I am going to look at is the embedded computer in a digital camera.
Figure 10 shows a picture of a digital camera. The screen of the camera is displaying a picture that has previously been stored in a memory card within the camera. This memory card is not the camera computer's main memory, nor is it the secondary memory used to hold the computer's program; it is a form of removable secondary memory where the computer stores the images taken. Next to the camera in Figure 10 is an example of the memory card that this particular camera uses to store its images. The memory card can be unplugged from the camera and another memory card inserted.
When the user presses the button to take a picture with a digital camera, its shutter opens, and the lens system focuses light from the image being photographed onto a device called a charge-coupled device or CCD. The CCD consists of a two-dimensional array of tiny light-sensitive cells that convert light into electrical charge. Figure 11 shows this array of cells and how the CCD is located behind the camera lens. The brighter the light that hits a cell, the greater the electrical charge that accumulates at that site. Once the camera shutter has closed, the information stored in the form of electrical charge at each cell is converted into a binary code and stored in the form of 1s and 0s in the camera's memory, and this forms the image captured by the camera. To collect colour information a system of colour filters is placed over the cells of the CCD.
This stored raw data representing the image is then processed. The colour is reconstructed and adjusted, and techniques are used to sharpen the fine detail. The result of this process is a picture ready to be stored as a file in the camera's secondary memory. To reduce the amount of stored data, the file is usually compressed – that is, the number of bits used to represent the image is reduced. In some cameras the user can select options to choose the type of compression carried out. The process of compression is described later in this block.
Figure 12 shows the actions that the digital camera performs when taking a picture. Note that this diagram is not a functional block diagram of the camera but shows the actions that must occur to take and store the picture, in the order in which they must happen. The digital camera shown in Figure 10 has some buttons that allow the user to set particular conditions when taking a picture. In addition to the button to take a picture, there are buttons to set the flash, control the preview of the stored images on the screen and set the zoom ratio. As there is a flash facility, there must also be a light-level meter incorporated into the camera; the level of light falling on the meter determines whether the flash will operate.
As with the PC and the electronic kitchen scales, a specific form of Figure 3 can be created for this digital camera.
Activity 9 (Exploratory)
Using the information about the digital camera given above, draw the specific form of Figure 3 for this camera.
This computer has main memory and also two items of secondary memory: the removable memory card and the internal secondary memory. The input devices are the CCD plus the buttons to take a picture, preview the stored images, set the flash and set the zoom ratio. The light meter is also an input device. The output devices are the camera's screen, the flash mechanism, the zoom and the control to open and shut the shutter. (If you are a camera enthusiast you may also have thought of the various controls for the shutter aperture, the focus etc., but as I have not explicitly mentioned them in the text I have not included them in my answer.)
Figure 13 shows all of this. Notice that this computer is complex, with a very wide range of input and output devices.
Click on 'View document' link below to see a larger version of the 'Functional block' diagram.
Normally the images stored in the camera are downloaded to a PC for viewing on the monitor and printing. One way of doing this is to take the memory card from the camera and put it into a memory card reader connected to the PC. If this is done, the memory card shifts from being a secondary memory of the camera to a secondary memory of the PC.
Another way of downloading the images is to use a USB cable, supplied with the camera, to connect the camera directly to the PC for data transfer.