Imagine the scene. Your marketing department comes to you, the design engineer, with an idea: to add video-capture capability to an existing product. How difficult can it be? After all, even a non-engineer can attach a web camera to a PC and turn it into an internet video terminal!
If this happens to you, beware. It can certainly be done. In fact, very effective capture of moving or still images can be achieved at relatively low cost. But it is not simple, and it requires some understanding of the protocols and standards employed by the imaging industry.
This article outlines the key issues from the point of view of a design engineer who has not previously worked with video.
To illustrate the issues, this article will use as its example an imaginary project to add moving video-capture capability to a simple PIR intruder detector. In a conventional burglar alarm system, the sensor detects the presence of an unauthorised person, and triggers an audible alarm, and often also transmits an alert to the police or a security company.
The design example illustrated in this article shows how, in addition, the sensor device would be able to capture images of the burglar for later use by the police and courts of law.
The camera will need particular attributes in such a system:
- Ultra-low cost " since there are multiple sensors and multiple cameras in a building, a low unit cost for the camera is essential if the overall system cost is to be kept to a reasonable level;
- Small size " the devices must not be visually intrusive, particularly for residential use;
- Adequate image quality " sufficient to enable identification of an intruder in a wide variety of lighting conditions, to a standard required by the law. The system must offer high dynamic range in order to work as well in bright sunlight as in near-darkness.
These attributes dictate the use of a low-cost image sensor. While the latest consumer cameras boast image sensors with more than 10Mpixels, this design can use a device with as few as 1Mpixels. Such devices are available at very low cost, thanks to the proliferation of image sensors in mobile phones and vehicles.
As the article will show, however, this presents a number of challenges to the system designer, particularly in:
- Interfacing the sensor's output to a host processor or microcontroller;
- Capturing a suitable image under lighting conditions that are far from ideal.
