What do we want? Information.
Tuesday, December 2, 2008
Web cams are not the latest technology. In fact, according to my research, you could pick one up for the paltry sum of $99 in 1996. What is new and improved is residential and collegiate access to so-called high speed Internet connections which make streaming video accessible to the masses. To review, streaming video is sent in packets or frames. Instead of downloading an entire QuickTime file for example, you only receive the set of frames you are viewing, like a broadcast television program. Web cams can be used to capture a video file, like a traditional video camera or to send streaming video. There are two general categories of web cams. The first is the most common, a tethered camera connected to a laptop or desktop computer. The second, called an IP camera, uses a built-in web server and static IP address to send video as a stand alone device.
A web cam is a video camera. It has a lens, an iris and a sensor chip. It senses light, like any video camera and digitizes or captures it, frame by frame. Standard definition American television captures video at a rate of 29.97 frames per second (for our purposes here let's call it 30 frames per second). This is important to note because when we think about capturing 30 snapshots in one second, we are suddenly accumulating an enormous amount of data. Video captured from a high end video camera produces enormous files, somewhere in the neighborhood of 99 megabytes per second. Streaming this signal on the Internet would require dedicated fiber optic cable. To create smaller video streams we do two things, compress the signal (more in a moment) and reduce the frame rate, the amount of data per second. With this in mind, web cams are designed to capture video at rates lower than 30 frames per second. Some as low as five fps. Video at this rate is very choppy and looks stroboscopic. Somewhere in the range of 15 fps becomes palatable.
A professional photographer will tell you that the lens makes all the difference. Because of their compact nature, web cams have a small lens. A small lens means that the distance between the glass of the lens and the sensor that detects light is short. Generally, this means that the focal length of a web cam is short which in turn means that depth of field is also short. Translation: the camera can only see what is directly three feet in front of it.
The sensor chip, which is usually a CMOS chip (some use a CCD), is simply a light detector. When a portion of the chip, called a picture element (pixel), is excited by a photon of light, it converts the intensity and frequency of that light into an electrical signal. The pattern of these excitations becomes the picture. This process is sampled. In other words, the camera reads the sensor chip information at regular intervals not continuously. This is called the refresh rate or sample rate. Better chips are capable of higher sample rates.
Larger chips have more picture elements and therefore have a higher resolution. Resolution is an interesting word that conjures many fallacies. Resolution, in the digital world, is not equivalent to quality. One more time, resolution is not equivalent to quality. In the digital world, resolution refers to scale. When we speak about quality we use words like contrast, focus, balance, composition and the like. Resolution determines how many pixels are captured or sampled which often relates to quality but does not guarantee it. Typical resolutions for web cams range from 320 X 240 to 640 X 480 measured in pixels. A window that is 320 X 240 contains less pixel data and therefore produces a smaller stream. A larger window, while easier to see the subject, produces more pixel data. A resolution of 320 X 240 is generally acceptable for most purposes.
Web cams come in all shapes and sizes. It is important to think about where a web cam will be used. Many now come with what has been termed a 'claw' which is, in essence, a clamp that attaches to lap top monitors. Some come with suction cup bases and double-sided sticky tape. It's worth considering how you will use a web cam when considering how it is mounted. Usually, standard web cam composition is a 'head shot' (top of the head to neck or mid sternum shot). Centered on top of your monitor may work. Web cams built in to laptops are usually configured in this manner.
Most modern web cams utilize either a USB or FireWire interface. Early models used the serial port (nine pin 'D' connector) which was a problem for some users who also had a serial mouse and Windows 95. In the web cam world today, USB is by far the most common connection. Almost without exception, web cams get their power from the USB bus. Laptop owners should note this fact, though the drain on battery life is comparable to a wireless mouse (less than 50 milliamps, depending on the camera).
It should be clear at this point that video produced by a web cam is not cinema quality art. Nor is it meant to be. A web cam is a communication tool and an entertainment device. Video chat, distance learning and collaborative meetings and events can be enhanced with a modern web cam and a solid high speed connection to the web.