"Output" is when data from the computer is translated from digital to analog and expressed to the outside world. There are three major types of output devices: monitors, speakers, and printers.
Until the year 2000, CRTs (cathode ray tube displays) were the normal monitor used, but in the ten years between 2000 and 2010, they were almost completely replaced by thin, flat-screen LCD monitors. The primary reason was size: CRTs were huge, bulky, and heavy.
LCD (liquid crystal display) monitors allow for sharp, bright images to appear on a very flat screen.
The light from a single row of LED lights is spread across the whole screen by several layers of film. Next, a technology called "liquid crystal" is used. This is a chemical which allows light to pass normally, but when an electrical current is given, the light can be dimmed or turned off, to show black. By using electrical signals, each tiny part of the screen can become lighter or darker.
Finally, there is a layer of colored cells—in red, green, and blue—which add color to the display. For a detailed visual explanation, see this YouTube video.
A pixel is one dot on your screen. Each pixel his a separate color. Here is a pixel as you might see it in a graphics editing program, like Photoshop:
Each pixel is made up of three sub-pixel elements, one each of red, green, and blue. The pixel changes color by brightening or darkening each element.
As a demonstration, I photographed a monitor showing the LCJ Mail page. This is from the upper-left corner of the screen. Notice that the white box in the first two frames shows what is in the next image to the right, until you can see a single pixel with the three pixel elements:
In the image above, notice that the pixels in the "J" for "Japan" only have the red subpixel; the green and the blue are turned off. Meanwhile, for the white pixels in between the letters, all the subpixels are on at 100%, to make the white background.
Here is a microscopic image of pixels in an LCD monitor:
The exact same effect is used in giant video screens, like ones you see at sports stadiums. In those screens, they use red, green, and blue lights which are the size of your hand! However, from a long distance, they all blend together to make other colors. Below is a close-up of "pixels" from a "Jumbotron"-style stadium screen:
You can find a web page where various monitors' pixels are compared. Also, below is a video showing magnification of pixels on various iPads:
Pixel Size & Density
When pixels are very large, the quality of the image is poorer. For example, if you walked up close to a jumbo-sized stadium screen, you would not be able to see the picture clearly; the pixels would be huge!
The smaller the pixels are, the better the image. Below is a sample of three monitors created for handheld devices. The one on the left is 132 ppi (pixels per inch), meaning that 132 pixels fit across one inch of the screen. The second image has 244 ppi, almost double the number of pixels in each direction. The last one is 498 ppi, almost 4x the number of pixels in each direction. Notice that the quality improves each time the pixels get smaller and more numerous.
A few years ago, Apple began releasing iPhones, iPads, and laptops with something called the "Retina" display. This is simply a screen with smaller-than usual pixels. Previously, Apple's monitors were between 100 and 130 ppi; with Retina displays, that improved to 220 to 326 ppi.
Pixel densities are improving; this year, Sony released the Xperia Z model smartphone with a 5-inch screen with a pixel density of 443 ppi.
However, there is a limit to the usefulness of pixel density. Apple's "Retina" name is based on the idea that the pixels are smaller than the human eye can see. Beyond a certain size, there is no benefit of making the pixels even smaller.
The term "resolution" is very important. It means the total number of pixels on a monitor, or in an image.
Resolution is measured by the number of pixels on a monitor, horizontally and then vertically. A common monitor resolution is 1280 x 1024—meaning that if you could count the pixels from the left to the right, you would count 1,280 of them, and 1,024 from top to bottom. If you click on the chart below on the right, you will see an image demonstrating various resolutions.
When referring to an LCD monitor, native resolution is used to describe the number of physical pixels in the display. Monitors are also able to display resolutions lower than native resolution, but not higher.
Resolution can be used to describe the number of color points in a monitor, a sensor, or an image. For example, you could have a monitor with 1280 x 1024 resolution, or a digital camera with an 8-megapixel sensor which takes images with a resolution of 3264 x 2448 pixels.
The higher the resolution, the more information is on the screen, and the better-quality the image will be. In short, "higher" resolution is better.
Overall quality of a monitor, however, is determined by three factors: resolution (the number of pixels); pixel density (the size of the pixels); and viewing distance (how far you are from the screen).
For example, let's say that you have a 5-inch Xperia Z smartphone, and a 50-inch big-screen TV. Despite the great difference in size, both devices have the same quality image! The Xperia Z has a 1920 x 1080 resolution, exactly the same number of pixels as the big-screen TV. Therefore, the information in the image is exactly the same on both devices. The main difference: you must sit far away from the TV, but you hold the smartphone just inches from your face.
Other Monitor Terms
There are three other terms regarding monitors that you should know: one is the refresh rate; the other two terms are related, interlaced and progressive scanning.
These terms have to do with how an image is created on a monitor. Typically, the image is created line by line, from the top of the monitor to the bottom. This is called a scan.
In order to create the illusion of motion, many scans are created so quickly that the human eye sees them as a single fluid, moving picture.
The refresh rate (also called the scan rate or frame rate) is how many times a scan creates a picture on the monitor. Traditionally, TV has had a refresh rate of 60 Hertz (60 Hz). Since TV uses interlaced scanning (see below), that equals 30 fps (frames per second). Movies have traditionally used 24 fps.
In film projectors and CRT TV sets, the scans flicker—meaning that there is an image shown, then darkness, then another image, and so on. If the scan rate was not set correctly for CRT monitors, the flicker could cause your eyes to feel uncomfortable.
LCD monitors do not flicker. The picture is always there. As a result, the refresh rate can be increased, and the main result will be higher quality images.
Generally speaking, the higher the refresh rate, the better the quality will be. A recent movie, The Hobbit, was filmed and presented at 48 fps in order to increase the quality of the film. However, slower frame rates create more blur, which many consider to look natural. When you remove the blur, many viewers feel the image looks "too real."
Another use of higher refresh rates is 3-D display. In such a case, half of the scans create the image for the left eye, and half the scans create the image for the right eye. Many new 4K TV sets use a refresh rate of 120 Hz. New monitors are being developed with refresh rates of 300 Hz or higher.
Interlaced and Progressive
Very often, when you see numbers referring to resolution of TV images, you will see a small letter "p" or "i" at the end of the number, for example 1080i or 1080p. These numbers do make a difference.
The "i" stands for interlaced scanning. This means that the monitor uses two full scans to create one image (or "frame"). Half the lines are displayed in one scan (e.g., 1, 3, 5, 7, etc.) and then the other half are displayed in the second scan. This explains why TV shows 30 fps but has a refresh rate of 60 Hz. This is done because of legacy techological difficulties existing in television broadcasting. Interlacing reduces the clarity of the image.
The "p" stands for progressive scanning. In this method, each scan creates a complete image. In this case, 60 Hz equals 60 fps. Computer monitors have always been progressive, which is one reason why they were usually higher-quality than TV sets.
If you have a TV set and a Blu-ray player, you perhaps can see the difference also. Normal TV signals are broadcast in 1080i (1920 x 1080 resolution, with interlaced scanning). Blu-ray, however, is 1080p, which means there are 60 fps instead of 30. Not all movies benefit from the increase, but some do—especially computer-animated features. This is one reason why stores often display TV sets with such movies.
There have been four basic types of printers used over time.
The two older types fall into the same category: impact printers. These are printers which physically strike the paper, with an inked ribbon in between, to leave a mark on the paper.
Typewriters work by having metal keys with raised letters on the surface. These keys hit the inked ribbon and then the paper to leave an imprint of each character. These typewriters were limited to a single font, usually what today is called "Courier."
More modern typrewriters would not use metal keys, but instead would have a metal or plastic ball with raised letters all around the surface. The ball would be rotated to type each letter. Another style was the "daisy wheel" which had the keys on a plastic wheel which would spin to type each new letter. These balls and wheels could be replaced, allowing for different font types and styles.
Dot Matrix Printers
Dot matrix printers were common in the 1970's and 1980s. They used metal pins to strike the ink ribbon and paper. Letters would be formed by creating many small dots in that letter's shape. These printers produced very low quality text, but they were very flexible, and were the first commonly-used printer that could print images. However, they were extremely loud!
Laser printers use the same process as copy machines. The printer has a metallic drum which has an electrostatic charge. Lasers "paint" a negative of the image onto a rotating drum; where the laser hits, the electrostatic charge disappears. A finely-powdered ink called toner will stick to the areas which still have a charge. The toner ink is then transferred to a piece of paper using heat. Laser printers are best when you do a large number of black-and-white prints. Color laser printers are avilable, but they are more expensive.
Inkjet printers spray small dots of liquid ink on paper to create images. This spray is not precise, and under the microscope (see image at right), it looks fuzzy and messy. However, the dots are small enough that it appears as high-quality to the human eye. Like color laser printers, they usually use the CMYK (Cyan Magenta Yellow blacK) ink colors to create the printed images.
A warning about inkjets: printers may seem cheap, but that is because most of the profit is made selling ink! In fact, with some cheap printers, changing the ink just once may cost more than the printer did! When looking for a printer, also check out the cost of the ink cartridges sold for that printer.
While monitors are measured in ppi (pixels per inch), printing quality is usually measured in dpi (dots per inch). While dpi on a scanner is equal to ppi, printed quality is quite different. Printed "dpi" is much lower-quality than ppi. However, most printers today have a high-enough dpi that it does not make a noticeable difference for most people.
There's not much to say about speakers. Some Desktop computers have decent built-in speakers, but many have very poor ones. Most speakers on laptop computers do not have very good sound quality either. You will either want to listen with a good set of headphones, or else purchase a set of speakers which can be connected to the computer.