Contributor: Paul Broome
Company: Sensormatic Video Products Division

Picture Quality: A Pixel is worth a thousand words...

And it could take a thousand words to explain the various terms used to describe the quality of a CCTV image. There is a lot of confusion out there. You hear terms like; Scan Lines, TV Lines, Lines of Resolution, Digital Images, Analog Images and Pixels. What does it all mean?

CCTV is an industry in need of standards. Manufacturers of different equipment used in the same CCTV system often use differing terms to describe the quality of the image they produce. The "box processing" people, Quads and Multiplexers, stick to Pixels, as do manufacturers of computer equipment. VCR and Monitor manufacturers use Lines of Resolution. Camera manufacturers use both Pixels and Lines of Resolution.

One standard that we do have in the US is NTSC (National Television Standards Committee). This is the agreed upon configuration of transmitted video used in television broadcast. This standard (minus the carrier frequencies and additional data used in broadcast TV,) is the way that all video in coax cable is formatted. This is often referred to as composite video, or, base band video. It contains video, horizontal and vertical synchronization and other things necessary to give CCTV manufacturers a starting point from which to begin design of their contributions to the CCTV industry. This standard has been around and has remained relatively unchanged since the big depression, long before the advent of the digital electronics age. Some of today's confusion stems from the different measurements used in digital electronics.

Before digital electronics, manufacturers of cameras, monitors and VCRs, measured the end quality of their products by viewing a test pattern. The test pattern is a series of converging lines with a circle in the center. Smaller patterns of black and white lines are scattered about the image. All the line sets are calibrated to a known number. If a line set were extended across three quarters of the width of the screen, it would take the indicated number of lines to fill that area of the picture (200, 400, etc.). Why three quarters and not the full width of the screen? The ratio of the standard TV image is three high to four wide (a rectangle). By using three fourths of the width, lines of resolution could be compared horizontally and vertically. If you are old enough, or, if you find a TV station that shuts down for some period of the day, you will have seen this exact test pattern on the TV screen. It is usually accompanied by an audible tone. (It used to wake up us old timers when the TV stations shut down in the wee hours of the morning.)

To do the test, you looked at the converging sets of lines and estimate where on the pattern the formerly black and white lines turn into a mass of gray. Extremely accurate? Hardly. However, you eventually reach a consensus among technical and marketing people and a number of "Lines of Resolution" is placed on the spec sheet. So

"Lines of Resolution" is an after the fact visual appraisal of the video picture based on the ability to distinguish between alternating black and white lines as they become smaller and closer together.

If you have grasped this concept, then you already know what "TV Lines" are. They are the same as lines of resolution. The terms are used interchangeably. TV Lines have nothing to do with Scan Lines.

"Scan Lines" are the individual electron beam tracings on a television, or, monitor. There are a total of 525 lines in the NTSC format consisting of odd and even fields at a rate of 60 fields per second (30 frames).

Up until now, we have been referring to "analog" pictures. By that I mean pictures produced in the traditional television format. Tube cameras, VCRs and TV monitors all did it this way. Contemporary CCD cameras and computer monitors use digital pixels. VCR technology and TV monitors are still analog.

Enter digital electronics into the picture

There is only so much you can do with an analog picture. Would you like to see two pictures on the same monitor? You can use a switcher to alternate two images, one at a time. You can use an analog splitter to overlay one image on another. This produces either two tall thin images, or, two short wide images. In order to get four symmetrical images on one monitor we need to perform some digital wizardry.