Picture Quality: A Pixel is worth a thousand words…

Picture Quality: A Pixel is worth a thousand words…

Contributor: Paul Broome
Company: Sensormatic Video Products Division

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.

With digital electronics, we sample a scan line some number of times. We continue to sample additional scan lines. This provides us with a matrix of the make-up of the original picture in digital format. We can then discard some of the pixels both horizontally and vertically, perform some color changing magic to compensate for lost pixels, convert the image back into NTSC format, then place four, nine, or, sixteen images on the same monitor. Do you lose some picture quality with each transition from one form to another? Yes you do. These are basics of how resizing is done. The particulars of how this is done make all the difference in the end result. So, what is a pixel?

A “pixel” is a picture element. This is the smallest portion of a digitized picture which can be changed. If you have ever taken a close look at one of those spectacular mosaic tile murals in a nice Mexican, or, Italian restaurant, you noticed that the detail is composed of small tile pieces which have been artfully arranged to provide the beautiful picture. Pixels are something like the tile pieces. Pixels are typically expressed as a matrix, like, 256×256, or, 512×464. This refers to the number of individual pixels, horizontally first, then vertically.

If one person is speaking in lines of resolution and another is preaching pixels, is there any hope? You can convert pixels to Lines of Resolution by multiplying pixels by .7. This seems to be the generally accepted conversion factor. Where did this come from? Pixels are the total number of individual elements across the picture (or top to bottom). Remember that Lines of Resolution are measured across three quarters of the horizontal picture. 75%, or, 70%. Its pretty close. The theory is that each pixel can display an alternate color line.

What is the best combination of equipment to comprise a CCTV system? This could be a long debate. Each manufacturer will say that their equipment is the right fit for the right application. Is the end user willing to pay any price to get the very best equipment on the market today? Don’t count on it. Sometimes doubling the price will only get you a small increase in quality. The real test of how good the final picture looks is to do side by side comparisons. Big numbers for lines of resolution and large numbers of pixels don’t count for much if the final result is a fuzzy picture. The quality of the image produced by any individual component in a CCTV system is directly related to the quality of the image provided by the proceeding component in the system. All components are important. The overall image quality is most limited by the weakest component in the system. Let’s discuss each component in a small CCTV system one at a time. For this discussion, we have cameras, a multiplexer, a VCR and a monitor.

Cameras: As we now know, almost all cameras produced for CCTV today are of the CCD type. This means that they are digital devices, meaning that the camera captures images in a pixel format, then converts it to NTSC. Since camera manufacturers are used to rating their devices in lines of resolution, they still do that and also rate them in pixels. If you use your calculator and compare the lines of resolution and the pixels, you will see that lines of resolution are between 70% and 75% of the horizontal pixels, depending on the manufacturer.

Lenses: Don’t forget lenses. If the image is not presented to the CCD pick up device of the camera accurately, everything else will suffer. Know what you will be looking at. Constant indoor lighting can provide good images from an inexpensive camera. Once you venture outside, a low lux level is required to provide adequate images in low light situations. The general rule is, auto iris in varying light conditions and fixed lenses in constant light (indoors).

Multiplexers: By nature, multiplexers digitize images from a number of cameras, resize them, then display them on a single monitor. Typically, a second “spot” monitor is available to display single full size images of a selected camera. With robot multiplexers, separate circuitry digitizes the camera images then displays them on the monitor and sends them to the VCR for recording. 512 horizontal pixels are included in the monitor image. 730 horizontal pixels are sent to the VCR. Images displayed on the spot monitor are not digitized. This image is simply redirected from the camera to the spot monitor. Images on the main monitor have been first digitized, then converted back to NTSC for monitor viewing. If you want to see how much the image is changed by digitizing it, select the same full size image on both the main and spot monitors. Remember that the monitors must both be adjusted properly. Most people don’t notice the difference.

VCRs: VCRs are traditionally the weakest link in the chain. The NTSC images are recorded on a magnetic media (the VCR tape). The physics of this technology is not as accurate as the other devices. The frequency response of the tape typically produces black and white at a higher quality than color, due to the required bandwidth. VCRs indicate the quality of their output in lines of resolution, usually stating black and white and color as different numbers. Once the images are recorded on tape, the process must be reversed to view them. The images on the tape must be processed and once again converted to NTSC format.

Multiplexers: Once again, the multiplexer must take the NTSC image from the VCR, convert it to pixels, process it, then convert it back to NTSC for the monitor.

Monitor: monitors take the NTSC input and display it on a large vacuum tube.

Cables: Don’t forget cables. Each time the NTSC image is moved from one device to another, it typically is transported through coax cable. Each cable has a connector at each end. If you have made one of these connections, you know why bad cables are the number one cause of failures in the CCTV industry. The quality of the cable, the cable distance and the connections can greatly effect the quality of the final image.

Even in the relatively simple system in our discussion, the video image has undergone eight separate transitions. Each time the image loses some of the original quality. If you were to ask how many lines of resolution are provided by a multiplexer, you could just take the horizontal pixel count and multiply by .7 as we discussed earlier. However, we know that the lines of resolution produced by the multiplexer is dependent on the quality of the image provided to it.

Which is the most important component in the system? The real answer is that they are all interdependent. Due to the technology used, the VCR usually is the most limiting factor.

How about SVHS video format? The standard color video consists of chroma (color) and luma (black and white). These are combined in the NTSC format. Some devices separate the chroma and luma to process separately. Quads, multiplexers and VCRs often process these signal separately. Quads and multiplexers, which typically separate the signals internally for processing, can send the signal separately to the VCR for SVHS processing. This eliminates one of the transitions and allows for improved quality with VCRs which have SVHS capability. VCRs which have SVHS capability are typically of better overall quality and usually produce better pictures even when the standard BNC connectors are used.

So, what have we learned? There are different methods used to express the quality of a video image. “Pixels” are used with digital devices. “Lines of Resolution” are used with analog devices. “TV lines” are the same as lines of resolution. You can convert from “Pixels” to “Lines of Resolution” by multiplying Pixels by .7. (If you run into someone who wants to express their output in Megahertz of bandwidth, you can guesstimate that by dividing “Lines of Resolution” by 80. Ex. 200/80 = 2.5Mhz). If someone points to a monitor image and asks if that is an analog, or, a digital image, the answer is that all standard monitor images are analog. The image may have been digitized (in fact several times) prior to display on the monitor. Typically, all modern camera images start out digitized. Quads and multiplexers digitize the multi-screen images. The spot monitor images on the multiplexer and the full size images on quads are typically passed through directly from the previous device. VCRs are usually the weakest link in the image chain due to the conversion to and from magnetic tape, with color suffering more than B&W. Several image conversions take place, even in a small CCTV system. The overall image quality is limited to the quality of the weakest link in the system. All system components are interdependent quality wise. Each produces a picture based from the information provided to it.

There is much information in these few pages which should provide more than is needed to make any customer feel that they are dealing with an industry expert. This information is rarely found in a single document. If you were to ask everyone you know in the CCTV industry, you might find a handful who could explain all this off the top of their head.

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