One of the aspects of digital video that is becoming more important with technology advancements is image resolution. As we move further into high-definition (HD) or megapixel (MP) cameras, it is important to understand source and viewing image resolutions, and how resolution relates to image quality.
It has long been understood, even from the days of coax and CRT monitors, that the higher the resolution of a camera or monitor, the better the picture would be. It has also been known that resolution alone is not the only determining factor for image quality, but it is a big one.
Some basic fundamentals of analog and digital video may help to sort through the confusion.
HOW ANALOG CONVERTS TO DIGITAL
Some of you new to the industry may ask, “Why do I care about analog? Isn’t this the world of IP?” The fact is that in many cases we are still progressing along the convergence path, but we are not fully in the digital universe yet. Many IP and even some MP cameras still utilize a CCD chip for imaging. The CCD by nature is an analog device. It takes light and changes it into electronic impulses, which are then encoded for IP cameras.
Also, many applications are still using analog cameras with external encoders. Either way, an understanding of analog resolution is a good thing.
If you’re not new and have been in this industry for any length of time, you probably understand that the resolution of an analog video signal is determined by the number of horizontal scan lines in the signal, or TVL (TV lines). Most recent color analog cameras are rated at about 480 TVL (although there are some newer “super hi-res” 540-TVL cameras out there.
Many people have asked if there is a direct relationship between analog TV lines and pixel-based digital resolutions. While this actually is a fairly complex question with lots of variables, the basic answer is yes.
When a 480-TVL camera is digitized, or encoded, the closest one-to-one match in digital resolution is 640 X 480. See a similarity here? 480 TVL roughly converts to 480 vertical pixels. Is this absolute? Not at all. Encoded video streams can be up- or down-converted as needed to fit the situation and encoding hardware, but a 640 X 480 (some people use 4CIF or D1 interchangeably, but that’s another discussion) digital image is approximately the same size as a standard analog camera video image.
RESOLUTION VS. IMAGE QUALITY
It’s important to realize resolution is NOT the determining factor in image quality. Resolution is strictly the size of an image in vertical and horizontal pixels (or lines). Higher resolution does not guarantee better quality. While resolution is most definitely one of the contributing factors, others include quality of the lens/camera combination (garbage in = garbage out), encoding or compression method (MPEG4, H.264, MJPEG) and the compression ratio (how much compression is applied). I can tell you, and I’m sure most of you have had similar experiences, I have seen very low quality 4CIF/D1 images and very nice looking 1CIF/320 X 240 images.
Different hardware and software video management platforms display video at different sizes and resolutions. Image quality can’t be measured on a specification sheet. The only way to truly objectively determine what platform will give you the best image quality is by using the one tool we all have, our eyes. There is no way to quantify image quality.
PUSHING THE RESOLUTION ENVELOPE
The advent of MP cameras makes the resolution discussion even more important. We’re no longer dealing with just CIF through 4CIF or D1. The term megapixel essentially means millions of pixels. The number of megapixels (1, 3, 5, etc.) is derived when the two numbers in the resolution of an image, such as 1,280 X 1,024 are multiplied together. In this case, 1,280 X 1,024 equals 1,310,720 pixels, or 1.3 megapixel. Not too complicated.
Most MP cameras in today’s security market are running between 1 and 3MP. There are some companies with higher megapixel resolutions, such as 5 and 16, and I have even seen a 21MP camera advertised, but they are still very expensive and not too common.
You may also see different designations for these higher resolutions; 1,280 X 1,024 is also referred to as SXGA, WUXGA is 2.3MP (or 1,920 X 1,200) and so on. As we move forward with megapixel and extremely high resolution cameras, however, there is one very important question you need to ask yourself: What am I going to view it on?
CONSIDER THE SOURCE … AND DISPLAY
Video is only as strong as its weakest link. The viewing device you choose must be capable of reproducing the resolutions you want to display on it. The resolutions of many MP cameras today are larger than the native resolutions of all but the biggest monitors available.
If you purchased a big fl at screen for your living room fairly recently, it is probably the consumer resolution standard of 1,080. This is actually HD 1,920 X 1,080 (or 2MP). The high-end laptop I use supports a native resolution of 1,920 X 1,440 (taking the wide screen into account). That means these monitor screens can only display up to a 2MP image at its native size. Many MP cameras have already outgrown that.
You need to be aware if you try to view a super high-res MP camera on any old fl at-screen monitor you find lying around you aren’t going to see what that camera is capable of. The monitor or display device is an equally important factor in the calculation of image quality. Make sure you match capabilities and compatibility all the way through.
