Coming Into Proper View
Deciding which camera resolution to use has long been an involved process, and has only been made more demanding with the advent of megapixel and HD technology. Here’s a clear-cut account of the necessary topics to heed in order to successfully meet customer expectations.
Since the inception of video surveillance cameras, customers have been faced with the challenge of deciding which camera resolution will best meet their requirements.
With analog cameras the choice was usually between standard (>330 TV lines or TVL) and high (480 TVL) resolution. Even higher resolutions became available with 540 TVL. Then digital recording emerged and terminology switched to CIF (Common Intermediate Format), 2CIF, 4CIF and D1. As customers demanded even higher resolutions, megapixel imagers were introduced. Nowadays, yet another term — high definition (HD) — is the latest resolution catchphrase.
So which resolution should you choose? It depends on what you are pointing your cameras at and factors such as storage and bandwidth. Several aspects must be considered to ensure that the overall solution meets the needs of the customer from a technical and financial standpoint.
Analog Resolution Up Close
So where to begin? Let’s start with the camera itself, and a brief discussion of what resolution is and how it is measured.
Analog camera resolutions are based on horizontal TVLs. The standard for an NTSC signal is 525 horizontal scan lines. However, not all the lines contain actual video information. Other lines are needed for vertical blanking, which is used to separate frames/fields and to store information. The resulting number of video lines required to create an NTSC picture is about 480, which is a fixed number. The detail that can be seen within each one of these horizontal lines is what we refer to as horizontal TVL.
Still not clear? Try this: Horizontal TVL refers to the number of black and white vertical lines that can be seen within a horizontal scan line. The “Camera Resolution Chart” below is an example of a test chart used to measure horizontal TVL. The point at which you can no longer determine the difference between black and white lines is your horizontal TVL. Camera specifications are based on how many black-and-white lines can be seen in three-fourths of the horizontal scan line (using a 4:3 aspect ratio).
Considering Digital Imaging
In the digital world, resolution is expressed as horizontal and vertical pixel counts. The standard formats we see for NTSC-based images are shown in the “Compression and Resolution Formats” table on page 93.
Some of these resolutions are based upon CIF and some are based on video graphics array (VGA). In both cases, these terms appeared with the introduction of DVRs. Today, these resolutions are applied to encoders and IP cameras. Which resolution is used depends on the application and the options within the edge and recording devices. QCIF (Quarter Common Intermediate Format) is usually considered unusable, as it provides an insufficient level of detail. To understand why, let’s look at the images that comprise “CIF Comparison” below, a native and expanded image resolution comparison using a Rotakin and other camera testing charts.
Each of the three images in “CIF Comparison” is shown at a 1:1 ratio, which is why the 4CIF image (704 X 480) is larger than the QCIF image (176 X 120). The QCIF image is small, so the natural tendency is to enlarge it to see more detail. However, as shown in Image 3 below, when the QCIF image is expanded, the detail does not improve and, in fact, could be perceived as being worse. For this reason, it is often a good rule to view images in their native 1:1 resolution. Remember, how images appear to you often depends on how they are scaled on a visual display.
One other point to be aware of is that with 4CIF and D1 resolutions, recording from a camera using interlaced video can result in motion smearing. This can be solved by deinterlacing techniques, but that approach may result in some resolution loss. Motion smearing is best solved by using a camera that provides a progressive scan output. Or, if an organization has to use interlaced cameras, they should simply stay with 2CIF images, which do not typically result in interlace issues.
Overall, if you want the best image, D1 offers the highest resolution. To go higher requires a different type of camera sensor; time to talk about megapixel and HD video.
Megapixel Under the Microscope
Megapixel, simply put, means 1 million pixels. The number of megapixels in an image is based on the size of the sensor. Typical megapixel camera resolutions are shown in the table above. Compared to a D1 image, a 1.3-megapixel image offers 274-percent more pixels, making megapixel cameras the ideal choice for applications with large surveillance areas and digital zoom requirements. Digital zoom functionality also enables a single megapixel camera to cover the same field of view as multiple non-megapixel cameras; the higher the resolution, the greater the coverage.
However, as the total number of pixels on an image sensor increases, the pixel size gets smaller. This results in better resolution but also means higher storage and bandwidth requirements, and diminished low light performance. Why does low light performance change? With CMOS sensors, each pixel is like a window. To fit extra pixels in the same sensor area, the pixel ‘window’ becomes smaller, which means there is less light to detect.
These examples highlight why video resolution affects more than just a camera or recorder specification. Often, the entire system design and cost is driven by storage and bandwidth, which is why H.264 video compression is essential to help offset these requirements. H.264 enables more efficient transmission and storage of higher resolution video than MPEG-4 or MJPEG and therefore helps lower system costs while ensuring high performance.
HD Has Its Advantages
HD is a term we know from the consumer industry, but how does it differ from megapixel? Megapixel sensors that have more than 1 million pixels provide a 4:3 aspect ratio, may be limited on real-time frame rates and do not conform to standards such as color reproduction. The higher the megapixel value the better the ability to perform forensic zooming, and the higher the bandwidth and storage needed.
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