Megapixel ≠ HD

Megapixel ≠ HD

While 2009 may be the year of megapixel cameras, it may also be the year where confusion between high-definition (HD) and megapixel cameras arose. This article clarifies the two terms.

When Axis Communications started marketing its new camera as having high-definition television (HDTV) performance, some raised their eyebrows out of confusion. While all HD cameras are megapixel, not all megapixel cameras are HD:

* Maximum resolution for HD to date is 2.1 megapixel; regular megapixel can reach 16 to 21, and the most common ones on the market now are 3 to 5.
* HD format is 1,280 x 720 or 1,920 x 1,080, whereas megapixel cameras can offer many more formats.
* HD aspect ratio is 16:9, compared to 5:4 or 4:3 in other surveillance cameras.
* HD frame rate is 30 or 25, whereas regular megapixel cameras often deliver 3 to 15 frames.
* HDTV has quality compliance standards, whereas megapixel simply specifies the number of pixels.

Which is better is not a clear-cut case. Some applications do need more than 2.1 megapixel (the maximum for HDTV today) and do not need high frame rates; these should opt for non-HD megapixel cameras. Others care more about frame rates and image quality; in these cases, HD cameras would be the safer bet.

Aspect Ratio and Frame Rates
For most applications, the 16:9 aspect ratio of HD is more efficient than the 5:4 aspect of regular megapixel cameras. In surveillance, width is usually more important than height. The reason is that people and vehicles are only so tall but they can be anywhere across a wide area. However, this does not mean you have to use an HD camera. For instance, if you use a three-megapixel camera (2,048 x 1,536) and crop the bottom and top, you have the same effective aspect ratio and resolution of a 1,080p HD camera. This camera may also be less expensive.

Most applications can do with 12 frames or less, which is typical for three-megapixel cameras. While much attention is drawn to “full frame rate,” the reality is that surveillance applications have traditionally used far fewer frames per second and been successful. On the other hand, there will be certain applications (like casinos or PoS) where milliseconds count. A 12-frame-per-second camera captures an image every 83 milliseconds; a 30-frame-per-second camera every 33 milliseconds. An HD camera cuts out 50 milliseconds, meaning only in circumstances where very fast, tiny movements are key will HDTV make a difference.

One thing users should take heed of is the implicit claims to HDTV quality. At trade shows, some HD cameras deliver great image quality, while others can only render webcam quality.

So far, 1,080p HD cameras are being priced at a significant premium to regular three-megapixel cameras. Axis' manufacturer-suggested retail price (MSRP) for its HD camera is US$1,495. Cisco's is sold at $1,400. Contrast this to an Arecont three-megapixel, H.264 camera, which sells on the Internet for less than $900 and probably with an MSRP of no greater than $1,100. Is a 30-percent premium worth it?

Given Axis' market influence and the general appeal of HDTV, the branding of HD surveillance cameras is almost certainly here to stay. People will have to adapt to this new emphasis, either through matching HD products or enhanced education of tradeoffs/alternatives.

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