Q: Resolution is confusing because it’s specified in so many different ways. What do they all mean?
A: With analogue cameras, resolution is usually specified in TV lines (TVL). A PAL picture – the standard for broadcast TV in the UK – is equivalent to 625 horizontal scan lines, viewed top to bottom on the screen (of which normally only 576 are visible), with most CCTV cameras delivering 400-600 lines of actual resolution across the width of the picture. For digital cameras, resolution is quoted as the total number of pixels making up the image. For example, a 4CIF image is 702×576 (approximately 0.4 megapixel) and D1 (DVD) 720×576 (0.5 megapixel). True High Definition is 1920×1080 (2 megapixel)
Q: Is it best to use the highest resolution camera available?
A: Not necessarily! Remember that apart from costing more, high-resolution cameras need more bandwidth on the network used to connect them, and recording images from them takes up more storage space. For example, a single 2-megapixel HD camera operating at 25 fps might need a bandwidth of 12 Mbps (compressed), so it doesn’t take many of them to overload a typical 100 Mbps network! Also, images from this camera need five times as much storage capacity as images from a 4CIF camera.
Q: So when are high-resolution cameras useful?
A: They are useful wherever it is necessary to capture detailed images. For example, in areas open to the public, a lot of detail is needed for facial recognition, as the subject will usually be unknown to the CCTV operators. Where high levels of detail are needed, a good solution is often to use a high-resolution camera but to minimise bandwidth and storage issues by reducing the frame rate. Depending on the application, rates as low as 2-3 fps may be perfectly satisfactory.
Q: What about mixing high and lower resolution cameras in an installation?
A: This is often a good approach. In a retail store, for example, high-resolution cameras at the entry and exit points will capture detailed images of people entering and leaving the store, allowing lower resolution cameras to be used for tracking them inside the store. Most modern CCTV systems will allow high and low resolution cameras to be mixed but it’s always worth checking before buying.
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