Originally Posted by
intelliGEORGE
Whilst the big numbers look good in the glossy print and end-users think bigger is better, 540(H)TVL is the practical limit in analog CCTV systems.
The number "540" is not a magical number, CCTV manufacturers didn't sit around a camp fire and vote unanimously for 540TVL. This number is the result of an analog signal passing through a low-pass filter found in the composite input circuitry (BNC or RCA) of monitors and DVR's.
Analog signals pass through a low-pass filter to prevent any cross talk between the color sub carrier and the luminance information before they are converted to digital signals. It is this conversion process that limits the resolution on any composite signal to 540TVL.
The conversion process samples analog video signals and converts them to digital based on the Rec. 601 (CCIR 601) industry standard (720 luminance samples per horizontal line).
Therefore, 720 x 0.75 (limitation to 3/4 of the width due to aspect ratio) = 540HTVL.
Then there is (V)TVL (Vertical TV Lines), which are not quoted in marketing paraphernalia and for very good reason; Total Resolution.
The total resolution of an image is essentially HTVL x VTVL and up until megapixel cameras, VTVL's have not been openly promoted as they are limited by the technology used in image capture. For example, some CCD sensors use line-pair summation for interlaced video capture. This process also acts like a low-pass filter to overcome image flicker, however it decreases "effective" vertical resolution by 25%.
Therefore, a 480 line CCD has an effective vertical resolution of 360 lines making the 'Total Resolution' reduced by 25% and doesn't look too good on glossy brochures.
Progressive scan CCD's (most commonly found in IP/Megapixel cameras) on the other hand do not suffer from this, enabling them to utilise all 480 lines.
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