TVL - Television Lines and the Marketing Hype

[intelliGEORGE]

How many of you are being bombarded with brochures from CCTV manufacturers and/or suppliers claiming they have cameras with 600, 700 and in some cases 800TVL? I know I have which has prompted me to highlight these false claims.

Whilst these 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.

In order to view more than 540TVL you will require professional series studio monitors, CRT to be precise, that offer 800TVL when using an S-Video signal. In reality, none of our installations will utilise these monitors nor will they utilise S-Video signal feeds.

But... Let's assume that we did utilise a $4000 JVC CRT capable of 900TVL, the camera signal is still required to be recorded on a DVR which again is limited to 540TVL making the high resolution image only useful for Live Viewing.

To understand why DVR's limit resolution to 540TVL, one must understand the signal conversion process that happens in a DVR. As stated above, composite video inputs (BNC or RCA) have a low-pass filter to prevent any cross talk between the color sub carrier and the luminance information. The conversion process samples analog video signals and converts them to digital based on the Rec. 601 (CCIR 601) industry standard. This widely used standard limits horizontal resolution to 720 luminance samples per horizontal line. Therefore, 720 x 0.75*(*limitation to 3/4 of the width due to aspect ratio) = 540HTVL. In saying this, I have not found a DVR on the market capable of more than 540TVL.

Then we have VTVL (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 on the other hand do not suffer from this, enabling them to utilise all 480 lines.

So, unless you are viewing one of these "super-duper high resolution" cameras on a $4000 professional studio monitor (before passing through a DVR) you are not going to benefit from anything above 540TVL in analog CCTV installations.

[Drift]

I've been having that very same argument with suppliers, techs and clients since these magical yet mostly pointless higher res devices became available. For the most part, people are brain washed in to believing the bigger number must be better than the smaller number, yet few actually take the time to understand the real limitations of the products they're selling or installing.

The looks on suppliers faces when you ask them just how youre to take advantage of that additional resolution are always priceless. As are some of the excuses that follow.

Alas, until such time as mediocrity is no longer accepted as the norm in this industry, we're destined to have this sort of carry on continue saving the dwindling pool of individuals with an intrinsic understanding of their product.

[MegapixelMan]

HI,

I have done some research and have been speaking with a few suppliers myself and honeslty many of them have no idea, Dan and Video Security Products has sent me a email he received from a Camera Manufactuer that says that anything over 540tvl running through DVR's with VGA outputs and also through standard 4:3 VGA monitors is pointless. I will attach the file once I find it as it is very Interesting....

Intelligeorge - In your findings do have you heard anyone say that the DVR with a inbuilt upscaller fix the issues if using a HDMI cable and HDMI monitor....

I say Megapixel all the way.... Sony Exmor CMOS Rocks.

Jamie