CCTV Tech Notes

1. What is a CCTV System?

CCTV stands for Closed Circuit Television. The System does not broadcast any TV signals but transmit the signal on a closed circuit via the coaxial cables linking the cameras to the Digital Video Recorder.

2. What is CCD Chip?

A CCD (Charged Coupled Device) consists of several hundred thousand individual picture elements on a tiny 1/2", 1/3" and 1/4" chip. Each picture element (pixel) responds to light falling on it by storing a tiny charge of electricity. The charges are carried to the camera's video processing circuitry where the signal is processed and transmitted to the DVR as a video signal. CMOS is the other technology used for capturing images. Compared to CCD, video captured using the CMOS chips tends to be inferior in image sharpness and quality.

3. What does resolution means?

The resolution of CCTV system, usually measured by TV Lines (TVL), is a measure of the quality of the image and hence the camera. A camera with a resolution of 380 TVL is not as good as a camera with a resolution of 530 TVL. Typical resolution is 420 TVL.

4. What does Lux indicate?

Lux is the measure of light intensity. A camera with a Lux specification of 1 Lux means that the camera can still capture the image in an environment where the light intensity is 1 Lux. To give you a feel of how much light are we talking about, the following are some examples of natural light expressed in LUX.

Full daylight 10,000 Lux
Very dark day 100 Lux
Twilight 10 Lux
Deep twilight 1 Lux
Full moon 0.1 Lux

5. How is focal length and viewing angle related?

Focal length is defined as the distance between the center of the lens to the focal point. The smaller the focal length, the wider the viewing angle. Hence a 6 mm and a 5 mm lens would give a viewing angle of 56.8 and 66.0 degrees respectively. For a 1/3" CCD, the following formula is a simple way to determine the focal length of a lens needled.

Focal Length = (Distance to subject / Width of subject) * 4.5 mm

6. What is Video Motion Recording?

Video Motion Recording is a feature of the Digital Video Recorder, designed to start recording only when motion is detected. hence increasing the total recording time of the hard disk. The DVR compares the image with a previous image and following its algorithm, if there is a substantial change in the image, it will generate an alert and starts the recording for a pre-defined time period, typically 10 seconds.

7. What is data compression?

Video images are memory hungry and take up much hard disk space. To conserve the hard disk, special algorithm are created to compress and reduce the storage size requirement. Common compression method is the MPEG 4 compression.

8. What is DDNs and why do we need it?

To view the DVR remotely over the internet, the DVR will need to be assigned an address by your internet provider. Unfortunately for most of us, the plan that we signed up wit the internet provider is a dynamic (as opposed to static) plan where the internet provider will share the address with many other users. There is no exclusivity and the address that is given to you will change. Depending on how heavy is the network traffic, you may see your address change as often as 10 times within an hour. This presents a problem for anyone wishing to view the DVR remotely.

This is where ddns service provider comes in. You can program and instruct your router to tell the ddns service provider your new address whenever there is a change. The service provider will assign you a url address such as http://securevision.dvrdns.org, where any address changes is assigned to this url. So to view remotely, all you need to do is type in "http://securevision.dvrdns.org" and the service provider will check its index and point you back to your DVR.

 

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