This article outlines explains how the quality of the image on an analog CCTV system depends on the real time display and recording resolution of the DVR. Therefore the DVR you choose has a big impact on the overall performance of your surveillance system.
Please note that generally Trinity CCTV no longer supply DVR recorders as part of a new or upgraded installation (HDCVI recorders are used instead) but this page will still be useful for people researching existing systems.Please call us on 0508 11 00 22 or email us if you have further questions.
This diagram shows the different PAL image resolutions that are used in analog DVRs.
In New Zealand resolution is defined by the PAL (Phase Alternating Lines) analogue video standard. The maximum amount of pixels that can be created in the image is based on the number of TV lines (TVL) available to be digitized. PAL has a maximum resolution of 576 TVL and a refresh rate of 25 frames per second.
Therefore the maximum resolution that an analogue surveillance system can provide is D1 resolution. The most commonly used is 4CIF which has a resolution of 704X576 pixels or 0.4 megapixel. Older technology DVRs use CIF recording which provides a very low resolution picture.
The image below shows the difference between a 4CIF and CIF image. Note that we have pasted each image at pixel size onto an A4 sized piece of paper to give you an idea of the size of the image. We then expanded the 4CIF image to an A4 size of paper to show more of the resolution detail.
Because the quality of the image is dependent on the real time display and recording resolution of the DVR even if a higher resolution security camera is connected, the DVR will still only be able to play and record at its own resolution. For example a 700TVL surveillance camera has a resolution of 1020X596 pixels. However when it is used with a 4CIF DVR the image will display and record at the DVR’s resolution of 704X576 pixels. In the image below the inner box highlighted in blue is the 4CIF image that will be displayed and recorded by the DVR whereas the larger image behind that is what the CCTV camera actually sees.
This is an important point to note if upgrading an existing surveillance system – don’t replace all your cameras without first taking into account the resolution of your DVR. It may be more important to replace the DVR than the security cameras.
Recording Bit Rate
Another factor that affects the recorded image quality is the maximum bit rate available for each recorded channel. On some DVRs the system is able to display a relatively high resolution image such as 4CIF meaning you see a nice clear picture when viewing live however when playing back the image is of a lower quality. This is often due to the maximum recording bit rate available which is the maximum amount of data the machine can process and is measured in bits per second. The specs of the DVR – especially lower quality DVRs, can state a recording resolution of D1 or 4CIF on each channel however the maximum bit rate is not high enough to record all channels at full resolution resulting in lower quality recordings on some channels.