What are the differences between Analogue and Digital Systems?

Analogue

A traditional CCTV system is known as Analogue. This means that video signals are passed from the CCTV camera in a continuous electronic wave, very similar to radio (or television) waves. These waves may be intercepted by a CCTV monitor to display live video from the camera and/or the waves may be recorded on a VCR (video recorder).

Many CCTV installations have more than one camera and use a multiplexor to channel each video signal in turn to a VCR or monitor.

The main attribute of an Analogue system is that the video signal is continuous - requiring constant monitoring or recording and playback in order not to miss images of interest.

VCR technology records the analogue video signal on a moving magnetic tape. The video signal can be recreated by playing back the tape, however the signal quality degrades in the process of recording. This degradation increases as the video tape is re-used even past the point when the playback signal is unusable.

A VCR can only record one video signal. To record multiple signals a multiplexor passes each video signal in turn to the VCR. This means the VCR is actually recording snippets of video from each camera in turn. These snippets must be replayed through the multiplexor which re-assembles the snippets from each camera. A single VCR cannot record and playback at the same time. A multiplexor that can record on one VCR and playback from a different VCR at the same time is known as Duplex.

Digital

A Digital system converts the continuous analogue video signal into a sequence of separate images or snapshots. Each snapshot contains a complete still video image as at a single moment in time. The snapshot image is defined as a regular matrix of points in the image, the content of each point (or pixel) can be described by a number - hence the digital description. The resolution of the image is defined by the number of pixels, the higher the digital resolution the more numbers required to describe the image.

A digital video snapshot is in the same form as computer data, it is therefore possible to use computer technology to store and process this data. We will come onto some of the significant advantages of digital data but one of its disadvantages is that each image contains a comparatively large amount of data, which have implications in processing and storage.

It is important to note that analogue and digital are different. Digital is not just an 'improved' analogue system. There may be instances where a digital system is inappropriate (for example if a camera is linked just to a monitor and no recording or transmission is required), although in most installations the digital recording unit offers significant advantages when configured appropriately.