Multiple Screen Displays
Any system that combines more than one video signal is technically a multiplexer. These days it is customary to refer to multiplexers as equipment that can simultaneously combine eight or more signals, otherwise they are known as screen splitters or quad splitters.
There will be many occasions when it will be advantageous to display more than one camera on the monitor at once. One example is if an incident occurs but it is not certain just where it originated. With a simple switching device, it would be a tedious business to review all the cameras recorded in sequence. In addition, as stated previously, essential information may be lost. However, if all the cameras were recorded simultaneously and could be displayed simultaneously then reviewing and finding the sequence of events would be very much easier. In addition, virtually no information would be lost and the relevant scenes can then be analysed with full screen pictures.
The essential benefit therefore of recording in the various multiple screen formats is that no information is lost due to dwells in switching.
Analogue and Digital Displays
The picture received directly from a camera and displayed on a monitor is an analogue representation of the scene. The picture information has been converted directly to a video signal and reconverted to the same scene on the monitor. The clarity of the picture is dependent on the quality of the camera, the lens, the transmission system and the monitor.
To display or record more than one picture at a time it is necessary on most systems to convert the analogue signal to a digital form. This is known as analogue to digital conversion. After processing, the signal then has to be converted back to analogue form to be displayed on a monitor. This process introduces the possibility of degradation to the original picture. Definition can be lost through the complicated conversion processes and noise can be added to the signal. Also, the final quality is dependent on the resolution in terms of the number of pixels comprising the digital information.
Picture In Picture
This is a simple system by which one scene can be inserted in another. The camera outputs are connected to a controller that allows one camera to be designated as the main picture. The other camera is designated as the inserted picture. The inserted picture may be positioned and sized anywhere on the screen as shown in Diagram 13.1. Usually either camera may be displayed as a full screen picture.
The normal controls for the inserted picture are: Horizontal size, vertical size, horizontal position and vertical position. Note that only the inserted picture may be altered, the background camera is always shown full screen. Note that, where the inserted picture is analogue the cameras need to be synchronised. This can be from an external sync generator or one camera can be synchronised from the other.
This is similar to a picture in picture inserter except that both camera scenes can be adjusted to compose the most useful combination. A screen splitter refers to a combination of two cameras. The split can be arranged either horizontally or vertically. The degree of overlap of either camera can also be adjusted. Screen splitters also require the cameras to be synchronised.
Quad Screen Splitters
As the name implies, this system allows the presentation of four cameras on the one screen. The majority of quad splitters now incorporate digital image processing. This means that it is not necessary to synchronise the cameras and the picture is digitally compressed to a quarter of its size. The four images are then displayed on a single screen. Note that each picture will only be 25% of the screen resolution.
There are many features that may be available with quad screen splitters and it is essential to check with manufacturers’ literature for particular models. As always, the more features it provides the more expensive a unit is likely to be. It is possible to spend more than necessary if poor selection of a piece of equipment includes more features than are required. Another factor to check out is the resolution of the displayed pictures.
Some features that may or may not be included are as follows.
By definition, quad splitters will have four inputs but there are units available that can have eight inputs. These usually display blocks of four cameras in sequence.
When a camera is shown in full screen this is a method of electronically enlarging a quarter of the screen to a full screen view. The area in view may be ‘panned’ around any part of the original picture. Note though that this will produce a very grainy looking picture. This is because each pixel in the enlarged view will be four times as large as in the full screen scene.
For instance, if the full screen picture is made up of 512 x 512 pixels then a quarter screen will contain 256 x 256 pixels. When ‘zoomed’ the new full screen picture will be made up of the 256 x 256 pixels.
This is the capability to provide either a quad display or sequencing through each full screen picture.
The capability of providing dual monitor outputs, one with a quad display, the other a sequence display.
Some quad splitters offer the capability to accept alarm inputs. The treatment on receipt of an alarm can vary. For instance, it can hold the associated camera on full screen until deactivated or it could override a sequence and switch to quad display.
Alarm outputs are sometimes provided. These can be used to switch a video recorder to real time or operate any other ancillary equipment.
Another option sometimes available is the facility to insert camera numbers and titles on the screen. These can usually be moved around the screen to prevent obscuring an important part of the scene. Not all systems allow the positioning of individual camera titles. Some only provide a fixed position for all cameras. The number of characters available for titles varies between models.
As with switchers, some models provide loop through facilities with switchable termination. The same comments apply to ensure correct termination when looping through video signals.
On Screen Menu
Some of the systems with more facilities provide the capability of setting up the various functions from on screen prompts.
Video Loss Alarm
This feature can provide a warning, both visible and audible, if there is a loss of video signal from any of the cameras
This article is an extract from chapter 13 of 'The Principles & Practice of CCTV' which is recognised as the benchmark for CCTV installation in the UK.