Help on Video Switching

[j37prakash]

Hi Friends, can anyone help me build a simple and fast acting video switch that could be used to select one of four video inputs and output a single video with four split screens with the four video signals. I want to view all the four videos simultaneously on the four split screens. Kindly suggest me a suitable circuit or circuits.

 

[betwixt]

You can not do this with a simple video switch.

If you want a single, 2x2 (four image) screen you have to digitize each of the four inputs and replay them at double horizontal and double vertical scan rates.

It isn't easy to do and unless you are expert at design I would respectfully suggest you buy a ready made system. They are available from many sources as security camera controllers and recorders.

Brian.

 

[j37prakash]

Thanks a lot Brian. Can you suggest me any such IC that I can put to use ? I am sorry I am not well experienced.

 

[betwixt]

I am not aware of any single IC that can do what you want.

The problem has to be broken down into small steps to understand why the task is s complicated:

1. Each video source produces picture and synchronizing information but unless you have 'Genlock' sources, which is very rare, they will be out of step with each other. For example, one picture could be starting its scan at the top left corner of the picture while another was half way down the picture. There could also be slight scan speed variation between sources which would mean the relative positions of the scans would always be changing.

2. The viewing screen has to see a full picture from each of the sources so each individual image has to be reduced to half height and half width.

3. The four smaller images have to be synchronized to the viewing screen.

The method used to achieve this is to digitize each of the incoming pictures into its own memory. The writing (storing) of the picture is controlled by the synchronizing signals from that video source so the image is always correctly 'aligned' in the memory space. The type of memory used is called 'dual-ported memory' because it has two independent address and control bus connections. The idea is that the video source writes into the memory at the same time the as the display device reads from it on the second port. The reading is synchronized to the viewers own synchronizing source so the image show correctly on the output screen. This is how digital video sync and also timebase correction (TBC) works.

Lets call the four video sources V1, V2, V3 and V4.
To display all four of them on one screen, you have to read the top line of memory from V1 for half the line of the viewing screen, then read the top line of memory for V2 for the remainder of the line. Next you move to the second line of memory and do the same thing. When half way down the viewing screen you switch to reading the top line of memory for V3 and then V4 in the same way.
You may have noticed that the number of lines in each video memory is the same as in the display screen so they wont fit. the trick used is to discard every second pixel (memory address) when reading across the lines so the picture reduces to half width and also discard every second line when reading down the source memory so the picture appears half height. When each picture is half width and height they will all fit into the single viewing screen. You do lose half the picture resolution doing this but there is no other way.

If you want to do this yourself, I would suggest you need a good knowledge of how video and especially synchronizing works. The timing signals can be generated in a gate array. I have done something similar using FPGA devices from Lattice but I can not give you the design I'm afraid.

Brian.