Projects -- 8x8x3 Audio/Video Switch

This project is an audio/video switch, that allows any audio and video input to be connected to zero or more outputs. It's organized as an 8x8x3 matrix. This means that there are 8 inputs and 8 outputs, duplicated 3 times (so, there's a "video plane" and two (stereo) "audio planes"). Signals cannot cross planes.

I've been working on this project on and off over the last 6 months (more off than on, obviously). What follows is a step-by-step description of building the project.

I started by looking at specifications. Initially, I was going to build the switch using discrete CMOS demultiplexors (4000-series), but then talked myself out of it when I thought about how many actual wires would be required (each CMOS demux is a one-of-four, so that would necessitate 2 packages per output, times 24 outputs, plus buffering, plus plus plus...)

However, Maxim Semiconductor makes an excellent product. The MAX4456 "Low-Cost 8x8 Video Crosspoint Switch". Coupled with the MAX497 "375 MHz Quad Closed-Loop Video Buffers, Av = +2", and most of the heavy lifting was already done for me!

Except for the actual construction. The circuit diagram is pretty much the same as given in the app note for the 4456.

The Empty Project Case

This is an empty case I bought three or four of at an auction a long time ago; it features a 12V centre-tapped transformer with a 1A rating, a fuse, power switch, and 7805 5-volt regulator. I asked nicely at work one day, and they drilled out the 48 holes I needed for the RCA jacks (3 planes of 8 inputs + 8 outputs = 3x16=48) and also punched out the DB-25 hole (centre).

The Power Supply

Using more junk I had lying around, I cobbled together a small power supply board to generate the -5V, and also to hold the filter capacitor and the bridge rectifier. In order to generate plus and minus voltage, I connected the centre winding of the transformer to chassis and power ground, thus the output of the bridge rectifier was positive on one end and negative on the other.

The Power Supply Mounted in the Case

And this is what the complete power section of the project looks like.