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

Empty box for 8x8x3 Audio/Video Switch
Empty box for 8x8x3 Audio/Video Switch

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

Small PCB containing the power supply
Small PCB containing 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

Small PCB containing the power supply mounted in the audio/video switch project box case
Small PCB containing the power supply mounted in the audio/video switch project box case

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

The Video Section

Next, using some more scrap parts I had, I began work on the video switch portion of the project. Both the video and the audio portions are similar, the only major difference is the type of wire used and the values of the resistors. For the video section, I used 75 ohm coax, and for the audio section I used twisted pairs of stranded wire.

The Video Switch Board

Prototype board layout for the video switch section of the 8x8x3 Audio/Video switch
Prototype board layout for the video switch section of the 8x8x3 Audio/Video switch

These prototyping boards came from Datem, a company that I used to work for and that's no longer around. These were used for in-house prototypes, and I still have a stack of them, so I use them for all my electronics projects. The 40-pin socket is for the 4456 8x8 crosspoint switch, and the two 16-pin sockets are for the 497 buffers.

The Interconnects

Connection from RCA jacks, in progress, part of the 8x8x3 Audio/Video switch
Connection from RCA jacks, in progress, part of the 8x8x3 Audio/Video switch

As you can see in the picture, from the back side, the top row of RCA jacks is for the video (I used two different thicknesses of coax cable; both, however, are 75 ohm). The bottom two rows are for the audio, and I used twisted pairs of stranded wire. Note also that the inputs all have impedence matching resistors directly soldered across the RCA jacks (75 ohms for the video, and 10k for the audio).

The Completed Video Section

The completed video section, showing the prototype board installed, part of the 8x8x3 Audio/Video switch
The completed video section, showing the prototype board installed, part of the 8x8x3 Audio/Video switch

Finally, this is the completed (but untested) video section. Dealing with coax cables is a royal pain in the ass — they are very stiff and don't bend easily.

More as I have it!