The Mux

The main mux is the huge black thing in the AV rack. It routes video to different places, and also has analogue TV demodulator inputs. It's controlled by an outboard box which sits on the Director/Vision Mixer's desk.

Functions

• 32 input to 4 output matrix

Faults & unknowns

• 2 composite inputs to the matrix are dead (23 and an unknown)

Explanation of operation

This was originally based around a PIC16F84 with an 8MHz crystal containing code written by Rob Sprowson, the code for which is now lost.

A new PIC, a PIC18F2455 using a 20MHz crystal, has replaced this PIC through use of an extra 'daughter' board. This change in PIC was to allow easier coding, using C instead of assembler, and to provide USB capabilities. The source code has also been lost.

It now runs on an installed Arduino, and is one of the first YVP Protocol compatible devices. This does mean a computer is needed to control it however, connected to the now real, RS-232 compliant serial port on the back.

Power supply

This is very much the weak point of the mux, and is tied up with why there are loads of unconnected BNCs on the back. - The Mux used to hold a distribution amplifier for creating 8 outputs of each main output from the matrix. Sadly though, this experienced some heavy cross-talk, and was misdiagnosed as "electric fields". The original creators now propose that simply adding better decoupling to the power supply would have solved the problem.

There are two mains transformers:

• 2x6V
  • 50VA
  • powers + and - 5V rails

• 2x6V
  • 3VA
  • wired as 1x12V, 3VA to provide the +12V rail.

All the supplies are smoothed with big electrolitics, and then regulated with 78/79 series regulators.

All this is built on a length of chocolate block connector strip, with a lot of stranded hookup wire.