An ASCII text version of this page is available here

Introduction

     The VMUX series of video switchers are designed for video installations in art museums and for routing video signals in small studios. They are matrix switchers with 8 inputs and either 8, 16, 24, or 32 outputs. They are controlled by an external computer, typically an IBM style PC though at least one customer has controlled them using a Mac (we do not provide Mac support ourselves). They are controlled by a stream of characters sent out of the computer's parallel port. 

Inputs

     The inputs on the VMUX switchers are "loop-thru" style pairs of BNC connectors. This allows a signal to be connected to the switcher input and then looped to another device. If an input is not looped through to another device, it must be terminated by placing a 75 ohm teminator on the second BNC connector. The inputs accept any standard 1 volt peak to peak video signal in any of the world video standards (NTSC, PAL, SECAM, or industrial video).

Channel One

     Input number one of the VMUX switcher has a dual purpose. Besides being an input to the switching matrix like all other inputs on the VMUX, the sync signal of this input is used to define the vertical interval for vertical interval switching. You must have a video signal connected to input one even if you do not have signals that are synchronized to the vertical interval. Besides this special function, input number one is the same as all other inputs and the quality of this input is not affected by the vertical sensing circuitry in any way. 

Input jumpers

     There are three jumpers inside the VMUX switcher for each of the inputs. These jumpers can be set differently for each input and each set of jumpers only affects that specific input. To set these jumpers you must remove the top cover panel of the VMUX switcher. For safety it is recommended that you disconnect the power cable from the switcher before removing the cover. This will prevent damage to the switcher if anything falls inside and any hazards to the person changing the jumpers from accidentally touching the power supply. It also reduces the chance of damage from static electricity damaging the switcher. 

     The three jumpers per input are: 

• Input isolation
• Input coupling
• Settling time

     The input isolation jumper is used to select if the input's ground signal is directly connected to the switcher ground or if the input has differentially received, isolated ground for noise reduction. There is a label at the top of the circuit board showing the position of the jumper for each setting. This jumper has three posible positions. 

• None - There is no isolation. The input's ground is directly connected to the switcher's ground.
• Semi - There is a 200 ohm resistor connected between the input's ground and the switcher's ground. This is the default setting. It gives some isolation from ground loops and allows the input's ground to be used by the differential input receiver to remove noise that was picked up by the BNC cable. This is ideal for field setups where there might be long runs of cables to the switcher.
• Full - The input's ground is isolated from the switcher's ground by the 3000 ohm input impedance of the differential receiver. For all intents and purposes this is fully isolated from ground. The input's differential receiver will not only reject noise on the input ground but will also remove hum of up to about 5 volts peak to peak. This setting can be used for studios where there is a lot of crosstalk between cables lying in piles on the floor. It is not recommended for situations where the ground might have huge AC differences on them, like in field operations.

     The input coupling jumper is used to select between directly coupled and either AC coupled or DC-restored coupling. The direct coupling setting can be used if your video signal is connected to a device with a known DC stable source since this will minimize any distortion to the signal from the DC restoration circuitry. This setting should only be used if you are an engineer and have confirmed that the video source has a stable DC level. The default setting is the AC/DC-Restored position. This position is used for both AC coupling and DC restoration. 

     The third input jumper is the Settling Time, or speed of coupling. This jumper has no effect if you have chosen direct coupling. It only has an effect if you have selected AC/DC-Restoration on the Input Coupling jumper. This jumper has three positions, Fast, Medium, and Slow. If you set this jumper to Slow, then the input is AC coupled. The DC restoration circuitry will still slowly adjust the DC level of the input so that over time a constant image will maintain a predefined sync tip voltage, but any rapid changes in the video will bounce the DC level around as if it were AC coupled. If the jumper is set to Medium then the input will be DC restored. This is the default setting and is best for normal switching. In this setting the DC restoration circuitry samples the sync tips over an average of several lines of video and stabilizes the sync tips to a defined DC level. If the video signal has single lines of video where the sync level fluctuates, this setting will ignore them and pass them through as is. The third setting is the Fast setting. This will sample the level of each line's sync tip and use it to adjust the DC level of the video signal. This clamps the video tightly to a DC level but can cause problems in the image if the sync level changes from line to line since the video black level will then change from line to line. This is only recommended for signals that have very stable sync level information. This setting can also cause a slight shade change from the left edge of the screen to the right edge of the screen. This slight darkening from left to right is only about 1 or 2 percent but can be noticable in critical applications. 

Input Offset

     Each input has one trimpot control that can be adjusted to set the DC level or "offset" of the input. This is used to match each of the inputs so that video black is at the same DC level for each. If this is set wrong then the sync and black levels of the output will jump each time the switcher is switched between different inputs.This should only be adjusted using an oscilloscope. Connect the oscilloscope to one of the outputs and switch between each input, then adjust each of the Offset trimpots so that the video black level is at zero volts on the output. If the sync levels of the inputs vary widely then you may want to adjust the Offset so that each of the sync tips are at the same level. That will prevent monitors from rolling when the switcher changes patches. If you don't have an oscilloscope, but notice that the monitor is rolling when you switch to a given input, you can carefully change the Offset control for that input until the monitor doesn't roll during switching. You do not have to move the Offset control very much, so do this with care. 

Outputs

     The outputs of the VMUX switcher pass through the video directly from each input, they do not do any sync processing on them. The outputs of the VMUX switcher are designed to drive 75 ohm terminated inputs with one volt peak to peak video. The devices that each output is connected to must be terminated by 75 ohms or there may be oscillations on the outputs. Most video devices have either a single terminated input, or a dual connector "loop-thru" input. The loop-thru inputs may have a terminator switch. If so, and that device is the last input on a chain of devices, then it should be set to "75 ohms". If it does not have a switch then it should have a 75 ohm terminator placed on the second input. If you loop the output through several devices you need to make sure that only the last one is terminated. Any loop-thru inputs that have a terminator switch, and that are not the last device in the chain, should have that switch set to "Hi-Z". Only the last one shoud be set to "75". 

Computer Interface

     The computer input on the VMUX switcher is normally connected to the parallel port of a computer using a special cable. These inputs are optically isolated from the rest of the video circuits of the switcher. This means that the noisy ground of a typical computer will not affect the video signals. This also means that there could be a dangerous voltage potential between the computer and the video circuitry. Because of this it is recommended that the computer's power cable be connected to the same power grounding system as the video devices in the system. This does not have to be the same power strip or even the same power line as the video circuitry, as long as they share a common ground within the building that they are installed in. 

Computer Software

     Which software you use will depend on your application. Simple patching and routing software for IBM style computers is included with each VMUX switcher. Advanced software for specific applications is also available, or you can write your own (see the page on writing software for the VMUX). 

Power Source

     The VMUX switchers can run off of either 110 or 220 volts. The power inlet is an IEC international power inlet and uses the same type of power cable as most computers. A power cable can be purchased in almost any country in the world at a local computer or electronics store. Before power is connected you must make sure that the voltage selector switch on the back of the VMUX is set to the correct voltage. Insert a small screwdriver into the voltage selector switch and rotate it to either "110" or "220". If you feed 220 volts in while this is set to 110, you will blow the fuse. The fuse is in a small drawer under the power plug, and the power plug must be removed before this drawer can be opened. Replace this fuse only with a 1 amp slow blow fuse (if the VMUX will only be used on 220 volts then this can be a 1/2 amp fuse).