Virtual Grand Master

Grand Master Image

What is Virtual Grand Master?

Virtual Grand Master (or Virtual GM) is a software emulation of a Grand Master stage lighting control.

OK, what is a Grand Master then?

The original Grand Master was a typical lighting control being installed in theatres from the 1920s until the 1950s. It was made by the Strand Electric Company Limited - the dominant lighting manufacturer in the UK for most of that period.

Why should I be interested?

The way the Grand Master worked was heavily constrained by the technology available at the time. In order to understand lighting of the period, it is essential to understand theses constraints - and what better way than to actually use a Grand Master to do some lighting?

This is not easy to arrange with a real Grand Master - there are few left, and even fewer that are able to be used. And they are large, and heavy, which makes them less than convenient for study. Enter the Virtual Grand Master, which is operationally identical to the real thing. You can either just play with it on the computer screen, or (by using a LanBox) link it via DMX to real dimmers and lighting.

How does it work?

In order to understand the real Grand Master, it is important to appreciate the limitations of the technology which was available at the time. Dimmers used a variable resistance to alter the current through the lamp, and the dimmer level was controlled mechanically by a lever. This meant that any kind of presetting or memory control was not possible: the dimmer levers were ‘live’ so that any change had an immediate effect on the lighting on stage.

The Grand Master divided the total number of lighting circuits (typically 40 to 80) into six groups, with the levers of each dimmer driven by a shaft. Levers could be locked onto their shaft, so that rotating the shaft drove the locked-on dimmers either up or down depending on the direction of rotation. Any dimmers not locked on didn’t move, while a clutch mechanism allowed the lock to slip when the dimmer reached the end of its travel, ensuring that the shaft could go on turning to complete the travel of any dimmers still to reach top or bottom.

A further drive system linked all the shafts to a single Grand Master wheel (hence the name) so that they could all be driven at once. The gears could be set so that individual shafts would turn in either direction or not at all when the grand master wheel was turned. Thus one control could operate all the dimmers - a major advance over preceding types of control.

Of course, there were substantial limitations; in particular, dimmers on the same shaft could not move in opposite directions. Choosing which dimmers were on which shaft was therefore crucial. At the time, wash lighting with flood battens was usual, and these were often coloured with the three primaries (red, green, blue) and white. Sometimes amber replaced green. All the red circuits would be put on the same shaft, as would all the green circuits, etc. Thus it would be possible to do a cross fade from a red state to a blue one by setting the gearing on the red and blue shaft masters to turn in opposite directions wheel the grand master was operated. Odd circuits would be placed on the Auxiliary shafts.

Another significant limitation was that the maximum speed of a cue was limited by the strength of the operator; on a large Grand Master with all the dimmers moving in a cue, the mechanical resistance to movement was considerable. A three second cue was probably a realistic maximum. To provide snap blackouts, and to allow for a certain amount of presetting just before a cue, individual circuits could be switched either off, full on (direct from the mains supply) or via the dimmer. In addition, master switches could black out the circuits on each shaft separately, or the whole lot.

Virtual Grand Master replicates these controls, and if you don’t have a LanBox, you can see the output in the Stage window.

Using VirtualGM with the LanBox LC

Virtual GM will communicate with the LanBox via an Ethernet connection, using the UDP protocol. You can set the port number and IP address of your LanBox in Virtual GM's Preferences window. By default, the port and IP address are set to the LanBox's default values, so you should be able to connect the LanBox, set your computer's networking settings according to the LanBox manual's instructions for using UDP, select "Connect via Ethernet" in Virtual GM's "LanBox" menu, and everything will work.


Virtual GM is part of my PhD research into lighting control systems and their relationship with lighting design and performance. For more information, contact me or visit my web site (see below).

Technical stuff

Virtual Grand Master is built with REALbasic, the object oriented rapid application development tool for the Macintosh. It is offered as freeware as both a built application and as source code, so do what you like with it. Just don’t expect me to take responsibility for the consequences!

Problems? Want to know more?

Contact me by email:

Or visit my web site:

Find out about the LanBox:

Find out about REALbasic:


Nick Hunt, April 2006