THEORY OF OPERATION OF LIGHTING CONSOLES
There are two primary control philosophies used in the operation of entertainment lighting consoles, Pile-On, wherein the highest level instruction given to a channel is output to associated dimmers and Last Action, wherein the last level given to a channel is output to the dimmers.
These two disparate philosophies each have advantages and disadvantages in their application to the industry. Explaining the evolution and heritage of the two styles will help in understanding their operation.
PILE-ON OR HIGHEST LEVEL TAKES PRECEDENCE (HTP) OPERATION
The Pile-on control philosophy traces its heritage to multi-scene preset boards which evolved with the onset of remote controlled dimming. They embody the concept of recording each cue in the show as a complete look, with levels for every channel recorded in the memory for that cue. Essentially, each cue is a stand alone look; with all active channels represented. This allows the fading between one look to the next with cross fades.
Since each cue contains levels for all of the channels, regardless of whether they are moved in that cue, it is easy to fade cues in or out in any order, say Q1, Q10, Q2, etc...
The typical record operation of a HTP system is a "snapshot record". It records all of the current levels only in the cue selected, not effecting the levels in the following cues, and not dependent on levels from previous cues.
Most HTP systems have incorporated "tracking" type record functions which emulate the standard record operation of last action systems, allowing channel level modifications to track forward through cues, stopping when a new level is encountered.
When a cue is recorded, the channels remain on the keyboard. The cue is loaded to a fader and played back before those channels are actually fader channels.
Cue Playback Faders
Another significant attribute of HTP systems is that all faders can contribute levels to the stage look. If channels are present in more than one fader, the highest level produced by the faders is sent to the dimmers.
Additionally, when a cue is played back on a fader, its levels remain associated with that fader, allowing it to be cancelled or cleared. On last action systems, a cue is associated with a fader only during execution.
Keyboard override of channels is more difficult in HTP systems than in Last Takes Precedence (LTP) systems, since there are so many paths or faders which can determine the level of a channel. It is not possible to pile-on the keyboard level, since it would make lowering channel levels impossible.
To overcome this, a channel controlled by the keyboard or wheel is "captured" by the keyboard and the levels produced by the submasters and cue faders are ignored, and replaced by the level provided by the wheel. These channels will remain under control of the keyboard until they are intentionally "released" back to the fader and submaster control.
Submasters are a natural fit into a HTP system, since they are defined as pile-on devices.
LAST ACTION (LTP) OR MOVE OPERATION
Last Action consoles find their roots in single scene control systems such as resistance plate dimmers (Piano boards) found largely on Broadway. The basic thesis is that a channel is moved to a level and stays there until it is moved again. Therefore, a cue contains only those channels which are hanging levels in that cue.
Since only the levels changing in a cue are recorded, the system usually implements a "tracking" record mode. If, for example, a level is set for a channel in cue 1 and is not changed until cue 10, there will be no recorded level in cues 2-9 for that channel. Therefore, changing the level for that channel in cue 1 will affect the look of cues 2-9 (the track forward stops when the move instruction in cue 10 is encountered). A move to 0% is recorded for a channel to fade out. The system makes a distinction between no level recorded (no change will happen) and a level of 0% (the channel will fade out).
It is common to record "block cues" where all channels are recorded as move instructions. This prevents changes in previous cues from possibly effecting the selected cue. A block cue is often written at the beginning of a scene or act.
A typical variation of the record process in last action system is called "Cue only record". This option emulates the snapshot record style of HTP systems by making the change only in the cue being recorded; leaving the previous level in subsequent cues, if applicable.
When a cue is recorded, the cue is simultaneously placed on stage.. Channels that were contributed by the keypad are now considered fader channels.
A channel may be under the control of only one playback fader or cue at any time, and that will be the last cue activated which had a level for that channel. A channel is either moving up, moving down or at a steady level.
When a cue is played back on a fader, its levels are associated with that fader only during the fade execution. Once the fade is complete, the channels are no longer under fader control. Channel levels may be changed via a keyboard, submasters or another cue, but the cue cannot be "cancelled".
Keyboard Override is simple in a last action system since moving a channel by the wheel is considered the "last action". Subsequent actions made by executing a cue will move the channel from the level set by the wheel to that set by the cue. Interactions between the keyboard and submasters in last action style consoles are more complex.
Overlapping submasters by definition are pile-on devices. If the same channel is controlled by multiple submasters and possibly the cue playback faders, the highest level for the channel produced by any of these routes is sent to the dimmers. This conflicts with the simple operation of the last action system.
The greatest problem occurs with the keypad override of levels produced by submasters. Since the system does not incorporate "captured channels", which temporarily take the control for a channel away from the faders, the system typically removes the changed channel from the control of any submaster which is currently at a level until such time that the submaster is returned to a level of 0%. At this point the submaster regains control of that channel, and piles its level on to the other faders and keypad.
To overcome this problem, features such as "update submaster" have been developed, returning control to the submaster at the modified level.
COMPARATIVE ADVANTAGES OF PILE-ON AND LAST ACTION OPERATION
Pile-On [Expression line consoles]
Pile-on or HTP consoles offer advantages in application which require live operation, and running cues out of their recorded order. Also, since most HTP consoles have dual playback faders, looks can be added together easily. Additionally, the ease of interaction with the submasters allows for more efficient and flexible operation when "winging it" is required. Some areas where this is preferable:
§ Live shows such as music and concert work
§ Live television productions which can be unpredictable (Beauty pageants)
§ Mixed or flexible production venues which offer less set-up and tech time
§ Television production facilities where extensive submaster use is needed
Last Action [Obsession line consoles]
The last action consoles find the best acceptance in uses which are essentially cue to cue (in sequence) operations. The power of the tracking function and only dealing with the channels which are moving in a cue make programming and editing of such productions much more efficient. Examples of such uses include:
§ Legitimate or live theatre (Broadway)
§ Musical Comedy venues
§ Rehearsed television productions
§ Long running productions (Las Vegas)