HTP vs LTP – Lighting Desk Basics 5

Wednesday, March 12th, 2008 - Learn Stage Lighting - by:

One of elements of learning intelligent lighting control, is grasping the difference in the two ways a lighting desk treats control channels. The concept of Highest Takes Precedence (HTP) and Latest Takes Precedence (LTP) is actually very simple but is often misunderstood and explaining it can make it seem more complicated. On Stage Lighting always tries to make things simple to here goes…….

 

What are you talking about, anyway?

When a stage lighting desk controls channels for intensity, position, colour etc. it treats them in one of two ways – HTP and LTP. In the days before waggly mirrors and nodding buckets (that’s moving lights BTW) every stage lighting control channel was output on a Highest Takes Precedence (HTP) basis.

Intelligent lighting came along, with all it’s fancy effects and a-wagglin’ about all over the place and HTP channel control was just going to make a mess of all this new information that the lighting desk was spitting out. Latest Takes Precedence (LTP) was born to make it possible to cleanly control intelligent lighting using the traditional lighting control system – channel no. @ channel level ( 0-100%).

So, do I need to get my desk modified for this LTP thing or what?

No, HTP and LTP are just the ways in which the lighting control organises its playback of your fades and scenes. It is not a different kind of DMX nor does it come out of a special socket on the back of the lighting desk.

In fact, you could pretty much get away with not really caring about it much at all and just be safe in the knowledge that your lighting desk (probably) sorts it all out. Forget I mentioned it. Go and do something more interesting instead!!!

What is HTP?

HTP (don’t forget – Highest Takes Precedence) means that if you have a light faded up on one fader and the same light in another fader too, the channel level that is highest (nearer 100%) is output to the stage. So, with HTP:

Channel 1 (fader 1) @ 75%
Channel 2 (fader 2) @ 50%

Then the light output on stage would be 75%.

And if you lowered fader 1 to zero the light would fade down to meet the 50% of fader 2 and then stay at 50%. HTP control channels are used for light intensity and simple lighting controls that are for controlling dimmers always use it.

HTP control channel – “Whoever shouts loudest, is right”.

What is LTP

LTP (Latest Takes Precedence) is a handy way of controlling intelligent lighting parameters that don’t related to intensity or colour mixing. It you need a colour wheel to change from blue to red, or a position to move from up to down, you want it to happen as you expect.

LTP control channels send the latest instruction to an intelligent fixture parameter and nothing changes until it sends another one on that channel. No mixing or adding, nothing. LTP control channels are used in the majority of fixture parameters other than intensity and can be thought of as more of a switch, though it is sometimes possible to fade from one instruction to the next.

LTP control channel – “Whatever was just said – let’s do that !!”

 

Where is each channel set to HTP or LTP?

The way that channels are played back is set within the lighting desk itself. Information in the control patch, fixture personality files or desk software sets the behaviour and may or may not be edited the user. Cheaper intelligent lighting controls can have more restrictions on how you can set up your behaviours.

Anything else?

The popularity of LED colour mixing fixtures has brought some common questions from new users of LED fixtures – such as “Why are my LED lights behaving strangely” or “Should I use HTP or LTP for my LED colour channels”.

The colour mixing from LED fixtures is produced by the intensity of the Red, Green and Blue LEDs. This would make you assume that the RGB intensity channels would be controlled using HTP but many desks and fixture personality files set LED colour mixing similar to the colour wheel in a moving spot – LTP.

The choice of either using HTP or LTP for control of LED fixtures is down to personal preference – my preference depends on what I’m doing. Some points to remember are:

HTP with LED gives you the ability to “mix” scenes on different faders in a way that makes sense. A Red scene and a Blue scene mixed with give you a pinky, magenta look. The downside to using HTP is that when manually fading between two distinct scenes you often get an odd colour mix in the middle of the fade, as the two “add up”.

LTP with LED means that the fixtures do what they are told and wait for further instructions. This can be useful when used with a “tracking” lighting desk (one that replays changes only, rather than whole scenes). LTP can also be confusing when you fade up an LED scene and the LED stay lit, even when you pull the fader down again. Odd colour mixing during fades can also occur, depending on the channels involved, but during timed fades this can be improved by use of Up/Down fades times and follow on cues.

Although the majority of lighting desks handle the difference between HTP and LTP for you, it is still useful to understand how this information is being processed inside the box. This can lead to better programming and playback of fades/scenes and improve your programming of intelligent lighting. It can also help you work out why your rig is not behaving as you expect.

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Rob is a Lighting Designer and Moving Light Programmer and currently Senior Lecturer in Technical Theatre Production at Bath Spa University in the UK. He is also the Editor of On Stage Lighting and runs workshops in stage lighting practice.

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8 Comments

  1. tarsaim kumar agarwal:

    the article is very useful for begginers.as am i.thanks for helping begginers
    T.K.Agarwal

  2. Rob:

    Thanks T.K

    Keep visiting – there is a lots more help coming in the future.

  3. Tarsaim Kumar Agarwal:

    I have visited this site about a year back,and learned a lot for operating intelligent lights.The tips available are very useful.I want to learn more on programming.Now a days I am in London for
    a month,Pl.convey me any such contact/reference. Thanks.

    T.K.Agarwal

  4. Alon Cohen:

    Could there be a situation where one playback is running channels as LTP and another playback is running the same channels as HTP?
    If yes, what happens when the LTP fader initiates?

    Is there any reason for setting other moving light attributes, such as Pan and Tilt as HTP?

  5. Rob Sayer:

    Hi Alon

    No lighting console that I know about will allow you to patch the same DMX address/channel to two different controls so there would not be a situation where a particular DMX address ran as HTP and LTP simultaneously.

    Along with Dimmer attributes, Red, Green and Blue of an RGB might be useful as HTP, basically because they are also dimmer attributes. I can’t see any reason why there would be any benefit in setting anything else, like Pan and Tilt, as HTP. In fact, before we had proper moving light desks, we often used to run scanners from a simple DMX desk with faders that all ran HTP. It wasn’t fun.

  6. Alon Cohen:

    Hi Rob,
    Thank you for your reply.

    Sure I understand with the Patch, but what I meant was what if the playback owns that option, like most modern monster boards do.
    I noticed on the gMA2 if you have a cue on one fader with the same channel on another cue on another fader, one can be defined HTP and the other LTP.
    There is a set of rules there which I could not follow exactly the reason for.

    I am still researching…

  7. Rob Sayer:

    You can define things like IPCB faders, Use HTP etc. on many consoles. In the case of playbacks, precedence is often set by a series of priorities and also usual LTP HTP rules. If you have an HTP playback running then that obviously wins over anything HTP, until it’s released and the next one takes over or a new level is set via LTP. I’ve never had a conflict with these rules running multiple playbacks, where I didn’t know what to expect. If you use fader position for CMY in one cue, then fire another LTP cue that contains different colour data, LTP still applies in every case I’ve tried because at Patch level they are still LTP channels. The question comes down to the way the the desk handles playback priority.

    The point I made about the patch is pertinent because that is the only place where a conflict might occur – if only it was allowed.

  8. Alon Cohen:

    Thank you Rob for the enlightening responses.

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