Basic questions about number of channels

[Avalon93]

Hi there!

 

When I read the specifications of lighting desks, I usually read that e.g. the Jester ML 48 has 512 DMX outputs (channels) but 48 control channels, and the capability to control up to 30 moving lights.

 

The thing I don't understand is what are phisically the DMX outputs... and how can I control 512 devices if I got 48 phisical faders and 30 moving lights: 48 + 30 = 78, not 512!

 

I know that through a DMX cable I can send 512 different signals, I can control a 512 channels dimmer so set the levels of 512 fixtures... but if I have only 48 brightness parameters to set and 30 brightness, beamshape, colour and position parameters, how can I reach the 512 control channels?

 

I believe my question can appear quite stupid, but if I could by now operate on the consoles without asking about how they were connected to the devices and how a net of devices can be created, the situation in which I'm going operate needs me to know how works a net built by a desk, a dimmer and some devices (traditional Fresnels and modern intelligent lights).

 

Big thanks, bye!

 

Isaac

[Wol]

How do you control 512 channels? The simple answer is: You don't.

 

Smaller desks can usually patch to any of the 512 channels possible, but can only patch a limited number of things to them.

 

E.g. you can pick out 78 different channels to control, and patch those to any 78 of those 512 channels available on the DMX output, rather than controlling 512 different things. Larger desks aimed at larger rigs will support whole, and multiple universes. Smaller desks usually don't need to use an entire universe to control what they were designed for.

 

Also, DMX doesn't create a "net" as such, but a chain. One output from the desk goes into one fixture/dimmer. The output from that fixture/dimmer then goes to the next, and so forth. Have a look around the BR wiki, as there should be loads of info on there about this sort of stuff :-)

[smalljoshua]

Also, a fixture doesn't necessarily use a single channel. A moving head can use over 20. E.G. Pan, Tilt, Colour 1, Gobo 1, etc. Also, on the ML48 if you patched 29 movers with 16 channels along with the 48 generic channels you'd have filled up the desk without having more than 78 fixtures in the rig.

 

Josh

[pritch]

Also to add that one fader doesn't necessarily equal one DMX channel. The Jester, for example, will actually allow you to control all 512 channels off one fader!

 

That example might be a bit extreme, but I've been known to combine two dimmer channels to one fader for simplicity, when I'm absolutely certain that I'll always want both channels to do exactly the same thing.

[paulears]

I dreamed this up for when I had to explain DMX to totally non-technical kids who used to visit our college from school. It's out of place here - but it worked for me.

 

Imagine DMX as a typical high street in a town. The postman delivers the letters starting at number one. If you live at number 3, then you only get to read letters addressed to you there. You watch the rest of the post bag go past, and you couldn't care less what's in those letters. You and your neighbours all live in nice simple houses. However, a few doors away is a block of flats, only a single door to get in, starting at No. 6, but the ten flats inside soak up 10 Addresses. The next house is No. 16. This house looks normal - a big 16 on the front door - but the man from MI5 lives there, and as the bloke in number 3 is a spy, the Royal Mail deliver a copy of all number 3's mail here. So in effect, this house is kind of out of sequence, and is just interested in No. 3's mail. The MI5 person doesn't get any mail addressed to No. 16.

 

DMX works in a very similar way - the DMX data is like the postman's sack - it contains information for 512 addresses. Some addresses have just the single postbox, while the big users need the ability to get information for many address. A dimmer needs just a single address, a scanner with left-right/up-down/colour/gobo needs 4. If some addresses are duplicated this is fine - each fixture set to the same address reads the same information at the same time.

 

512 addresses seems a lot (and once was!) but modern clever fixtures eat them up quickly - hence why multiple universes (universe=512 DMX addresses) have become quite normal.

 

Sorry if this is a bit patronising - but I always found you cannot assume people know even the correct vocabulary. Even if the above is rather daft and child like - it's easy to then up-scale it to our own level of technology. The kids rather liked the man from MI5 sneakily reading other people's mail - so I think it worked for them, and once done, we forgot about it quickly - but they did remember it!

 

P

[Ynot]

When explaining DMX to newbies in the past I've often used the precursor to the whole PCM principle, and the basis on which all TDM systems are based - the old uniselector.

 

Bit difficult to demonstrate in print here, (and don't have time to try now) but was quite an easy principle once you got over the ideas.

In fact many years ago, as a project, I built a back to back uni to uni set up with a battery one end and small 6v lamps the other... With a variable speed so you could show one lamp lighting at a time, but run it fast enough and you got the impression of all of them pretty much lighting at once.

 

No idea where that went though - probably in a dark cupboard somewhere in a BT building...!

[Matthew Robinson]

When explaining DMX to newbies in the past I've often used the precursor to the whole PCM principle, and the basis on which all TDM systems are based - the old uniselector.

 

Bit difficult to demonstrate in print here, (and don't have time to try now) but was quite an easy principle once you got over the ideas.

In fact many years ago, as a project, I built a back to back uni to uni set up with a battery one end and small 6v lamps the other... With a variable speed so you could show one lamp lighting at a time, but run it fast enough and you got the impression of all of them pretty much lighting at once.

 

No idea where that went though - probably in a dark cupboard somewhere in a BT building...!

 

Doesn't that demonstrate multiplexing, rather than DMX which is a serial data transmission protocol?

[Andrew C]

I think we're all going a bit deep here; I think the OP wants to know how 48 channels is practically patched into 512 outputs. If so, you might decide that all your red backlights will be used together. That might be 4 dimmers numbered 1-4, these could be patched to control channel 1. The blues (5-8) patched to control 2, and so on until all of your groups of similar lights are patched. Then you would allocate your specials so that each individual light had a control channel of its own.

 

This way quite a large rig can be controlled with a relatively small number of control channels.

 

I'll leave it to someone with experience of the Jester to tell us if the 30 moving lights (which could require 20+ control channels each) are in addition to the 48 dimmer channels.

[pritch]

Doesn't that demonstrate multiplexing, rather than DMX which is a serial data transmission protocol?

 

Remind me what the MX stands for in DMX?