jcblincs Posted January 18, 2009 Share Posted January 18, 2009 Hi - just joined the forum today !I have been involved in am dram technical for a while and we have always used pulsar analogue gear. WE have a 12 channel desk and two 6 channel rack packs which are hard wired into our village hall. I have a connector box on the wall at the back of the hall and a corresponding box at the stage end. This was installed last year to make things more permanent and neater. I simply plug in the 8-pin pulsar lead to a socket at the back of the hall and power up the dimmers and away we go. We also put in a prewired lighting bar which looks very neat (the ceiling is low so this bar made a huge impact on the asthetics of the set up) Anyway - we now find ourselves a the point where the next production is going to be a major challenge on the lighting front. We simply will not have enough channels (6 are for use over the stage and 6 front of house pointing at the stage). We also find we have some funds available (up to £1000). This is my plan - I woudl welcome any advice...I plan to buy a DMX desk (probably 48 channel)I plan to buy a showtec demux box (or similar)I can then use my existing dimmer packs without having to undo all the hard work of the hard wiring we did. our connection boxes already have XLR and speaker connections - I plan to use the XLR connections to feed the DMX signal down the hall to the demux unit.I will then buy 2 or three of the cheap and cheerful portable 4 channel DMX dimmer packs and daisy chain them with the demux unit. this will give more flexibilty as these can be positioned where we need them (under the stage for footlights and behind the flats for ambient light when doors are opened etc) can anyone see and flaws in my plan. I have left a lot in teh budget for cables, connectors, clamps etc as I know these can add up. Have also budgeted for some parcan16 birdies to use as small light behind the scenery or as uplighters/footlights. the only possible issue I can see is the cable in the roof - I know technically its not DMX cable but it is screened mic cable. it passes over the main lighting bar and above the house lights which are flourescents (although these will not be on during production). total cable length will be about 30 meters to the demux and then another 10m to the next dimmer in the chain. aslo I have read about balancing the DMX chain with a resistor at the end so as not to leave it open to reflections. is this stricty necessary ? sorry this is a bit long winded but I dont want to sepnd the money and regret it !! I am on a very tight timescale and need to order the kit in the next 48hrs so any help woudl be gratefuly received, cheers ! Link to comment Share on other sites More sharing options...
PaulDF Posted January 18, 2009 Share Posted January 18, 2009 For what it is worth buying a pre wired DMX cable from somewhere like CPC isn't that much money, never used MIC cable so can't comment on it. http://cpc.farnell.com/jsp/search/browse.j...55249&No=50Showtec Demux will output 10v, 12v or 15v analogue (+ or -) by setting the switches on the back, it will not output a mixture of signals tho. CPC do a demux very similar to the showtec one http://cpc.farnell.com/elation/multi-data-...ount/dp/DP30320.If you go with the Showtec you'll need to make some 25 pin D - Type leads to interface the analogue dimmer packs. I'd strongly recommend using the 120ohm resistor across the end of the DMX network. Link to comment Share on other sites More sharing options...
Davethsparky Posted January 18, 2009 Share Posted January 18, 2009 I plan to buy a showtec demux box (or similar) the only possible issue I can see is the cable in the roof - I know technically its not DMX cable but it is screened mic cable. it passes over the main lighting bar and above the house lights which are flourescents (although these will not be on during production). total cable length will be about 30 meters to the demux and then another 10m to the next dimmer in the chain. aslo I have read about balancing the DMX chain with a resistor at the end so as not to leave it open to reflections. is this stricty necessary ? Pulsar use a different pin arrangement on their analogue connections to most other manufacturers so your demux to dimmer cables will need to adapt from one to the other. Irrelevant of length or route you really shouldn't use the wrong cable to carry your DMX signals, a quick search should bring up plenty of info on the reasons for this. It may work ok with the mic cable, but are you happy to take the risk? If you dont mind the possibility of DMX gremlins getting in and causing some impromptu lightning effects in the middle of a dark moody scene then go for it, but otherwise get the real stuff. Terminating (balancing as you called it) is a must. The magic recipe is a 120ohm 1/2 watt resistor soldered across pins 2 and 3 of an XLR plug, then plugged in to the last device in the chain, or you may be lucky and have a built in terminator in your last unit in which case it is just a flick of the switch. Not terminating could also lead to gremlins getting in and causing havoc. Hopefully this will help you, but I'm sure others will be along offering their advice too. Link to comment Share on other sites More sharing options...
back_ache Posted January 18, 2009 Share Posted January 18, 2009 This is my plan - I woudl welcome any advice...I plan to buy a DMX desk (probably 48 channel)I plan to buy a showtec demux box (or similar)I can then use my existing dimmer packs<snip>buy 2 or three of the cheap and cheerful portable 4 channel DMX dimmer packs<snip> can anyone see and flaws in my plan<snip> the cable in the roof <snip> not DMX cable but it is screened mic cable. it passes over the main lighting bar and above the house lights which are flourescents (although these will not be on during production). total cable length will be about 30 meters to the demux and then another 10m to the next dimmer in the chain. aslo I have read about balancing the DMX chain with a resistor at the end so as not to leave it open to reflections. is this stricty necessary ? sorry this is a bit long winded but I dont want to sepnd the money and regret it !! I am on a very tight timescale and need to order the kit in the next 48hrs so any help woudl be gratefuly received, cheers ! What you are suggesting should work just fine, you can get away without a terminator but to be honest they are not expensive and you can make them yourself easily enough. Link to comment Share on other sites More sharing options...
jcblincs Posted January 18, 2009 Author Share Posted January 18, 2009 thanks. its not the cost of the DMX cable its more the hassle at the moment. the box is there with XLR on both ends and cable running through the loft. there is no time to change the cable. I can run a lead down the hall and tape it to the floor if I have to. The more I read about the more I am torn - loads of people say mic cable is fine. Loads say not !!! whats the actual difference ? as long as the mic cable is screened twin core cable and all three pins are connected is there actually any difference ? termination - is it literally a 2 pence resistor from maplin soldered across the pins of a plug that is plugged into the last device ? Link to comment Share on other sites More sharing options...
niclights Posted January 18, 2009 Share Posted January 18, 2009 termination - is it literally a 2 pence resistor from maplin soldered across the pins of a plug that is plugged into the last device ?Yes. I would just try the existing cable first and see. Chances are it will be fine and never cause any problems, especially if you are only running generics. If it does then you can look to upgrade. Link to comment Share on other sites More sharing options...
Dmills Posted January 18, 2009 Share Posted January 18, 2009 The more I read about the more I am torn - loads of people say mic cable is fine. Loads say not !!!whats the actual difference ? as long as the mic cable is screened twin core cable and all three pins are connected is there actually any difference ?termination - is it literally a 2 pence resistor from maplin soldered across the pins of a plug that is plugged into the last device ? I think you will find that most of the people doing things where reliability really matters will be saying not! Mic cable is one of those things that works fine, MOST of the time, but do you want to be chancing your show on it not starting to misbehave? I think a lot of the differences of opinion on this one comes down to the theatre side (where not having any glitches is fairly important), Vs the club and disco side where an occasional glitch is far less of an issue. Incidentally, cheap cat 5 network cable does just fine for DMX as does AES digital audio cable. The difference is the characteristic impedance of the cable, at the frequencies DMX operates you need to think of a cable as a transmission line, and if there is a change in the impedance then energy will be reflected back down the cable potentially interfering with devices earlier in the chain. Termination, a 120 ohm 0.5 watt resistor between pins 2 and three, job done. Regards, Dan. Link to comment Share on other sites More sharing options...
Jivemaster Posted January 18, 2009 Share Posted January 18, 2009 Always check that the hall infrastructure will support the proposed changes for power supply and load bearing capability. Link to comment Share on other sites More sharing options...
jcblincs Posted January 18, 2009 Author Share Posted January 18, 2009 so if I buy this stuff I will be OK ? http://www.10outof10.co.uk/acatalog/10_out...s_88.html#a2360 but then it looks like the cable I have in the loft - two core screened cable can someone explain the difference please ? A concurrent post has been automatically merged from this point on. Always check that the hall infrastructure will support the proposed changes for power supply and load bearing capability. thanks. I thought about that earlier. we are pushing it close in theory but in practice we are nowhere near. the light we use are all 500w so it will take 30 lamps before we get close to the 60A rating on the breakers. dont think we will ever have that many lights on at once !! Link to comment Share on other sites More sharing options...
smalljoshua Posted January 18, 2009 Share Posted January 18, 2009 DMX cable is of the same make up I.E 2 core + screen but it acts differently when a signal is put down it. A mic cable is designed for 20Hz-20kHz not the much higher speed that DMX runs at. This is why you need a cable designed for the higher frequencies. Josh Link to comment Share on other sites More sharing options...
Davethsparky Posted January 18, 2009 Share Posted January 18, 2009 you can get away without a terminator Is it about 'getting away with it' or is it about doing a job properly? Link to comment Share on other sites More sharing options...
jcblincs Posted January 18, 2009 Author Share Posted January 18, 2009 ok - update. I have just dismantled the connector box on the wall. I did not install it and realised I was making an assumption about the cabling. I found that a 9 core cable has been used to connect 3 XLR sockets. Its groups in 3's and each group a foil screening on it. The sceen is connected to the 3rd XLR pin. in my case I would only use one of the groups and the others would not have a signal. I think I will buy a pre-wired DMX cable with 3 pin XLR on it and use this. I will run it down the side off the hall and duck tape it down. I need about 24/25m so will buy a 30m pre wired DMX cable. 10outof10 do it for about £30. does this seem like a safer option ?when I get more time I can lay the cable in the loft and make a neater job of it ! Link to comment Share on other sites More sharing options...
niclights Posted January 18, 2009 Share Posted January 18, 2009 If there's something to anchor it to I'd use cable ties instead of gaffa. It doesn't tend to hold particularly well and will leave a mess. Link to comment Share on other sites More sharing options...
boatman Posted January 19, 2009 Share Posted January 19, 2009 ..I think I will buy a pre-wired DMX cable with 3 pin XLR on it and use this. I will run it down the side off the hall and duck tape it down. I need about 24/25m so will buy a 30m pre wired DMX cable. 10outof10 do it for about £30. does this seem like a safer option ?when I get more time I can lay the cable in the loft and make a neater job of it ! One thing that seems to have been overlooked in this thread is that a DMX cable should have 5-pin XLR connectors and not 3-pin. Your new 48-channel desk will almost certainly have a 5-pin output socket and the Showtec demux has both 5-pin and 3-pin inputs. CPC do a 5-pin 30m cable for just over £20 including VAT. They also have a 50m version and, if you insist on going that way, 3-pin versions of both. Edit: for clarity. Link to comment Share on other sites More sharing options...
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