jfuller Posted August 5, 2009 Author Share Posted August 5, 2009 Firstly,thanks to everyone for all of the advice, it is very much appreciated. It turns out that all the system needed was some inductance in series with the power supply, as suggested above. I found a 240v/12v transformer, and tried it with the 12v winding. This worked well, but the 240v winding worked much better, to the point that the DTMF tones I was using to test were rather painful to listen to, so I will include some form of volume control. In fact, it looks as though I may not even need to worry with impedance matching, as I will just try to get higher ( nearer 120 ohm) impedance headsets, and the audio was so painfully loud that the system may work even if I am not able to do this. Part of the problem is that one of the telephones that I was using for the test is slightly broken. I had already removed the handset before the project, as I needed an electret microphone for another project, so I used the speakerphone instead. However, it looks like the speaker of the speakerphone is not connected, although microphone and DTMF tones work fine. I may also try the DTMF decoder idea, possibly with a push button across one of the contacts on phone's keypad - I am sure I have a DTMF decoder IC in a box somewhere. The DECT phone idea seems quite good as well, I may try that soon. Muting the microphone only should be easy, as most phones seem to have a button to do that. From experience, do you think a latching or push to make switch would be better? Link to comment Share on other sites More sharing options...
cedd Posted August 5, 2009 Share Posted August 5, 2009 Muting the microphone only should be easy, as most phones seem to have a button to do that. From experience, do you think a latching or push to make switch would be better? My preference would be a douple pole, centre off, toggle switch, with momentary action one way and latching the other way. The phone protocal is a bit of a pain due to the series connection. I'd originally thought it was just unbalanced audio, no such luck! I've had trouble connecting my astrolite headsets (moving coil mics I think) to the system - I just get no audio out of them at all. I had to put the original telephone mic in place in order to get anything out of them at all. Shame, as the astrolite mics are far nicer and are noise cancelling. I can't see what the problem is, surely the telephone mic is electret, so I should get a much larger signal out of the moving coil mic. It doesn't need a bias voltage. I guess it may be an impedance problem - the telaphone expecting a much higher impedance than the moving coil mic is offering. My main interest now is interfacing a telephone system with an off the shelf comms system. I've seen some telephone recording adapters which seem to give a line level input and output from a telephone line, guess these may do the job nicely. Link to comment Share on other sites More sharing options...
AndyL Posted August 5, 2009 Share Posted August 5, 2009 Firstly,thanks to everyone for all of the advice, it is very much appreciated. It turns out that all the system needed was some inductance in series with the power supply, as suggested above. I found a 240v/12v transformer, and tried it with the 12v winding. This worked well, but the 240v winding worked much better, to the point that the DTMF tones I was using to test were rather painful to listen to, so I will include some form of volume control. In fact, it looks as though I may not even need to worry with impedance matching, as I will just try to get higher ( nearer 120 ohm) impedance headsets, and the audio was so painfully loud that the system may work even if I am not able to do this. Muting the microphone only should be easy, as most phones seem to have a button to do that. From experience, do you think a latching or push to make switch would be better? Please: Beware that with that large inductance in the circuit if you make or break the circuit whilst it is energised there will be a lot of stored energy and some VERY BIG and painful voltage spikes. :) (V = -L * dI/dT..... if I remember correctly) You will probably find that, up to a point, you can reduce the 'volume' of the entire system just by reducing the supply voltage. Link to comment Share on other sites More sharing options...
mskyrme Posted August 5, 2009 Share Posted August 5, 2009 As for the mute, does it just mute the microphone or do you loose the audio from other stations as well? That could be a show stopper if you have to have your mic open to hear other people and may require a quick bodge with a switch........ Just to check then, the headset replaces the handset, the handset is redundant and no longer needed? Mute only mutes your mic, you can hear all other users (unless they've all got the things muted!). In fact you get a very quiet triple beep every 15-20 seconds, which I though would be annoying, but it is not. Handset is redundant. The lighting guy I'm currently working with already wants to nick the headsets for our next show - This'll save him shouting up to the lighting box when he's on the fly floor or focussing. The technician from the theatre also wants to have one so he can sit in the bar! This will be fine once I get it connected to the main comms system! I've been helping my Tech guys at work today putting in CCTV cabling. Again the system has proved invaluable when you are up in a roof space pulling through cables. Even when the system double beeped to warn of a low signal it sounded OK. Been running nearly 6 hours fine. Link to comment Share on other sites More sharing options...
AndyL Posted August 5, 2009 Share Posted August 5, 2009 I've had trouble connecting my astrolite headsets (moving coil mics I think) to the system - I just get no audio out of them at all. I had to put the original telephone mic in place in order to get anything out of them at all. Shame, as the astrolite mics are far nicer and are noise cancelling. I can't see what the problem is, surely the telephone mic is electret, so I should get a much larger signal out of the moving coil mic. It doesn't need a bias voltage. I guess it may be an impedance problem - the telaphone expecting a much higher impedance than the moving coil mic is offering. Nooo, sorry, moving coil output is always going to be way lower than electret, an order of around 20dB or x100 difference in voltage typically. You are actually going to need quite a bit of gain from somewhere to get a dynamic mic (i.e. moving coil)up to the level of an electret, it is more than just an impedance mismatch. As you say the dynamic mic doesn't need any bias voltage. Noise cancelling electret mic capsules are available. Link to comment Share on other sites More sharing options...
cedd Posted August 5, 2009 Share Posted August 5, 2009 Nooo, sorry, moving coil output is always going to be way lower than electret, an order of around 20dB or x100 difference in voltage typically. You are actually going to need quite a bit of gain from somewhere to get a dynamic mic (i.e. moving coil)up to the level of an electret, it is more than just an impedance mismatch. As you say the dynamic mic doesn't need any bias voltage. Noise cancelling electret mic capsules are available.Bugger, was sure in the back of my head that I'd got things the right way round! That would explain a lot then! Should have known that (kicks self), more to the point, did know that but chose to ignore it! At the minute, a quick mic change seems to have done the job, especially as my headsets are on 5 pin XLR's, so no chance of confsing them with standard comms system ones. Link to comment Share on other sites More sharing options...
jfuller Posted August 7, 2009 Author Share Posted August 7, 2009 Any suggestions on the connectors to use between the intercom and the power supply? At the moment I am thinking of cat5 cable with 4 pin XLRs on the end. I would use cat5 with RJ45s but all I do not particularly like making square holes in project boxes, and the connectors seem to 'flimsy' I would use 4 pin XLRs to ensure that this system is not plugged into anything else - this is the reason I would not use use 3 pin or 5 pin XLRs. I also thought about DIN sockets, however last time I used these they did not work well at all, and needed to be held in just the right place to work, though this may have been because I was using the shield as the only ground connection (for some self built LED lights). Any thoughts? Link to comment Share on other sites More sharing options...
david.elsbury Posted August 7, 2009 Share Posted August 7, 2009 It is possible to get locking DIN connectors I beleive, called bleecons IIRC. Link to comment Share on other sites More sharing options...
dbuckley Posted August 8, 2009 Share Posted August 8, 2009 Bleecons are a type of lockable DIN like connectors, they have the same pin layout and will mate, but not lock with DINs, lockable or not. Bleecons have a pull to unlock operation, lockable DINs are a twist to unlock. http://www.dmx512.com/web/light/analog/bleec1.gif http://www.canford.co.uk/images/itemimages/large/7906-01.jpg Link to comment Share on other sites More sharing options...
BoogieBear Posted August 8, 2009 Share Posted August 8, 2009 I would use 4 pin XLRs to ensure that this system is not plugged into anything else - this is the reason I would not use use 3 pin or 5 pin XLRs. Hi there XLR4 is industry standard connector for 12v DC to portable camera/TV equipment (Pin 1 -ve and Pin 4 12v +ve) and on some older stuff used pins 2 & 3 for certain non-standard switching and so on - Hitachi amongst others if my memory serves me well - we have quite a lot of this floating around in our venue Great idea this though - looking forward to seeing how you get it all working (monkey see, monkey do!) CheersAnton Link to comment Share on other sites More sharing options...
pritch Posted August 8, 2009 Share Posted August 8, 2009 At the moment I am thinking of cat5 cable with 4 pin XLRs on the end. I would use cat5 with RJ45s but all I do not particularly like making square holes in project boxes, and the connectors seem to 'flimsy' If you don't like making square holes, what about Ethercon? Standard Neutrik D-type cutout, and you can either use RJ45 or the more robust Ethercon metal shelled connectors. Link to comment Share on other sites More sharing options...
dbuckley Posted August 8, 2009 Share Posted August 8, 2009 The last time I saw telephones used as intercoms was in about 1966, and this was professional use, and they used standard GPO jacks for the connectors. Link to comment Share on other sites More sharing options...
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