Separate call signal for one leg of comms system

[pritch]

We're looking at our comms system - one of the things we'd like to do is extend it to the foyer, probably using the Tecpro HS591 handset station - the use case is basically for the stage manager to be able to quickly call box office to check how things are looking out front before starting the performance.

As we're only going to have one intercom line, I don't want the ringer on the box office station going off every time someone presses a call button. My thoughts are that there could be a button for the stage manager that sends a call signal down the FOH line, but that the we block the call signal from everyone else. If my understanding is correct, the old-style Tecpro call signal is sent by just shorting the power line onto the audio? If that's the case, sending the signal is easy enough.

The question, then, is how do I block the signal from everyone else?

We're unlikely to find ourselves using new-style Tecpro anywhere other than the handset in box office, so I don't need to worry about blocking the high frequency tone for the call signal, I just need to block the old-style DC signal. Getting a call signal back from FOH to stage isn't important.

[boatman]

It's not just a short of the power rail to the bus, a little bit more sophisticated than that. The attached picture is the call button section of Bluecom. I think you might get away with just building that section and isolating it from the bus with a 1uF capacitor so that the call voltage doesn't feed back to the rest of the comms system.

I can't guarantee that it will work but it shouldn't do any damage!

[pritch]

Thanks Boatman - I was rather hoping for a solution that involved a switch and a couple of resistors or capacitors. I guess that's not going to be on the cards.

Regarding your suggestion about isolating it from the bus with a 1uF capacitor, though - are you saying that if I just put that capacitor in the audio line and allow the ground and power lines to continue uninterrupted that it should give me a split call signal on that part of the bus? That might allow me to repurpose an old beltpack just for generating the call signal to FOH.

 

[sunray]

I'm a little lost here.

I think I understand adding the new handset with the high frequency 'call' function and isolating the DC signal from it, what I don't understand is why Boatman has offered the circuitry for the DC call function or where the high frequency call signal is being generated.

 

As I see this; adding a modified new style unit at SM location to will provide the HF call signal and additionally detect a 'call button' from box office.

1 hour ago, pritch said:

Regarding your suggestion about isolating it from the bus with a 1uF capacitor, though - are you saying that if I just put that capacitor in the audio line and allow the ground and power lines to continue uninterrupted that it should give me a split call signal on that part of the bus? That might allow me to repurpose an old beltpack just for generating the call signal to FOH.

That will work if it's a new style beltpack and possibly include the capacitor to prevent it calling across the system.

 

 

Apologies if I've missed the point somewhwere.

Another thought may be to add a diode across the capacitor to allow a 'call' by box office to call across the system normally.

[richardash1981]

My understanding is based on two things:

1. All of the existing intercom is old-style tecpro, i.e. DC call only.
2. The new Tecpro HS591 handset station responds to DC call (the description on Canford website is not completely clear on sends vs. responds), as well as HF call.

Given these, nothing on the system will send HF calls, so we can ignore that.

The capacitor in the audio line (which probably needs to be 1uF (minimum) non-polarised) blocks DC call signals from the main intercom line, producing a break point in call terms, whilst passing audio. Adding a diode with anode to the box office, cathode to the main comms system will allow one-way calling as Sunray says. It would also mean that calling the box office calls the rest of the comms system ...

On the comms leg between the capacitor and the box office something needs to generate a call signal:

• A spare (old DC call) beltpack.
• Boatman's circuit from SW2 (the call switch) to TR2 and associated parts. If you don't fit the diode but want the SM to be able to see that the box office is calling, you would want TR3, TR4 and LED1 (the call light) - otherwise you can ommit them.

A 1k resistor plus a switch between the audio line and VCC would work in terms of causing the system to call, but (because it would have very sharp edges) cause massive thumps/clicks on the audio line for everyone. This is what the various components around TR2 avoid, by making the DC call pulse ramp up and down slowly enough that the thumps are not heard.

[sunray]

1 hour ago, richardash1981 said:

My understanding is based on two things:

1. All of the existing intercom is old-style tecpro, i.e. DC call only.
2. The new Tecpro HS591 handset station responds to DC call (the description on Canford website is not completely clear on sends vs. responds), as well as HF call.

The new format belt packs send and receive DC and HF, some programming is included - ie to disable the buzzer - but I don't know about selecting call formats. If it's possible to disable responding to DC then no mods are required.

1 hour ago, richardash1981 said:

Given these, nothing on the system will send HF calls, so we can ignore that.

The capacitor in the audio line (which probably needs to be 1uF (minimum) non-polarised) blocks DC call signals from the main intercom line, producing a break point in call terms, whilst passing audio. Adding a diode with anode to the box office, cathode to the main comms system will allow one-way calling as Sunray says. It would also mean that calling the box office calls the rest of the comms system ...

Yes my suggestion of the diode is to allow the box office to call the rest of the system, but I'm not sure I see why calling the box office will call the rest of the system. However my interpretation of the system is something like this layout which may be different to you and indeed may be wrong:

With the cap and diode in the mystery box

I've shown an additional 'new format' handset at SM position in red but in reality all thats required is the HF signal generator element (could simply be a 555 and a bit of filtering) and responder lamp/buzzer etc (if required).

 

[boatman]

9 hours ago, pritch said:

Thanks Boatman - I was rather hoping for a solution that involved a switch and a couple of resistors or capacitors. I guess that's not going to be on the cards.

Regarding your suggestion about isolating it from the bus with a 1uF capacitor, though - are you saying that if I just put that capacitor in the audio line and allow the ground and power lines to continue uninterrupted that it should give me a split call signal on that part of the bus? That might allow me to repurpose an old beltpack just for generating the call signal to FOH.

 

Yes, a non-polarised capacitor between the existing system and the mystery box will prevent the DC calling signal to the new handset from going back down the line to the existing beltpacks. It needs to been a minimum of 1uF/50V but anything larger will be fine.

As the new box is DC isolated from the rest of the system you might need the LED indicator which will only light up when called from the new handset. If you don't need that then it's only four resistors, two capacitors and a PNP transistor and should only take half an hour to build on a bit of stripboard. If it helps I have all the necessary components and could knock something up for you to try.

[J Pearce]

This would probably be much easier, cleaner, and have better longevity, if you upgraded the SM desk position to a 2 ch station, and ran ChB as the link to box office. It might mean running a new bit of cable but longer term would provide a more flexible and more resilient system, without a one-off solution waiting to confuse matters in future years.

[sunray]

1 hour ago, J Pearce said:

This would probably be much easier, cleaner, and have better longevity, if you upgraded the SM desk position to a 2 ch station, and ran ChB as the link to box office. It might mean running a new bit of cable but longer term would provide a more flexible and more resilient system, without a one-off solution waiting to confuse matters in future years.

The irony is that was in my first reply but reading OP again I deleted it as a single line seemed to be a given.

I'd still like to understand where SM sits within the system and if DC or HF signalling is required.

[WxmMike]

Our solution - use the telephone extension adjacent to the DSM desk to call box office.

Our contingency plan if "cans" system fails is to use phone to call LX/Sound box!

 

[pritch]

Thanks everyone - some food for thought there.

@sunray - re your comment on HF signalling, that does sound rather interesting. My original thought was to just use DC  (I guess because I thought it would be easy).

Going with DC would have meant running the intercom line from the SM position to box office, which wouldn't have been ideal because the SM would have been somewhere in the middle of the line - but if I could go HF then it could mean having the SM position somewhere in the middle of the bus (like your diagram a few posts back) which would be simpler to install in terms of physical cable runs.

[sunray]

5 hours ago, pritch said:

Thanks everyone - some food for thought there.

@sunray - re your comment on HF signalling, that does sound rather interesting. My original thought was to just use DC  (I guess because I thought it would be easy).

Going with DC would have meant running the intercom line from the SM position to box office, which wouldn't have been ideal because the SM would have been somewhere in the middle of the line - but if I could go HF then it could mean having the SM position somewhere in the middle of the bus (like your diagram a few posts back) which would be simpler to install in terms of physical cable runs.

Thanks for the clarifications, that clarifies the situation.

I'd always understood you wished to extend the existing FOH (which I took to be the sound/lighting tech position).

If you're happy to run a separate cable have you thought about a completely independant intercom? As an example this was the first to appear in my google search

https://www.diy.com/departments/wired-door-entry-phone-receiver-system-100m-range-security-intercom-outdoor/5055538112478_BQ.prd

[sandall]

Not sure if they are still available, but some years ago I put an Eagle T 1.2 "Interphone" (basically just a pair of handsets with 2-wire connection) into a public hall where they needed a very simple link from stage to sound/lighting. Worked fine. 

[sunray]

11 minutes ago, sandall said:

Not sure if they are still available, but some years ago I put an Eagle T 1.2 "Interphone" (basically just a pair of handsets with 2-wire connection) into a public hall where they needed a very simple link from stage to sound/lighting. Worked fine. 

That sort of thing was more what I had in mind but the door entry system was the first hit on google.

Eagle are well gone now but I'm sure similar items must be available.

 

EDIT: in fact putting T 1.2 "Interphone into google comes up with:  https://www.amazon.co.uk/Telephone-Intercom-Handsets-Conversations-Restaurants/dp/B000NJ0L1G/ref=asc_df_B000NJ0L1G/?tag=googshopuk-21&linkCode=df0&hvadid=205228866603&hvpos=&hvnetw=g&hvrand=15407623639463784092&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9045130&hvtargid=pla-434328631827&psc=1&mcid=13480835e2b73b7db65498f3eef24ef4

Which is more like I was thinking about.

Edited June 4 by sunray

[sandall]

Yes, same thing, newer styling. It wasn't actually an Eagle product; it just had the 'Eagle T 1.2' label stuck on an 'Interphone' box.