Audio conferencing interface

[WaveyDipole]

How does one go about interfacing a PA system to a telephone socket for the purpose of tele-conferencing events? Does one have to buy a BT approved box of some kind? If so what kind of box is required? On the other hand, is it possible and permissable to build one?

The charity I'm asking the question for is currently using an induction pickup wired into a telephone headset socket (RJ11 I think). This picks up the signal from a wire wrapped around it from that leads to the deaf loop and works in principle, but unfortunately, and unsurprisingly, picks up all manner of electrical noise as well.

I am hoping that there is a better way to do this both safely and reliably but also relatively inexpensively. Any thoughts appreciated.

[AndiiT]

Hi,
Something like this plugged in to the telephone line and then connected to your PA system might do the job, there are alternatives on the same site depending on your exact requirements.

Regards

Andrew

[OscarFoxtrot]

You need a BE (or CE) approved telephone interface which will split out the 2-wire BT line into separate send and receive connections you can interface to the PA system.

http://www.canford.co.uk/GLENSOUND-T...IGITAL-HYBRIDS

If only required for occasional use, these can be hired.

If you want to build your own then use a transformer-isolated circuit and some suggestions are at
http://www.epanorama.net/circuits/te....html#audioint

[Nickthedentist]

If you're on a shoestring budget, find a secondhand corded phone with a "loudspeaking" facility in a charity shop or what have you, and connect your amplifier in place of its speaker via an isolation transformer, e.g. the ones that you used to find in 1980s TVs which had live chassis but also a headphone socket, such as the Philips KT3, CTX-E and so on.

Not technically approved, but perfectly safe if you do it correctly.

See also here: https://www.vintage-radio.net/forum/...d.php?t=102485

[Nicklyons2]

The BT Converse 220 has the very facilities Nick suggests and even has a 'headphone socket' underneath (using RJ type connector); there is an amplify function for receive. A decent-ish mic well placed and the headphone earpiece feed via, as suggested via transformer, to the P.A. input. There is very little chance of 'live bits' in this setup - more hum loop problems so even a decent audio driver transformer (as long as it's fully isolated pri - sec) will do for coupling.

One further point get everyone near the set up to SHUT OFF their mobile phones - if they don't you WILL end up with the 'machine gunnning' digital interference. You'll find people think and say they've shut off their mobile and they actually put it on silent and fib - it'll catch you out! On one professional job I had to get security to confiscate a couple of mobiles because the owners just wouldn't turn them off and I couldn't get a clean recording.

By the way the official name for the piece of professional gear is a Telephone Hybrid. Now, most are digital but some early ones were analogue and you 'fine tuned' the send and receive balance. In the hands of a careful operator (IMHO) the analogue ones were capable of very good results possibly better than the digital ones, but in the hands of those less familiar the digi ones are possibly a bit easier to set up and 'bomb proof'.

[dagskarlsen]

Originally Posted by OscarFoxtrot

If you want to build your own then use a transformer-isolated circuit and some suggestions are athttp://www.epanorama.net/circuits/te....html#audioint

The last link here are good, but it depends of your skills to make it. The main issue are the impedance matching, what kind of input possibilities do you have on your PA system?

The simplest diagram just under "Audio interfaces without transformer isolation" may be good enough.

dsk

How about Skype and a laptop?

[Nickthedentist]

How about the call failing 50% of the time, if my experience is anything to go by

[julie_m]

How about fitting a scrap PC with an FXO card, and installing Asterisk and Linphone on it?

(Yes, I know I seem to propose Asterisk as the solution to most telephonical problems. But that's because it usually is .....)

[G6Tanuki]

You can get "hybrid couplers" with RJ11? connectors that fit into the lead between the handset and the phone-unit itself.

You unplug the handset-cable from the phone, plug the adapter into the phone then plug the handset-cable into the adapter.

Then connect your 'parasite' amplifier/mic to the other port.

Something like this: http://www.handset-solutions.com/sho...ch-p-1108.html

I've used these in the past as a kludge to provide 'teleconference' facilities for clients who are not prepared to spend money on a proper Polycom "spider-phone" or where their corporate PABX is of an age that doesn't support any kind of third-millennium digital technology.

[Stuart R]

I got caught out with this once, but it was over 20 years ago, teleconferencing technology may have changed. We were asked to use a Sonifex Hybrid to bring a teleconference in from the US. All went well until a voice at the other end asked that if anyone wanted to ask a question, they should press a key on their conferencing unit's keypad.

The Sonifex device has a single divert button, the line is either connected to the adjacent telephone handset, or sent to the audio interface.

By switching the Hybrid back to the handset, I was able to make our request, but this removed the sound from the auditorium speakers for a short moment.

The sound got removed for an uncomfortably long time when I later accidentally switched the line back to the hung-up handset and dropped the call.

It took an embarrassingly long time to dial-in to the US again!

Just something to be aware of, but it doesn't seem like it's been a problem for you so far.

SR

[WaveyDipole]

Thank you for the many replies, the circuits and transformer information. They are on a shoestring budget for this so I'm not sure that I will be able to purchase the Glensound interface suggested but I would be willing to build one.

There seems to have been a gravitation towards the subject of being able to record a conversation from the phone line but neverthess some very usefull information. I am actually looking at sending audio in the opposite direction, i.e. from the PA system down the telephone line to the conferencing provider. Elderly homebound listeners dial in to the conferencing provider to listen in.

We have been considering an IP solution and this is being developped in parrallel as the modern way forward. Asterisk has already been brought to my attention, but my understanding is that you have to be something of a SIP specialist to set it up and use it. However I do intend to download a copy along with Linphone to have a look at.

In the meantime, there are still some elderly individuals who do not have access to the Internet and for whom the only way forward is the telephone conferencing option, hence the need to retain a PSTN solution but improve upon it.

Nick, that link to Farnell did not work, but I did do a search on telephone adapter and did find a gadget to record and play. I would like to find a solution that would allow me to adjust the audio levels as well though.

Regarding the PA amp, I think I have the option of using one AUX out. There are two DIN AUX In/Out sockets. One connects to a computer sound card for playback and recording. The other is used to connect a CD player in, but the output is unused.

I had wondered whether I could salvage the 600ohm isolation transformer from an old telephone perhaps but never considered that one might be found inside an old TV!. I probably have most of the bits I need except for the isolation transformer. I will probably look at building an op amp circuit as illustrated on that ePanorama link but with appropriate input level control.

Is there any further safety advice I need to follow when using a mains powered interface?

[dseymo1]

Depending on the frequency of the sessions, something like Webex might be the simplest solution - you could ignore the video facility, take an audio feed direct from your PA desk to a Pc, and invite participents to dial-in to the 'conference' on a listen-only basis.
Nothing much to set-up (in physical terms), and just a laptop or similar required.

[OscarFoxtrot]

This circuit

http://www.rane.com/note150.html

says it can be used to send audio to the phone line.

You're more likely to find an isolation transformer in an old modem than in an old phone.

Couple of points -

- filter the feed to the phone line to remove sound outside the main speech frequency band

- the feed to your induction loop may have an 'ambient' microphone feed so that loop users don't feeel isolated when no-one is speaking. This 'ambient' microphone won't be fed to the main PA system. Therefore the induction loop feed may be more appropriate for feeding to the phone unit.

[WaveyDipole]

OscarFoxtrot, thanks for that link. Very helpful and good explanation of why each component is used. The point about filtering is an interesting one that I hadn't considered but which makes sense. I now need to figure out how to do that.

I also was thinking about how to hold the line open? Would simply connecting the transformer circuit on to the line keep it open once the conference service has been dialled using the phone? Is it a simple matter of including a two way on/off switch to couple/decouple the circuit from the line?

[OscarFoxtrot]

Originally Posted by WaveyDipole

I also was thinking about how to hold the line open? Would simply connecting the transformer circuit on to the line keep it open once the conference service has been dialled using the phone?

On the Rane circuit R1 180 ohm across the line holds the line open.

I would suggest adding a line-powered LED across the line as a reminder that the line's in use, otherwise someone might leave the interface holding the line and come back a week later to a rather large phone bill.

Lots of circuits online including
http://www.zen22142.zen.co.uk/Circuits/Misc/tiuc.htm
which would connect permanently across the line.

For filtering consider a graphic equalizer, effects send from the mixer desk, or band-pass filter. The hearing loop system should ideally be speech filtered and compressed and probably give the best source, if it has an aux send.

Incidentally, you might also try the Blue Room theatrical tech forums, they have knowledgeable people on there for event audio.

[WaveyDipole]

I had wondered whether the 180 ohm resistor was for that purpose as it does not appear to be commented on in the linked page. However I just discovered that it is commendted on in the PDF document that is linked from that page. I had also considered including an indicator LED. Although the equipment is usually powered down after the session and power to the interface would be off, the switch would still hold the line open if it were left in the online position. As you say, the result could be a rather large bill. I'm thinking another option would be to use a relay which keeps the line open while energized and would automatically drop it when the interface was powered down. Power for the indicator LED could be tapped from the circuit that enerizes the relay. This would not change when the received was lifted as per the circuit in your link but would be sufficient to indicate that the interface was online.

[WaveyDipole]

I've been slowly working on a circuit design for my interface and this is the effort so far. I set up the input and output OP amps to have a gain of 2 to provide some control over the input and output levels and compensate for any losses. In practice these will probably be set up using presets rather than having front panel controls. As it is a full duplex circuit, balancing of input vs output is provided by IC3/C. I have also included a headphone amp for monitoring purposes. I am considering adding and unbalanced to balanced output and possibly and input, but since I will be building this on veroboard or similar, I'm not sure whether space will permit and it is probably not essential as apart from the telephone wire, the cable runs will be relatively short.

One area where I am a little unsure is whether to use inverting or non-inverting inputs on the OP amps. I have spent some considerable time researching this and have looked over plenty of example circuits and am still confused over which way to go. In any case, I have now reviewed and revised the circuit several times and think I am just about ready to try and build it. However I would appreciate another fresh pair of eyes to have a look over it in case I have overlooked something or done something silly!

Incidentally, does it cost much to get someone to print a proper circuit board for it? I have heard that software exists that will automatically create the circuit board design?

There are lots of 'PCB Pool' outfits, some give PCB software away too (they only give output in their format, fair enough). I have yet to meet an automatic PCB designer program though. Yours looks simple enough to be built 'dead bug' style if size isn't a factor. Use one of the veroboard design tools that can be had for a download. Or make it on spot board (vero with only dots of PCB and holes) and 'draw' it in components and wire, quite satisfying.

[AndiiT]

Originally Posted by WaveyDipole

....in case I have overlooked something...

Hi,

With reference to the power supply section, shouldn't the mains transformer be centre tapped to allow the use of +ve and -ve supply rails, or is it just the way that your software has drawn the circuit?

Regards

Andrew