Resistive hybrid for soundcard to enable speakerphone skype calls

[arithmetix]

I think it should be possible to use a resistive hybrid to convert from 4-wire to 2-wire, and plug an ordinary telephone network speakerphone into the hybrid to allow hands-free operation.
Has anyone any thoughts on this, or even better, experience of such a system?

 

[berkeman]

I think it should be possible to use a resistive hybrid to convert from 4-wire to 2-wire, and plug an ordinary telephone network speakerphone into the hybrid to allow hands-free operation.
Has anyone any thoughts on this, or even better, experience of such a system?

Could you say a little more about what you want to do? Are you wanting to make a full-duplex speakerphone? Where is the microphone? There are more issues with full-duplex speakerphones than just the singing problem...

 

[arithmetix]

Thank you for your reply, Berkeman.
I am thinking of using a commercially available speakerphone, designed for the telephone network, for full duplex operation. As for such issues as singing/feedback, I hope that the solutions already implemented by the engineers who designed the phone for the public telephone network will be effective.
For me, there don't seem to be any obvious issues with using the resistive hybrid to go from 4 wire to 2 wire operation, providing the hybrid has been properly balanced.
There may however be issues which I have not anticipated, which is why, in the spirit of cautious conservatism, I have posted my question. And theory has never been quite the same as practise in my experience.
(I was originally trained in telephony, and have worked in telephone transmission stations.)

 

[berkeman]

Thank you for your reply, Berkeman.
I am thinking of using a commercially available speakerphone, designed for the telephone network, for full duplex operation. As for such issues as singing/feedback, I hope that the solutions already implemented by the engineers who designed the phone for the public telephone network will be effective.
For me, there don't seem to be any obvious issues with using the resistive hybrid to go from 4 wire to 2 wire operation, providing the hybrid has been properly balanced.
There may however be issues which I have not anticipated, which is why, in the spirit of cautious conservatism, I have posted my question. And theory has never been quite the same as practise in my experience.
(I was originally trained in telephony, and have worked in telephone transmission stations.)

Standard speakerphones are not full duplex, they are half duplex. The full duplex speakerphones are designed to work in pairs, and include lots of DSP to deal with echos and singing.

I suppose you might be able to run a standard speakerphone in full duplex mode, if you knew for sure that the phone on the other end was a handset only and not a speakerphone too.

 

[arithmetix]

Aha! Shows how much experience of speakerphones I have! Thank you very much.
If you have anything else to say, I'm listening.

 

[berkeman]

I suppose you might be able to run a standard speakerphone in full duplex mode, if you knew for sure that the phone on the other end was a handset only and not a speakerphone too.

Actually, I don't think even this would work. The person with the handset would hear their echos. You really need the same DSP cooperating at both ends in order to get even one full-duplex speakerphone to work

As an aside, I was part of a focus group about 20 years ago involving speakerphones. There were about 15 people in the group, and I was probably the most technical person there (EE), and definitely knew the most about phones and telephony (used to work for Bell Labs). I hadn't heard about the work on DSP-based full duplex speakerphones, though. So when the facilitator started presenting the full duplex speakerphone to us, I politely interrupted and mentioned that speakerphones weren't full duplex because of <list of problems>. The facilitator just smiled, and said, oh yes, our speakerphones are definitely full duplex -- we've solved those problems. I asked how much they cost, and was truly astounded by how inexpensive they were for doing so much DSP (especially 20 years ago).

Amazing. Oh the company that made the speakerphones was Polycom, and I think they were a startup at that time. Doing pretty well now, I see.

 

[arithmetix]

Thank you again Berkeman.
About the only thing left of my incompletely conceived idea is the hybrid, and I'm very glad not to have had to go through the time-consuming and costly exercise of finding these things out the hard way...
... that is, unless I find there is some reason I can't use the hybrid either!