Hi there!
I have a design like a telephone but when I use it in full duplex, it produces echoes and howling..Can anyone help me or suggest what IC will I add to the design to make it possible in full duplex.

 

And why is it urgent? Are you receiving a call today afternoon and want to be ready?

 

I have a design like a telephone but when I use it in full duplex, it produces echoes and howling

A schematic would give some vital information.

 

Hi please see the block diagram..I think I should add a handfree IC.. What are your suggestions?? thanks

 

Very expensive full duplex speakerphones like Polycom use a DSP group of ICs and some very complicated programming for them to try to cancel received sounds from being re-transmitted and to try to cancel the local mic from coming from the local speaker.
It isn't perfect since conditions change in the room when the door might be open or closed and a large number of people absorb sounds but a few people allow many echoes in the room. Sometimes people come and go during a teleconference and doors open and close so the DSP must make changes to its model of the room during the teleconference.

The full-duplex speakerphone needs to be "trained" to the room by having it produce pink noise for a few seconds. When it is training it makes a model of the acoustics of the room and of the delays of the telephone line.

Full duplex speakerphones are not perfect so they cheat and use a little voice switching which reduces the speaker volume when a local person is speaking and reduces the mic gain when the speaker is producing sounds from the distant end.

I always laughed whenever the DSP got confused and produced "weird sounds".

Do you know that when you transmit into a telephone line then some of your sound comes back and is received? You need a "hybrid transformer" or special circuit to reduce it so it doesn't cause acoustical feedback howling in the room. The DSP echo canceller does not make a good hybrid transformer.

 

Couldn't help it but you have definitely scared the OP this time Guru

 

If you need a full duplex system to work over a standard telephone line, then you will need to check out the DSP concepts that Audioguru mentions.

If, however, you are simply needing to connect two locations with a full duplex audio system, then a 3rd wire can make life a lot easier!

Check out some of the comments in this thread http://forum.allaboutcircuits.com/showthread.php?t=42632

 

I built a voice-switching speakerphone using Motorola's MC34018 IC in about 1981.
It worked pretty well and sounded good (because I made it boost frequencies around 3.5kHz) but it was not full duplex.

Northern Telecom developed a similar voice-switching IC that they used in their speakerphone. Then another speakerphone manufacturer bought used Northern Telecom speakerphones and used the IC in their different-looking and more expensive speakerphones.

I worked with the manufacturer that stole Northern Telecom ICs and with two full-duplex digital teleconferencing manufacturers. I also worked with Polycom full duplex digital speakerphones.

Today the Cirrus digital full-duplex speakerphone IC can be used.
www.cirrus.com/cn/pubs/proDatasheet/CS6422_F1.pdf
It occasionally slips into half-duplex when the DSP becomes confused.

I hope the OP is not scared.

 

Hi Audioguru,

Thanks for your comment but can you suggest another technique or IC, because CS6422 is expensive.. Thanks

 

Before Cirus made their speakerphone IC I worked with two manufacturers who made extremely expensive digital full-duplex tele-conferencing systems. I also worked with Polycom who makes expensive full-duplex speakerphones.
They used a microprocessor, separate ROM and RAM memory and a very complicated program.

Did you read the datasheet for the Cirus IC to see how complicated it is to make a full-duplex speakerphone? Of course it is not cheap.

The Motorola (Freescale) ICs are much less expensive and are used in "cheap" voice-switching speakerphones.

 

Hi Audioguru,
Thank you very much for your advice,, Do you have a reference schematic using motorola chip for handsfree?

 

A good schematic using any IC is in its datasheet. Semiconductor manufacturers also have application bulletins for their ICs.

I made my speakerphone 30 years ago so its IC is obsolete and not available today.
Motorola (Freescale) make at least two other speakerphone ICs today.