Industrial intercom

[mingyee]

Hi, I'm having a problem where I'm doing a full duplex voice communication. Both the mic & spk are placed in a same chasis (stainless steel casing for industrial application). Once they are fixed into the casing it has feedback noise. I have already put a layer of fibre wool in the casing & also behind the speaker. The feedback noise is still there. I've also tried putting a 0.1uF cap across the speaker. It still doesn't help. What can be done? I've attached my schematic.Thanks.

 

[Russlk]

You have to have a transmit/recieve switch. When transmitting, the speaker is not working and vice-versa. The other alternative is a telephone handset. There is enough isolation to reduce feedback. Another possibility is to make a VOX circuit which will kill the mike when the sound is coming from the speaker. It may be possible to feed some of the speaker signal to the mike input 180 degrees out of phase to kill the feedback.

 

[audioguru]

Full-duplex speakerphones use DSP close-distance and far-distance echo-cancellation to cancel local and far acoustical feedback.

 

[mingyee]

It may be possible to feed some of the speaker signal to the mike input 180 degrees out of phase to kill the feedback.

Could you please tell me how do i do the phase shift to kill the feedback?

 

[mingyee]

Full-duplex speakerphones use DSP close-distance and far-distance echo-cancellation to cancel local and far acoustical feedback.

Besides dsp, are there any idea using analog method?

 

[audioguru]

The phase of a microphone and a speaker varies all over the place due to resonances and reflections.
Until DSP came along, intercoms used manual or voice-switching for half-duplex to stop acoustical feedback.
DSP trains to the phase relationships and cancels feedback very well.

 

[mingyee]

The phase of a microphone and a speaker varies all over the place due to resonances and reflections.
Until DSP came along, intercoms used manual or voice-switching for half-duplex to stop acoustical feedback.
DSP trains to the phase relationships and cancels feedback very well.

I guess it's too late for me to change as I've already came out with the pcb. I'm just thinking are there any solution in the analog way. I would be keen to change to DSP the next round.

 

[audioguru]

You can try severely chopping off the low audio frequencies where most of the speaker's phase-shift occurs, and severely chopping off the high audio frequencies where the distance between the mic and the speaker causes a phase-shift, then try cancelling the mic's pickup of the output of the speaker with a variable phase-shift circuit.

Next design, refer to this White Paper description of the functions of an Acoustic Echo Canceller that is done in DSP:
**broken link removed**

 

[mingyee]

You can try severely chopping off the low audio frequencies where most of the speaker's phase-shift occurs, and severely chopping off the high audio frequencies where the distance between the mic and the speaker causes a phase-shift, then try cancelling the mic's pickup of the output of the speaker with a variable phase-shift circuit.

Thanks a lot for the info, but do you have any phaseshifting example? or where can i look for them? thanks.

 

[haerifar]

it is very very simple you used same ground for spk and mic and it is not right schematic ...
if you want cancel this feedback you should have two seprate ground .
or you can use hybrid .
or use low frequency for modulation because mic and spk are the same band.

 

[mingyee]

it is very very simple you used same ground for spk and mic and it is not right schematic ...
if you want cancel this feedback you should have two seprate ground .
or you can use hybrid .
or use low frequency for modulation because mic and spk are the same band.

Hi may i know any recommendation of books or info on the web to read up about this? thanks.