I'm working with a factory on a speakerphone type of product but am getting feedback. We are using full-duplex.

This is what we are seeing/hearing:

  1. User A talks into unit A
  2. Unit B's speaker plays User A's voice...HOWEVER Unit B's mic picks up the audio and sends User A's voice BACK to unit A!
  3. Unit A starts to hear the feedback!
  4. Unit B starts to get it too!

How do speakerphones do it? Do they somehow turn off the speaker (for a really short amount of time) when they detect that the mic is being used? What happens if the speaker phones are right next to each other?


Cordless speaker-phones use half Duplex so the "loudest talker" controls the direction of audio, thus preventing feedback and echo.

Skype and telephony networks use a white noise or spread spectrum signal to equalize the amplitude and phase of the distorted channel so that the active mic "Side tone" can be cancelled out in the network when making a long distance connection. this is a short test just prior to a connection being made. ( You may recall Faxes and Modems sound the same with test tones then white noise before making a data connection

But higher-end conference room phones use full duplex analog with high quality microphones so that when the impedance is balanced the energy from the mic is cancelled using a differential amplifier with phase inversion and amplitude adjustments to cancel the local signal or the received signal from being echoed back.

The conditions for oscillation or feedback howling is 360 deg or 0 deg phase shift and gain >=1 at any frequency.

  • \$\begingroup\$ Thank Tony. We are using full duplex analog technology (I believe) in a cheap consumer product. Is there a cheap way of feedback cancellation? We can not afford high quality mics. \$\endgroup\$ – milesmeow Oct 9 '16 at 19:22
  • \$\begingroup\$ have you compared the speaker signal to the mic signal and try to cancel it out ( so that gain controls to mic/speaker have min. effect) \$\endgroup\$ – Tony Stewart EE75 Oct 9 '16 at 19:32
  • \$\begingroup\$ it must be done at both ends otherwise attenuation in distance or HDX is needed \$\endgroup\$ – Tony Stewart EE75 Oct 9 '16 at 19:36
  • \$\begingroup\$ Separation in distance between mic and speaker? What is HDX? Half duplex? \$\endgroup\$ – milesmeow Oct 9 '16 at 19:39
  • \$\begingroup\$ Half DupleX .. . . \$\endgroup\$ – Tony Stewart EE75 Oct 9 '16 at 19:40

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