Feedback,interference and humming sound problem in audio system

Discussion in 'General Electronics Chat' started by sadatan, Sep 9, 2010.

  1. sadatan

    Thread Starter New Member

    Feb 24, 2010
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    Iam using drv134 and INA137 as transmitter and reciever and and tpa6211a1 for amplificaton of recieved sound tl072 for preamp. please suggest me some solutionns for feedback noise which is coming when iam connecting speaker on second side,please help me out.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    We really need to see an accurate schematic to have some idea of the problem.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    Acoustical feedback occurs when the mic hears the speaker and the sound then goes around and around. Each time the sound goes around it is amplified which makes the feedback worse.

    Simply turn down the volume of the speaker's amplifier, point the mic away from the speaker or provide more distance between the mic and speaker.

    If the mic and/or speaker are poor quality then there will be a resonant frequency that will produce feedback. There might be a resonance at the mic's location in the room which is fixed by moving the mic to another location in the room.
     
  4. sadatan

    Thread Starter New Member

    Feb 24, 2010
    15
    0
    Iam using drv134 and INA137 as transmitter and reciever and and tpa6211a1 for amplificaton of recieved sound tl072 for preamp. please suggest me some solutionns for feedback noise which is coming when iam connecting speaker on second side,please help me out.
    [​IMG] [​IMG]
     
  5. sadatan

    Thread Starter New Member

    Feb 24, 2010
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    actually the feedback noise starts when i connect second side speaker one way communication is perfect as soon as i connect speaker on another side it starts vitling sound and some noise.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    Does the acoustical feedback noise go away when you turn off the microphone?
     
  7. marshallf3

    Well-Known Member

    Jul 26, 2010
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  8. Audioguru

    New Member

    Dec 20, 2007
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    He says he has a transmitter and a receiver but the parts are just differential amplifiers.
    Before I looked at their datasheetrs I thought they were a wireless microphone (transmitter) and a radio (receiver) for it.

    He has a low-noise opamp for amplifying received sound (He says it is a preamp and he talks about communication) so I assumed the communication was from a microphone.

    He talks about feedback noise so I assumed it is acoustical feedback.
     
  9. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I would have assumed the same thing but with numbers like that I had no clue.
     
  10. sadatan

    Thread Starter New Member

    Feb 24, 2010
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    Actually iam communicating between two similar board having 1 trans. and 1 recieve section through cat5 cable transmitter section is having MIC->Preamp->Differential transmitter and receive section have Differential receiver->Amplifier ->Speaker .Now when i put only 1 speaker its works fine and as soon as i connect speaker on other side it stars giving whistle sound. one more thing i noticed as soon as i increase the the distance between mic and speaker it goes away but problem is that i have to keep them close as the requirement is.please tell me how i should kill this feedback noise even keeping the speaker and mic closer.am using drv134 and INA137 as transmitter and reciever and and tpa6211a1 for amplificaton of recieved sound tl072 for preamp.
     
  11. sadatan

    Thread Starter New Member

    Feb 24, 2010
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    Is it a problem of pick up in the mic should i need to do echo cancellation,if so can some one suggest me a simple ic for echo cancellation or some simple op-amp circuitry for the same.
     
  12. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Oh, those "audio/video over CAT5" adapters.

    First thing I know about them is that the CAT5 needs to be wired exactly as CAT5 is connected when used in networking so the pairs end up on the right pins.

    Second thing I know about them is they require healthy input signals.

    please tell me how i should kill this feedback noise even keeping the speaker and mic closer.

    You really can't run a microphone near a speaker that recreates it's input, you come up with an endless loop thus the feedback. Some microphones are more directional than others and just as with any PA or other audio situation the microphone needs to be kept at a reasonable distance from the speaker. There are sophisticated mixers used by bands in stage settings that help to reduce the effect through phase compensation but it's all hit/miss and requires a lot of experimentation in setup and operation. Even if you do find a point where feedback is cancelled out merely moving the mike a few inches can put you right back where you started.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    Since the feedback whistling sound goes away when you increase the distance between the mic and the speaker then it IS acoustical feedback. Acoustical feedback is normal when the mic and speaker are close together and the gain is high. It is also normal for the feedback to return when you add a second speaker because then the mic can hear the second speaker.

    Why are the mic and speaker too close together?
    Why is the gain so high that you have acoustical feedback?

    A speakerphone or intercom has the mic and speaker close together. It eliminates acoustical feedback by using a simple "voice switch" or a complicated digital echo canceller circuit.
    The voice switch turns off the speaker when the microphone hears a sound. Then it turns off the mic when a signal comes from the other end of the telephone line.
    The digital echo canceller circuit makes a model of the phases and delays in the path between the mic and speaker then cancels it from the speaker's signal.

    You forgot to tell us the function of your circuit. Is it a full duplex intercom?
    Here is a full duplex intercom that cancels the signal from going around and around. It uses shielded audio cable but it might work with your differential transmitter and receiver circuits without the shield. The complete project is here: http://www.redcircuits.com/Page78.htm
     
  14. sadatan

    Thread Starter New Member

    Feb 24, 2010
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    Thanks for the help,it clears many queries .
    Our system is Full duplex,now i just want to know can we do this feedback cancellation using OP-amp circuit (as we are using Tl072 for making Pre-amp).
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    You cannot cancel acoustical feedback with an opamp. The phase of a speaker is different at different frequencies and the microphone is the same. The surroundings reflect sounds and additionally mess with the phasing. So many frequencies will be in-phase and cause acoustical feedback.

    I used "feedback reducers" in boardrooms. One used a fairly complicated circuit that digitized the sounds then clocked out the data at a different rate. Then it converted and filtered the data into audio but at a slightly different frequency. It reduced acoustical feedback a little and sounded weird as the end of every sound (or spoken word) had its frequency sliding up or down while the sounds went around and around the loop.
    Another frequency reducer used a fairly complicated circuit that detected feedback and notched away that frequency. Then other frequencies caused feedback and it notched them away. All that was left were a few frequencies that had opposite phasing and sounded low level and weird. It was almost useless.

    A digital echo canceller uses a DSP IC, a microprocessor, memory and a complicated program. Speakerphone manufacturers like Polycom (look in Google) use a custom-made IC to do it and their own program.
    Frequently they cheat and combine a voice-activated-switch that cuts the microphone when there is sound in the speaker and cuts the speaker when there is sound in the microphone (half-duplex).
     
  16. sadatan

    Thread Starter New Member

    Feb 24, 2010
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    Thanks for the help.
    Now as have inserted a bandpass filter of 300-3khz frequency for clearing the speech between mic and preamp.now speech is quite clear but the feedback problem still persists which i thought could have reduced as i am only passing the speech frequencies,but at the time of feedback noise when i measured the frequency on the signal it has i feedback signal of around 1.2khz frequency.Please suggest me something to eliminate this feedback either by using some digital solution or some feedback eliminator.Can i do this using DSPIC? if yes please give me some reference circuit .Is it possible to do this using only DSPIC?
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    You must determine if the 1.2kHz frequency is caused by a peak at that frequency by the speaker, by the microphone or by a resonant chamber near the speaker or mic.
    A notch filter tuned to 1.2kHz in the speaker or in the mic amplifier will reduce the problem.

    Acoustical feedback howling is eliminated by simply turning down the gain of the speaker amp or the mic amp, or by separating the mic away from the speaker.
     
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  18. rogs

    Active Member

    Aug 28, 2009
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    There's a sketch for an opamp version of the idea that Audio Guru suggested in post #13, attached to this post from an earlier thread.

    Only one end shown. You need the same circuitry both ends.

    Unlike the shielded cable that Audio Guru mentions as being important when using the circuit he linked to, you can use Cat 5 cable here, because the audio line is transformer coupled, and thus effectively differential.

    Just an ideas sketch, but I have built it, so I know it works.
     
  19. sadatan

    Thread Starter New Member

    Feb 24, 2010
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    Actually i tested it further with different mics,but now i found that the frequency of feedback changes with the type and the position of the MIC.Please suggest some solution.Also please share some schematic of circuit using DSPIC33 if we can solve this problem using that.
     
  20. rogs

    Active Member

    Aug 28, 2009
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    Anything that changes within the feedback 'loop' - positon of mic and speaker - reflections from adjacent objects - is likely to change the dominate resonant frequency, and so the pitch of the 'howl round'. Filter out one frequency, change anything at all, and another takes it's place- and so on until you have virtually nothing left!

    The ciricuit I posted in the post above is a simple way of dealing with the problem, using opamps, as you requested earlier.

    The DSP approach is going to be a much more complex (and expensive) solution - as already mentioned by Audioguru.
    You're not likely to get any fully working schematics for that, on a hobby forum.

    You might like to have a look at this to give you some ideas of the problems and solutions for full duplex echo cancellation, over a single pair telephone cable.

    If you have access to the extra cable cores - which with Cat5 you do have -then I would suggest the opamp solution, similar to the one I illustrated, is a much simpler answer.

    Edit: Thinking about it further, as you have 4 pairs available with a CAT 5 cable, it would make sense to use separate audio pairs for each direction.
    This would allow the null attenuation depth of the mic /speaker combination at each end to not to be limited by the phase shift introduced by the transformer/ capacitive coupling of the line. Probably much more effective as a feedback eliminator.
    Still full duplex balanced audio, but with better feedback rejection, and no transformers.
    I can sketch something out, if it would be of any interest?

    Further edit: If this is an intercom system we are talking about here, then DRV134 and INA 137 are overkill for line drivers and receivers. Just use TLO72s for everything. Mic preamp, mixer, line drivers and receivers. Just use something like an LM386 as a power amp, and you'll be good to go! much simpler (and cheaper!)
     
    Last edited: Jun 7, 2011
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