Modulating Audio for Infrared

Discussion in 'General Electronics Chat' started by Catfeesh, Oct 3, 2011.

  1. Catfeesh

    Thread Starter New Member

    Aug 31, 2011
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    I'm trying to design an audio IR transmitter & receiver for a course project, and I wanted to know what would be the best/easiest method of modulating an audio signal for transmission? So far I was looking at Pulse Width Modulation using an 555 timer. Is this a simple/effective method? Is there any better or easier method? If I used a PIC microcontroller would it be significantly more complicated?

    Any responses will be appreciated
    Thanks,
     
  2. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    you can modulate the output by using an audio transformer connected to the audio source...... I have done a similar project with my son and we used a Laser pointer to transmit the audio and a solar cell to receive it...... here is a similar projet you can take a look at >>>> http://www.instructables.com/id/Send-Music-over-a-Laser-Beam/
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,451
    3,371
    Google delta modulation.

    If you want a quick and simple way to modulate a laser pointer, put a tiny mirror on a loudspeaker and reflect the laser beam off the mirror.
     
  4. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234

    I would like to see you aim something like that at a tiny receiver on the other end of the room :) ..... he needs to modulate the actual laser output, so the differences of the light intensities can easily be decoded on the opposite end with a simple solar cell, such as the ones found on solar calculators.... which by the way will work just as well with an infrared transmitter/receiver pair.... but without the proper optics and amplifying circuitry, it will be very difficult to line the transmitter to the receiver and get good transmission...
     
  5. Catfeesh

    Thread Starter New Member

    Aug 31, 2011
    7
    0
    @BMorse

    What exactly is the purpose the audio transformer? Is it just to amplify the signal to some appropriate level for transmission? I'm confused to how it actually modulates a signal. Cheers for the site though I will have a look at it later.

    @MrChips

    I did a search and found delta modulation seems feasible. So basically is converts the analog music signal to a discrete signal using just an integrator and a comparator? (and a D-latch?). And the original signal can be reconstructed using just an integrator and a LPF? Would using Delta Sigma modulation be any more effective in this case since we're transmitting an music (high-resolution analog) signal?

    Cheers,
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,451
    3,371
    If you wish high quality audio you will have to increase your clock frequency. If this is a school project I would stay with proof-of-concept and stay with simple Delta Modulation. The encoder and decoder are fairly simple to build and implement. The sound quality is surprisingly good. The encoder can also be built using a tracking ADC.
     
  7. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    19
    I am no expert but ....
    The transformer mentioned is a 'mixer' that modifies the carrier frequency.

    You are unlikely to get simple modulation, changing the brightness of the IR source, to work well, there will be too much background interference.
    You will need a modulated source and a tuned or filtered detector.
    [Look at IR remote circuits or IRDA for background]

    RE transmission you have some options ...
    AM Amplitude modulation of a fixed frequency carrier
    FM Frequency modulation.
    The modulation, be that amplitude or frequency change, is driven by the signal you want to transmit.
    (FM is harder but usually considered superior)

    Choices of signal / data ....
    Analogue (Easier / simpler)
    Digital (Far more complex but potentially far better) [Google: SPDIF / TOSLINK / DAB]

    The first thing you need to decide is what quality you are looking for, this will determine the bandwidth, how much info you want to transmit.

    You will not build a working device with what I have offered.

    Before you can ask a proper question to seek a qualified answer from someone much clever than I am you will need to do some research, lots of it.

    Sorry I can be more help but I hope this gets you started looking in the right direction.
    Sometimes re inventing the wheel is fun, and its always educational.
    Al
     
  8. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    It is used to vary the Laser's output using Amplitude modulation, with a good laser pointer and receiver unit you can get a pretty good audio signal upto 1/2 a mile, and around 300bps throughput (as the author claims anyway, I did not see what range we got with ours since it was just for a school project demonstration for my son)

    The transmitter is pretty easy to set up, the receiving end would have to have some thought put into it if you want quality sound, or you can check the link to see what they used as a receiver, our receiver just consisted of an 8ohm PC speaker and a small 1/2 volt solar cell.....
    here is a quick schematic showing how it is connected:
    LASER AUDIO MODULATOR.png
     
    Last edited: Oct 4, 2011
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