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Thread: Help designing analog filter designs for an audio activated 8x8x8 LED cube

  1. #1
    Associate Engineer
    Join Date
    Dec 2012

    Help designing analog filter designs for an audio activated 8x8x8 LED cube

    I am working on designing a music visualization system using LEDs. I would like to create an analog design that uses op amps to filter, amplify, and add DC bias to an audio signal so it can be read into the A/D of a 5V microprocessor. The input signal is a +/- 1.5V volt audio signal coming from an MP3 player. From this signal I need to produce three output signals. This means that I need to design three filters (a lowpass, a bandpass, and a highpass). These filters also need to amplify and bias the input signal so it will be a 0 to 5V signal that can be read by the processor's A/D at the maximum resolution. So basically what I have pictured is to first bias the signal, then amplify it, then filter it:

    MP3 Player Audio signal --> Bias --> Amplify --> Filter --> uP

    I am familiar with building filters with amplification, but I am not sure about the correct way to go about adding a DC bias to the filters with amplification.

    Lowpass (bass range): cutoff f @ ~500 Hz
    Bandpass (mid range): center f @ ~2250 Hz
    Highpass (high range): cutoff f @ ~4kHz

    The parts I have available for use are: LT1632 dual op-amps, resistors, 100nF capacitors
    Measuring the output signal from the iPhone: V(peak-to-peak): ~2.85V centered around 0V
    Required Gain: ~1.75
    Available Power supply units: +5V and +12V DC

    I've tried designing these filters by hand, and using TI's FilterPro software, however I have not been able to achieve the DC bias affect. I have been simulating the filters using LTSpice. I have the design files for the three filters though if it would help in describing my current problem. Any help in this area would be greatly appreciated.

  2. #2
    Technical Fellow Kelly_Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    Hummm..... Out of my league!

    Anybody else?

  3. #3
    Technical Fellow
    Join Date
    Feb 2011
    Got a funny feeling someone is inventing a wheel!!

    Would this pretty much give you what you need to feed into the uP???

    Search on this...
    Velleman 3 Channel Color Organ Kit K8017

    I built things like these 40+ years back. Referred to back then as "MusiColor" of "ColorOrgan" etc. Google, Google, Google.

  4. #4
    Principle Engineer
    Join Date
    Mar 2011
    Look up "DC restorer" which will clamp the negative value of the signal to ground.
    So your -1.5 to +1.5 will output 0 to 3 to the A/D.

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