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

[PatChang]

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.

 

[IRstuff]

?? If this is coming from an MP3, why not process the signal digitally?


As for DC bias, that cannot come from a filter; you need a separate thing like a summing amp.

TTFN
faq731-376

 

[PatChang]

Well I want the final product to be able to take in any audio signal regardless of the source and analyze it in real time so on-board source signal processing is not going to be an option. As far as DC biasing is concerned, I am currently utilizing a differential amplifier to accomplish this and that is working as of now, but I am having trouble achieving the gain, bias, and filtering combination.

Here is the schematic of my low-pass filter which is off-phase and not properly amplified. Also there is a little bit of noise:

Schematic

AC analysis

DC transient response

Any suggestions on how to get rid of the noise and bring the output in phase with the input signal?

 

[Skogsgurra]

Your low-pass filter doesn't work. The impedance at the inverting input of the opamp is zero. You need at least a T network. Better still a multi-pole filter. Read up on basic opamp circuits and analogue filter design. You are not on designer level. You need lots and lots of more education. You cannot get it here. It needs some work from you.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[PatChang]

Thanks for your response. I am still currently in school and am posting this question to gain insight and ask those more knowledgeable than myself in addition to asking my professors. I have tried reading up on opamp circuits and analogue filter design but have been unable to fully understand it. Can you point me to any sources you consider appropriate for such information?

 

[jimkirk]

Chip manufacturers have extensive libraries of data sheets, white papers and such. Most also have evaluation boards for particular chips so you can get some hands on experience. Here's an example paper from Texas Instruments on op amp basics. Have fun!

http://www.ti.com/lit/an/slod006b/slod006b.pdf

 

[PatChang]

Wow this is a very informative pdf! Thanks a lot, this is what I was looking for. How would I go about trying to find more of these papers from chip manufacturers? What should I search for?

 

[VEBill]

Adding the keyword PDF to a Google search can be very helpful in bringing up more informative results.

Other random hints:

You can often employ a little audio transformer if you need to increase voltage into an input with reasonably high input Z. But if your peak is only a single bit away from full scale, then don't bother. It's call headroom.

Biasing an input requires at most a capacitor and two resistors.

Assuming the processor has any processing horsepower at all, everything else can be done in software. Combined with the above hints, there's no need for any opamps at all... ...Not a good approach if this is in support of a class about opamps.

Student homework posting is generally not permitted.