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Old 15th August 2002, 08:06 PM   #1
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Question Switched capacitor filters

I'm curious if anybody has tried using switched capacitor filters for audio like the MF10? I'm in progress of making an audio system consisting of these speakers:
4 x 3 way (left/right front/rear) and 2 subwoofers (I have a soundblaster live with 4 outputs for surround sound).
All speakers are active active (yes, 3 monoamplifiers per speaker). The preamplifier is designed to filter all audio signals for the amplifiers and is controlled via an atmel microprocessor.
I just want to know if anybody has any tips to share with me?

btw: if you're interested in the schematics for this project: mail me and I will send them to you. If too many people want them, I will post them.

Thanks!
P.
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Old 15th August 2002, 09:00 PM   #2
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Default MF10

Hi
I just worked with the MF10 for a school project two months or so ago. The filter works with the "State var" filter type

After trying different setups of the device, I tried to make nutch filter (Band stop). It worked nicely, but the clockfreq of the system was very noticable on the output.

I would not use it for audio, build your active filters using opamps.

\Jens
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Old 15th August 2002, 09:37 PM   #3
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I wouldn't use them for high quality audio either.
They look attractive, but the clock bleed through is
totally unnacceptable.
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Old 15th August 2002, 10:16 PM   #4
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Ok, then what about this: The MF10's clock ratio can be set from 50:1 to 100:1. Suppose I want a low pass filter at 150Hz with clock ratio set to 1:100. That means the clock runs at 15Khz. If I then use an opamp filter at the output stage with it's -3dB point at let's say 1Khz, then where is the problem? The clock can't get through because it runs at a frequency 15x higher than the analogue lowpass filter. Next I use another 1Khz lowpass filter (for ease) at the input to ensure the input signal's frequency can't rise beyond half the samplerate to avoid aliasing.
Now I have a very good lowpass filter that is clock tunable without clock feedthrough or aliasing effects.
Am I right? Or am I missing something? This was the original setup I have for the amplifiers...
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Old 15th August 2002, 10:46 PM   #5
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Default Outputfilter

How high order filter will you use in the outputfilter ?

I think a first order gives you a osc feedthrough at about -20 dB

I still think it's a bad idear using a Switch Cap filter

\Jens
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Old 16th August 2002, 01:56 AM   #6
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Default Clock feedthrough

You can tune an additional LP filter by using a phase-lock-loop as a frequency multiplier -- or multiply the filter f3 point by 10e4 and divide by 100 twice
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Old 16th August 2002, 08:22 AM   #7
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hmm, explaion that a little more for me. I don't understand why the multiplication and the division?
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Old 16th August 2002, 12:29 PM   #8
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Default Multiply or divide

Let's say you are using an LTC1063 -- for a 200 Hz cutoff point you have to clock the filter at 20kHz. There's about 50uV of 20KHz clock feedthrough, however, so you would need to filter this at 100 * 20kHz -- you could cascade a couple decade counters to generate both frequencies, i.e. start with a xtal oscillator at 2MHz, divide down by 10 for 200kZ and 10 again for 20kHz.

Let's say you just have a 20kHz source, however, if you use a divide by 10 counter in a 4046 CMOS phase-lock-loop chip you multiply the output of the 4046 by 10.

Besides the clocking noise, there's other noise associated with these chips, but I find it a great device to work with, and as always, Linear's tech support is excellent.
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Old 16th August 2002, 02:39 PM   #9
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Nice aproach, but if you say that the switched cap filters have more noise than traditional opamp filters, I going for the first aproach to get as little noise as possible.
But the ltc1063 is a butterworth filter? I was taught that the only filters suitable for audio are the bessel filters because of it's constant delay? What about that? Does lineartech have clock tunable filters that you think are suited for audio use?
Or do you have another approach for my setup? I just want to be able to change cutoff frequencies with a microcontroller. Switched cap filters where the first I thaught of, but maybe there is more?

Thanks!
P.
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Old 16th August 2002, 03:24 PM   #10
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Default Analog and Digital Filters

If you want go analog, do it with a microprocessor cotrolling the digital potentiometers from Dallas (now Maxim), Analog Devices, Microchip etc. I was thinking to gang together several and writing a program to change the resistors on the fly. Bessel, Butterworth, Chebyshev -- whatever you want, knock yourself out. Figure that a 10K digital pot has about 39.1 ohms resolving.

The noise of some (not all) of these devices really gets down to the basement of the measuring equipment I happen to own.
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