Cheap Highpass Filter? (just add/subtract and bitshifts)

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Dwedit
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Cheap Highpass Filter? (just add/subtract and bitshifts)

Post by Dwedit »

I remember reading about a really cheap Highpass/DC cancellation filter that used only addition/subtraction and bit shifts. But I can't remember what the formula was, or where I heard about it from. I think emulators used this formula. Does anyone know it?
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lidnariq
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Re: Cheap Highpass Filter? (just add/subtract and bitshifts)

Post by lidnariq »

All digital filter design is relative to the sample rate, so I'd need more details before I could construct something similar to whatever you're thinking of...
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Dwedit
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Re: Cheap Highpass Filter? (just add/subtract and bitshifts)

Post by Dwedit »

Audio, like 44 or 48KHz.
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lidnariq
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Re: Cheap Highpass Filter? (just add/subtract and bitshifts)

Post by lidnariq »

You could do a really lazy first-order IIR highpass with something like
out[n] = out[n-1]*(1024-1) + in[n]/1024
that's a lowpass, one second while I do more math


Oh, man, it's been waaaay too long since I last remembered this.

Simple IIR highpass:
out[n] = out[n-1]*(1024-1)/1024 + in[n] - in[n-1]

End up with a corner frequency of [sample rate ÷ 1024]
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Dwedit
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Re: Cheap Highpass Filter? (just add/subtract and bitshifts)

Post by Dwedit »

Can the constant 1024 be doubled to cut the corner frequency in half?
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lidnariq
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Re: Cheap Highpass Filter? (just add/subtract and bitshifts)

Post by lidnariq »

Sure.
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Ben Boldt
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Re: Cheap Highpass Filter? (just add/subtract and bitshifts)

Post by Ben Boldt »

Do you see how the example's constant translates to a shift Dwedit?

out[n-1]*(1024-1)/1024

Is the same as:
out[n-1]-(out[n-1] >> 10)


Doubling the constant to 2048 would turn into:

out[n-1]*(2048-1)/2048
out[n-1]-(out[n-1] >> 11)
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Re: Cheap Highpass Filter? (just add/subtract and bitshifts)

Post by Zepper »

Dwedit wrote: Sat May 29, 2021 7:18 pm I remember reading about a really cheap Highpass/DC cancellation filter that used only addition/subtraction and bit shifts. But I can't remember what the formula was, or where I heard about it from. I think emulators used this formula. Does anyone know it?
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