Brickwall Oversampling Filter

toneburst · 2016-05-06 08:42:34 UTC

Excuse the naive question, but could anyone point me in the direction of a relatively simple filter function that could be applied to an oversampled oscillator before downsampling back to 48kHz?

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johannes · 2016-05-06 09:29:03 UTC

I think "Cascaded integrator comb" filters are a good candidate for oversampled oscillators, very efficient, but also quite cryptic, as they can only be implemented in integer math and make deliberate use of overflow.
Here's an implementation

github.com

cHemingway/CM4_CIC_Filter/blob/master/cic.c

#include "cic.h"
#include <stddef.h>
#include <string.h>


int cic_decimate_init_q15(cic_decimate_instance_q15 *S, uint16_t M, uint8_t N, q32_t *pState, uint32_t blockSize) {

	if((S==NULL) || (pState==NULL)) { /* Check for null pointers */
		return -1;
	}

	if (blockSize%M != 0) { /* Block size is not a multiple of M */
		return -1;
	}

	S->M = M;
	S->N = N;
	S->pState = pState;
	return 0; /* Success */
}

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but I suggest reading some theory too before trying
http://www.dspguru.com/sites/dspguru/files/cic.pdf
If you implement something, I'd suggest to validate it with a sine oscillator, and verify the stop-band attenuation.

toneburst · 2016-05-06 10:14:12 UTC

Thank you very much, Johannes. I will do a bit of reading.

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