Optimizations for FFT DSP Process STM32H730 (CORDIC, ETC)

[EPala.2]

Hi, I am working on a project doing realtime processing of FFT information on an audio signal on the STM32H730. In order to get better realtime performance I would like to optimize this process as much as possible.

What would you recommend to optimize the following code block? Is there anywhere I can replace math operations with CMSIS or CORDIC functions, and/or just write something to be more computationally efficient? 

 

 

 float32_t wrap_phase(float32_t phase_in){
 	if (phase_in >= 0){ return fmodf(phase_in + M_PI, 2.0 * M_PI) - M_PI; }
 	else{ return fmodf(phase_in - M_PI, -2.0 * M_PI) + M_PI; }
 }

 void pv_analyze(void){
 	for(int i = 0; i < fft_size; ++i){
 		fft_in[i] = input_ring[(i + index) % fft_size] * window[i];
 	}
 	arm_rfft_fast_f32(&fftHandler, fft_in, fft_out, 0);

 	for(int i = 0; i < fft_size; i+=2){
 		float32_t amp = sqrtf((fft_out[i] * fft_out[i]) + (fft_out[i+1] * fft_out[i+1]));
 		float32_t phase = atan2f(fft_out[i+1], fft_out[i]);

 		float32_t phase_diff = phase - last_phase_in[i]; //no divide for simplicity

 		float32_t bin_center_freq = M_PI * (float32_t)i / (float32_t)fft_size; //(2*pi*n/fft_size) i = n * 2 for arm fft format
 		phase_diff = wrap_phase(phase_diff - bin_center_freq * (float32_t)hop_size);

 	 float32_t bin_deviation = phase_diff * (float32_t)fft_size / (float32_t)hop_size / (2.0 * M_PI);
 	 pv_in[i+1] = (float32_t)(i/2) + bin_deviation;
 	 pv_in[i] = amp;
 	 last_phase_in[i] = phase;
 	}
 }

 void pv_process(void){
 	memset(pv_out, 0, fft_size * sizeof(float32_t));
 	float32_t shift = pitch_shift.get();
 	for(int i = 0; i < fft_size; i += 2){

 		int n = i / 2;
 		int new_bin = int(n * shift + 0.5) * 2;

 		if(new_bin > 0 && new_bin < fft_size){
 			if(pv_out[new_bin] < pv_in[i]){
 				pv_out[new_bin+1] = pv_in[i+1] * shift;
 				pv_out[new_bin] = pv_in[i];
 			}
 		}
 	}
 }

 void pv_synthesize(void){
 	for(int i = 0; i < fft_size; i+=2){
 		int n = i / 2;
 		float32_t amp = pv_out[i];
 		float32_t bin_deviation = pv_out[i+1] - (float32_t)n;

 		float32_t phase_diff = bin_deviation * 2.0 * M_PI * (float32_t)hop_size / (float32_t)fft_size;

 		float32_t bin_center_freq = M_PI * (float32_t)i / (float32_t)fft_size;
 		phase_diff += bin_center_freq * (float32_t)hop_size;

 		float32_t out_phase = wrap_phase(last_phase_out[i] + phase_diff);
 		last_phase_out[i] = out_phase;

 		fft_in[i] = amp * cosf(out_phase);
 		fft_in[i+1] = amp * sinf(out_phase);
 	}
 	arm_rfft_fast_f32(&fftHandler, fft_in, fft_out, 1);
 	for(int i = 0; i < fft_size; i++){
 output_ring[(i+index)%fft_size] += fft_out[i] * window[i] * 0.5;
 }
 }

 

Please advise. Thanks!

 

[EPala.2]

Bump

[EPala.2]

Bump

[Tesla DeLorean]

If you have to keep bumping posts you either haven't targeted them properly, or the forum has little intrinsic interest in the topic..

Add content and context with additional posts.

Looks to be several things that can be pulled out of the loops that might help general efficiency. Perhaps walk the compiler output code, looking at the quality, and identifying where the MCU is spending the bulk of it's time.

Perhaps discuss with your own DSP experts the extent to which the CORDIC functions may be relevant or useful. Review any available texts on the topics, and consult with those with specific expertise in the topic area.

[EPala.2]

I agree the question can be made more specific. I can post asking about optimization for different sections one at a time. 

However, speaking as the DSP expert, knowledge of how to program an FFT pitch shifter doesn't mean I have an in depth understanding of the acceleration features on a specific piece of hardware. There is no one I have direct access to with specific expertise on CORDIC, which is why I am asking here. 

More specifically then, can CORDIC offer increased performance in the following areas:

float32_t amp = sqrtf((fft_out[i] * fft_out[i]) + (fft_out[i+1] * fft_out[i+1]));
 		float32_t phase = atan2f(fft_out[i+1], fft_out[i]);

Can the magnitude and / or phase angle calculation be sped up by using CORDIC?

 

 

[etheory]

Absolutely it can. The CORDIC has a specific function for just that purpose. But you'll need to pipeline your operations to get the best performance. The chip specific documentation from ST covers this directly. Just read the CORDIC section thoroughly.