Audio: stft_process: Add conversion to polar format and back

This patch adds to stft_process component conversion to polar
(magnitude, angle) format for first FFT half from DC to Nyquist
frequency. The polar format is converted back to (real, imaginary)
complex and upper FFT half symmetry is applied. The magnitude domain
is commonly used for signal processing in frequency domain.

This change when enabled in Kconfig change increases load in MTL
platform from 72 MCPS to 202 MCPS with 1024 size FFT and hop of
256. Currently the build option STFT_PROCESS_MAGNITUDE_PHASE is
not set.

Signed-off-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>

Author: singalsu

src/audio/stft_process/Kconfig

@@ -1,14 +1,30 @@
 # SPDX-License-Identifier: BSD-3-Clause
 
 config COMP_STFT_PROCESS
-	tristate "Template example component"
+	tristate "STFT processing component"
 	default n
 	select MATH_FFT
 	select MATH_32BIT_FFT
 	select MATH_FFT_MULTI
 	help
-	  Select for stft_process component. Reason for existence
-	  is to provide a minimal component example and use as
-	  placeholder in processing pipelines. As example processing
-	  it swaps or reverses the channels when the switch control
-	  is enabled.
+	  Select for stft_process component. STFT acronym means
+	  short term Fourier transform. It converts audio
+	  to multiple FFTs with selected FFT size, hop, and
+	  window function. Possible signal processing can be
+	  done in it in frequency domain for FFTs that is
+	  efficient for more complex signal processing techniques.
+	  The component converts then the frequency domain
+	  version of signal back to normal PCM audio stream
+	  with inverse STFT.
+
+if COMP_STFT_PROCESS
+
+config STFT_PROCESS_MAGNITUDE_PHASE
+	bool "Convert FFTs to polar magnitude and phase"
+	default n
+	help
+	  Select for processing in polar magnitude and phase
+	  domain. Such complex values format is common for
+	  frequency domain signal processing.
+
+endif # COMP_STFT_PROCESS

src/audio/stft_process/stft_process.h

@@ -63,6 +63,7 @@ struct stft_process_buffer {
 struct stft_process_fft {
 	struct icomplex32 *fft_buf; /**< fft_padded_size */
 	struct icomplex32 *fft_out; /**< fft_padded_size */
+	struct ipolar32 *fft_polar;
 	struct fft_multi_plan *fft_plan;
 	struct fft_multi_plan *ifft_plan;
 	int fft_fill_start_idx; /**< Set to 0 for pad left, etc. */

src/audio/stft_process/stft_process_common.c

@@ -5,6 +5,7 @@
 #include <sof/audio/component.h>
 #include <sof/audio/audio_stream.h>
 #include <sof/math/auditory.h>
+#include <sof/math/icomplex32.h>
 #include <sof/math/matrix.h>
 #include <sof/math/sqrt.h>
 #include <sof/math/trig.h>
@@ -97,6 +98,36 @@ static void stft_do_ifft(struct stft_process_state *state, int ch)
 	stft_process_overlap_add_ifft_buffer(state, ch);
 }
 
+#if CONFIG_STFT_PROCESS_MAGNITUDE_PHASE
+static void stft_convert_to_polar(struct stft_process_fft *fft)
+{
+	int i;
+
+	for (i = 0; i < fft->half_fft_size; i++)
+		sofm_icomplex32_to_polar(&fft->fft_out[i], &fft->fft_polar[i]);
+}
+
+static void stft_convert_to_complex(struct stft_process_fft *fft)
+{
+	int i;
+
+	for (i = 0; i < fft->half_fft_size; i++)
+		sofm_ipolar32_to_complex(&fft->fft_polar[i], &fft->fft_out[i]);
+}
+
+static void stft_apply_fft_symmetry(struct stft_process_fft *fft)
+{
+	int i, j, k;
+
+	j = 2 * fft->half_fft_size - 2;
+	for (i = fft->half_fft_size; i < fft->fft_size; i++) {
+		k = j - i;
+		fft->fft_out[i].real = fft->fft_out[k].real;
+		fft->fft_out[i].imag = -fft->fft_out[k].imag;
+	}
+}
+#endif
+
 static void stft_do_fft_ifft(const struct processing_module *mod)
 {
 	struct stft_comp_data *cd = module_get_private_data(mod);
@@ -110,7 +141,12 @@ static void stft_do_fft_ifft(const struct processing_module *mod)
 		if (num_fft) {
 			stft_do_fft(state, ch);
 
-			/* stft_process(state) */
+#if CONFIG_STFT_PROCESS_MAGNITUDE_PHASE
+			/* Convert half-FFT to polar and back, and fix upper part */
+			stft_convert_to_polar(&state->fft);
+			stft_convert_to_complex(&state->fft);
+			stft_apply_fft_symmetry(&state->fft);
+#endif
 
 			stft_do_ifft(state, ch);
 			cd->fft_done = true;

src/audio/stft_process/stft_process_setup.c

@@ -163,6 +163,9 @@ int stft_process_setup(struct processing_module *mod, int max_frames,
 		goto free_fft_buf;
 	}
 
+	/* Share the fft_out buffer for polar format */
+	fft->fft_polar = (struct ipolar32 *)fft->fft_out;
+
 	fft->fft_fill_start_idx = 0; /* From config pad_type */
 
 	/* Setup FFT */