Audio: Optimize mel filterbank computation

This check-in optimizes the mel filterbank generation function by
precomputing mel values for each FFT frequency ahead of the main
processing loop.

By calculating the psy_hz_to_mel conversions once for each required
frequency and storing these values in fb->scratch_data1, we avoid
redundant computations within the core loops for filter bank coefficient
calculation. This optimization reduces computational overhead,
especially beneficial for large FFT bin sizes.

The aim is to enhance execution efficiency of the mel filterbank
generation without altering its functional output.

Impact:
- Decreased computation load and execution time during filterbank
  creation.
- Maintained functional behavior, ensuring performance gains do not
  compromise accuracy or result integrity.

Signed-off-by: Shriram Shastry <[email protected]>

src/math/auditory/auditory.c

@@ -92,7 +92,6 @@ int psy_get_mel_filterbank(struct psy_mel_filterbank *fb)
 	int32_t slope;
 	int32_t scale = ONE_Q16;
 	int32_t scale_inv = ONE_Q16;
-	int16_t *mel;
 	int16_t mel_start;
 	int16_t mel_end;
 	int16_t mel_step;
@@ -139,7 +138,8 @@ int psy_get_mel_filterbank(struct psy_mel_filterbank *fb)
 
 	fb->scale_log2 = 0;
 
-	mel = fb->scratch_data1;
+	/* Precompute all mel values outside of the inner loop */
+	int16_t *mel = fb->scratch_data1;
 	for (i = 0; i < fb->half_fft_bins; i++) {
 		f = fb->samplerate * i / fb->fft_bins;
 		mel[i] = psy_hz_to_mel(f);