Audio: Multiband DRC: Change emphasis filter to iir_df1_4th()

This patch changes in multiband DRC component the emphasis and
de-emphasis IIR filters to use the optimized 4th order IIR code.

The patch for crossover already covered the bands filter bank.
This change saves additional 2 MCPS in a HiFi5 build of the
component. From 85.17 MCPS to  83.44 MCPS.

The change is not restricting configuration. The existing filters
are hard-coded to 4th order (SOF_EMP_DEEMP_BIQUADS).

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

Author: singalsu

src/audio/multiband_drc/multiband_drc.c

@@ -66,6 +66,10 @@ static int multiband_drc_eq_init_coef_ch(struct sof_eq_iir_biquad *coef,
 {
 	int ret;
 
+	/* Ensure the LR4 can be processed with the simplified 4th order IIR */
+	if (SOF_EMP_DEEMP_BIQUADS != SOF_IIR_DF1_4TH_NUM_BIQUADS)
+		return -EINVAL;
+
 	eq->coef = rzalloc(SOF_MEM_ZONE_RUNTIME, 0, SOF_MEM_CAPS_RAM,
 			   sizeof(struct sof_eq_iir_biquad) * SOF_EMP_DEEMP_BIQUADS);
 	if (!eq->coef)

src/audio/multiband_drc/multiband_drc_generic.c

@@ -39,7 +39,7 @@ static void multiband_drc_process_emp_crossover(struct multiband_drc_state *stat
 		crossover_s = &state->crossover[ch];
 
 		if (enable_emp)
-			emp_out = iir_df1(emp_s, *buf_src);
+			emp_out = iir_df1_4th(emp_s, *buf_src);
 		else
 			emp_out = *buf_src;
 
@@ -178,7 +178,7 @@ static void multiband_drc_process_deemp(struct multiband_drc_state *state,
 		}
 
 		if (enable_deemp)
-			*buf_sink = iir_df1(deemp_s, mix_out);
+			*buf_sink = iir_df1_4th(deemp_s, mix_out);
 		else
 			*buf_sink = mix_out;