From bb35f2a0cc1235f2ab50c292ba88325b7f34a2cc Mon Sep 17 00:00:00 2001
From: Ian Scott <ian@polpo.org>
Date: Sat, 4 Apr 2026 16:22:24 -0600
Subject: [PATCH] HW interpolator for SB

---
 sw/CMakeLists.txt  |  1 +
 sw/sbdsp/sbdsp.cpp | 38 ++++++++++++++++++++++++++++++++++++--
 sw/sbdsp/sbdsp.h   |  2 ++
 3 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/sw/CMakeLists.txt b/sw/CMakeLists.txt
index a70d7aa..719b6b1 100644
--- a/sw/CMakeLists.txt
+++ b/sw/CMakeLists.txt
@@ -357,6 +357,7 @@ function(build_sb_dbopl3 TARGET_NAME MULTIFW)
         SB_BUFFERLESS_NG=1
         USE_CD_AUDIO_FIFO=1
         INTERP_VOLCTRL=1
+        INTERP_SB_LINEAR=1
     )
     target_link_libraries(${TARGET_NAME} resampler)
     target_link_libraries(${TARGET_NAME} hardware_interp)
diff --git a/sw/sbdsp/sbdsp.cpp b/sw/sbdsp/sbdsp.cpp
index f4b3a24..fbd3108 100644
--- a/sw/sbdsp/sbdsp.cpp
+++ b/sw/sbdsp/sbdsp.cpp
@@ -39,13 +39,14 @@ SPDX-License-Identifier: MIT
 extern uint LED_PIN;
 
 #include "isa/isa_dma.h"
+#ifdef INTERP_SB_LINEAR
+#include "hardware/interp.h"
+#endif
 
 #ifndef MAX
 #define MAX(x, y) (((x) > (y)) ? (x) : (y))
 #endif
 
-#define SB_RSM_FRAC 12
-
 static irq_handler_t SBDSP_DMA_isr_pt;
 static dma_inst_t dma_config;
 #define DMA_PIO_SM 2
@@ -225,6 +226,23 @@ uint32_t sbdsp_generate_sample() {
     }
 
     // interpolate sample
+#ifdef INTERP_SB_LINEAR
+    // interp0 blend: BASE0 + alpha * (BASE1 - BASE0) >> 8
+    // base01 packs older sample (low 16) and newer sample (high 16), sign-extended
+    interp0->accum[1] = sbdsp.rs.phase_acc;
+
+    // left channel: older_L in low 16, newer_L in high 16
+    interp0->base01 = (sbdsp.rs.buf[1] & 0xFFFF)
+                     | ((sbdsp.rs.buf[0] & 0xFFFF) << 16);
+    int32_t out_l = interp0->peek[1];
+
+    // right channel: older_R in low 16, newer_R in high 16
+    interp0->base01 = (sbdsp.rs.buf[1] >> 16)
+                     | (sbdsp.rs.buf[0] & 0xFFFF0000);
+    int32_t out_r = interp0->peek[1];
+
+    return (uint32_t)((out_l & 0xFFFF) | (out_r << 16));
+#else
     int32_t l0 = (int16_t)(sbdsp.rs.buf[0] & 0xFFFF);
     int32_t r0 = (int16_t)(sbdsp.rs.buf[0] >> 16);
     int32_t l1 = (int16_t)(sbdsp.rs.buf[1] & 0xFFFF);
@@ -235,6 +253,7 @@ uint32_t sbdsp_generate_sample() {
     int32_t out_r = ((r0 * phase_frac) + (r1 * ((1 << SB_RSM_FRAC) - phase_frac))) >> SB_RSM_FRAC;
 
     return (uint32_t)((out_l & 0xFFFF) | (out_r << 16));
+#endif
 }
 
 static void sbdsp_handle_time_constant(uint8_t tc) {
@@ -394,6 +413,21 @@ void sbdsp_init() {
     
     // Reset mixer
     sbmixer_reset();
+
+#ifdef INTERP_SB_LINEAR
+    // Configure interp0 blend mode for SB DSP PCM linear interpolation.
+    // Lane 0: blend=true. Lane 1: shift maps 12-bit phase fraction to 8-bit alpha.
+    {
+        interp_config cfg = interp_default_config();
+        interp_config_set_blend(&cfg, true);
+        interp_set_config(interp0, 0, &cfg);
+
+        cfg = interp_default_config();
+        interp_config_set_shift(&cfg, SB_RSM_FRAC - 8);
+        interp_config_set_signed(&cfg, true);
+        interp_set_config(interp0, 1, &cfg);
+    }
+#endif
 }
 
 
diff --git a/sw/sbdsp/sbdsp.h b/sw/sbdsp/sbdsp.h
index 5b7c358..5f9a6e0 100644
--- a/sw/sbdsp/sbdsp.h
+++ b/sw/sbdsp/sbdsp.h
@@ -94,6 +94,8 @@ typedef struct sbdsp_t {
         uint8_t b;
     } options;
 
+#define SB_RSM_FRAC 12
+
     volatile uint32_t cur_sample;  // packed stereo pair: L in low 16, R in high 16
 #if 0
     struct {