From fe49e8812238c92d316da697d24f74738cb218d9 Mon Sep 17 00:00:00 2001
From: Rob Riggs <rob@pangalactic.org>
Date: Sun, 20 Jun 2021 20:40:49 -0500
Subject: [PATCH] Update M17 modulator to use 72MHz clock for increased
 accuracy.

---
 TNC/M17Modulator.cpp |  6 +++---
 TNC/M17Modulator.h   | 18 ++++++++++++++----
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/TNC/M17Modulator.cpp b/TNC/M17Modulator.cpp
index eced929..de8cc49 100644
--- a/TNC/M17Modulator.cpp
+++ b/TNC/M17Modulator.cpp
@@ -11,10 +11,10 @@ void M17Modulator::init(const kiss::Hardware& hw)
 
     (void) hw; // unused
 
-    SysClock80();
+    SysClock72();
 
-    // Configure 80MHz clock for 48kHz.
-    htim7.Init.Period = 1665;
+    // Configure 72MHz clock for 48kHz.
+    htim7.Init.Period = 1499;
     htim7.Init.Prescaler = 0;
     if (HAL_TIM_Base_Init(&htim7) != HAL_OK)
     {
diff --git a/TNC/M17Modulator.h b/TNC/M17Modulator.h
index 5d95239..3ab6df5 100644
--- a/TNC/M17Modulator.h
+++ b/TNC/M17Modulator.h
@@ -26,7 +26,7 @@ struct M17Modulator : Modulator
     // Six buffers per M17 frame, or 12 half-buffer interrupts.
     static constexpr uint8_t UPSAMPLE = 10;
     static constexpr uint32_t BLOCKSIZE = 4;
-    static constexpr uint32_t STATE_SIZE = (m17::FILTER_TAP_NUM_15 / UPSAMPLE) + BLOCKSIZE - 1;
+    static constexpr uint32_t STATE_SIZE = (m17::FILTER_TAP_NUM_9 / UPSAMPLE) + BLOCKSIZE - 1;
     static constexpr int16_t DAC_BUFFER_LEN = 80;               // 8 symbols, 16 bits, 2 bytes.
     static constexpr int16_t TRANSFER_LEN = DAC_BUFFER_LEN / 2; // 4 symbols, 8 bits, 1 byte.
     static constexpr uint16_t VREF = 4095;
@@ -48,8 +48,8 @@ struct M17Modulator : Modulator
     : dacOutputQueueHandle_(queue), ptt_(ptt)
     {
         arm_fir_interpolate_init_f32(
-            &fir_interpolator, UPSAMPLE, m17::FILTER_TAP_NUM_15,
-            (float32_t*) m17::rrc_taps_f15.data(), fir_state.data(), BLOCKSIZE);
+            &fir_interpolator, UPSAMPLE, m17::FILTER_TAP_NUM_9,
+            (float32_t*) m17::rrc_taps_f9.data(), fir_state.data(), BLOCKSIZE);
     }
 
     ~M17Modulator() override {}
@@ -314,7 +314,7 @@ private:
             buffer[i] = adjust_level(tmp[i]);
         }
     }
-
+#if 0
     [[gnu::noinline]]
     void fill_empty(int16_t* buffer)
     {
@@ -328,6 +328,16 @@ private:
             buffer[i] = adjust_level(tmp[i]);
         }
     }
+#endif
+
+    [[gnu::noinline]]
+    void fill_empty(int16_t* buffer)
+    {
+        for (size_t i = 0; i != TRANSFER_LEN; ++i)
+        {
+            buffer[i] = 2048;
+        }
+    }
 };
 
 }} // mobilinkd::tnc