From 43f84ae9689346c33f029ef454062c94dbe297f1 Mon Sep 17 00:00:00 2001
From: Jonathan Naylor <naylorjs@yahoo.com>
Date: Mon, 29 Jun 2020 15:59:46 +0100
Subject: [PATCH] Simplify the pre-emphasis code for AX.25.

---
 AX25TX.cpp     | 20 +++++++++++---------
 AX25TX.h       |  3 ---
 SerialPort.cpp |  2 +-
 3 files changed, 12 insertions(+), 13 deletions(-)

diff --git a/AX25TX.cpp b/AX25TX.cpp
index da4a002..3296288 100644
--- a/AX25TX.cpp
+++ b/AX25TX.cpp
@@ -49,7 +49,6 @@ const q15_t AUDIO_TABLE_DATA[] = {
 };
 
 CAX25TX::CAX25TX() :
-m_twist(-6),
 m_poBuffer(),
 m_poLen(0U),
 m_poPtr(0U),
@@ -138,22 +137,25 @@ uint8_t CAX25TX::writeData(const uint8_t* data, uint16_t length)
 
 void CAX25TX::writeBit(bool b)
 {
-  q15_t in[AX25_RADIO_SYMBOL_LENGTH];
+  q15_t buffer[AX25_RADIO_SYMBOL_LENGTH];
   for (uint8_t i = 0U; i < AX25_RADIO_SYMBOL_LENGTH; i++) {
-    in[i] = AUDIO_TABLE_DATA[m_tablePtr];
-    if (b)
+    q15_t value = AUDIO_TABLE_DATA[m_tablePtr];
+
+    if (b) {
       m_tablePtr += 11U;
-    else
+    } else {
+      // De-emphasise the lower frequency by 6dB
+      value >>= 2;
       m_tablePtr += 6U;
+    }
+
+    buffer[i] = value;
 
     if (m_tablePtr >= AUDIO_TABLE_LEN)
       m_tablePtr -= AUDIO_TABLE_LEN;
   }
 
-  q15_t out[AX25_RADIO_SYMBOL_LENGTH];
-  m_twist.process(in, out, AX25_RADIO_SYMBOL_LENGTH);
-
-  io.write(STATE_AX25, out, AX25_RADIO_SYMBOL_LENGTH);
+  io.write(STATE_AX25, buffer, AX25_RADIO_SYMBOL_LENGTH);
 }
 
 void CAX25TX::setTXDelay(uint8_t delay)
diff --git a/AX25TX.h b/AX25TX.h
index 2c8eb8b..0a41757 100644
--- a/AX25TX.h
+++ b/AX25TX.h
@@ -21,8 +21,6 @@
 
 #include "Config.h"
 
-#include "AX25Twist.h"
-
 class CAX25TX {
 public:
   CAX25TX();
@@ -36,7 +34,6 @@ public:
   uint8_t getSpace() const;
 
 private:
-  CAX25Twist m_twist;
   uint8_t    m_poBuffer[600U];
   uint16_t   m_poLen;
   uint16_t   m_poPtr;
diff --git a/SerialPort.cpp b/SerialPort.cpp
index 7a81dc9..390338a 100644
--- a/SerialPort.cpp
+++ b/SerialPort.cpp
@@ -109,7 +109,7 @@ const uint8_t MMDVM_DEBUG5       = 0xF5U;
 #define	HW_TYPE	"MMDVM"
 #endif
 
-#define DESCRIPTION "20200628 (D-Star/DMR/System Fusion/P25/NXDN/POCSAG/FM/AX.25)"
+#define DESCRIPTION "20200629 (D-Star/DMR/System Fusion/P25/NXDN/POCSAG/FM/AX.25)"
 
 #if defined(GITVERSION)
 #define concat(h, a, b, c) h " " a " " b " GitID #" c ""