From f0c4b0fc9879c893c2ef2ed3a641b24b5d81e687 Mon Sep 17 00:00:00 2001
From: Jared Boone <jboone@earfeast.com>
Date: Mon, 29 Aug 2016 20:26:39 -0700
Subject: [PATCH] AIS: Doesn't use RRC filter -- use rect instead.

---
 firmware/baseband/proc_ais.hpp   |  2 +-
 firmware/common/ais_baseband.hpp | 22 +++++++---------------
 2 files changed, 8 insertions(+), 16 deletions(-)

diff --git a/firmware/baseband/proc_ais.hpp b/firmware/baseband/proc_ais.hpp
index eff71b4a..97e813ff 100644
--- a/firmware/baseband/proc_ais.hpp
+++ b/firmware/baseband/proc_ais.hpp
@@ -62,7 +62,7 @@ private:
 
 	dsp::decimate::FIRC8xR16x24FS4Decim8 decim_0;
 	dsp::decimate::FIRC16xR16x32Decim8 decim_1;
-	dsp::matched_filter::MatchedFilter mf { baseband::ais::rrc_taps_38k4_4t_p, 2 };
+	dsp::matched_filter::MatchedFilter mf { baseband::ais::square_taps_38k4_1t_p, 2 };
 
 	clock_recovery::ClockRecovery<clock_recovery::FixedErrorFilter> clock_recovery {
 		19200, 9600, { 0.0555f },
diff --git a/firmware/common/ais_baseband.hpp b/firmware/common/ais_baseband.hpp
index 1d7a2aa6..f8bbc0d8 100644
--- a/firmware/common/ais_baseband.hpp
+++ b/firmware/common/ais_baseband.hpp
@@ -32,21 +32,13 @@
 namespace baseband {
 namespace ais {
 
-// RRC length should be about 4 x the symbol length (4T) to do a good job of
-// cleaning up ISI.
-
-// Translate+RRC filter
-// sample=38.4k, deviation=2400, b=0.5, symbol=9600
-// Length: 16 taps, 4 symbols, 1 cycles of sinusoid
-constexpr std::array<std::complex<float>, 16> rrc_taps_38k4_4t_p { {
-	{  1.0619794019e-02f,  0.0000000000e+00f }, {  3.5758705854e-03f,  1.4811740938e-03f },
-	{ -1.3274742629e-02f, -1.3274742629e-02f }, { -1.5018657262e-02f, -3.6258246051e-02f },
-	{  0.0000000000e+00f, -2.6549484581e-02f }, { -1.5018657262e-02f,  3.6258246051e-02f },
-	{ -1.0237997393e-01f,  1.0237997393e-01f }, { -2.2527985355e-01f,  9.3313970669e-02f },
-	{ -2.8440842032e-01f,  0.0000000000e+00f }, { -2.2527985355e-01f, -9.3313970669e-02f },
-	{ -1.0237997393e-01f, -1.0237997393e-01f }, { -1.5018657262e-02f, -3.6258246051e-02f },
-	{  0.0000000000e+00f,  2.6549484581e-02f }, { -1.5018657262e-02f,  3.6258246051e-02f },
-	{ -1.3274742629e-02f,  1.3274742629e-02f }, {  3.5758705854e-03f, -1.4811740938e-03f },
+// Translate+Rectangular window filter
+// sample=38.4k, deviation=2400, symbol=9600
+// Length: 4 taps, 1 symbol, 1/4 cycle of sinusoid
+// Gain: 1.0 (sinusoid / len(taps))
+constexpr std::array<std::complex<float>, 4> square_taps_38k4_1t_p { {
+	{ 0.25000000f, 0.00000000f }, { 0.23096988f, 0.09567086f },
+	{ 0.17677670f, 0.17677670f }, { 0.09567086f, 0.23096988f },
 } };
 
 } /* namespace ais */