Add audio high pass filter

Gameboy.sv

@@ -538,7 +538,7 @@ wire [14:0] lcd_data;
 wire [1:0] lcd_mode;
 wire lcd_on;
 
-assign AUDIO_S = 0;
+assign AUDIO_S = 1'b1;
 
 wire reset = (RESET | status[0] | buttons[1] | cart_download | bk_loading);
 wire speed;
@@ -549,6 +549,32 @@ always @(posedge clk_sys) if(reset) begin
 	else if(cart_download) isGBC <= !filetype[7:4];
 end
 
+// Filter CE impacts frequency response
+reg [4:0] filter_cnt;
+always_ff @(posedge clk_sys) begin
+	if (ce_cpu2x)
+		filter_cnt<= filter_cnt + 1'b1;
+end
+
+wire [15:0] audio_l, audio_r;
+
+// Remove DC offset and convert to signed
+jt49_dcrm2 #(.sw(16)) dc_filter_l (
+	.clk  (clk_sys),
+	.cen  (ce_cpu2x & &filter_cnt),
+	.rst  (reset),
+	.din  (audio_l),
+	.dout (AUDIO_L)
+);
+
+jt49_dcrm2 #(.sw(16)) dc_filter_r(
+	.clk  (clk_sys),
+	.cen  (ce_cpu2x & &filter_cnt),
+	.rst  (reset),
+	.din  (audio_r),
+	.dout (AUDIO_R)
+);
+
 // the gameboy itself
 gb gb (
 	.reset	    ( reset      ),
@@ -577,8 +603,8 @@ gb gb (
 	.gbc_bios_do     ( bios_do    ),
 
 	// audio
-	.audio_l 	 ( AUDIO_L	  ),
-	.audio_r 	 ( AUDIO_R	  ),
+	.audio_l 	 ( audio_l	  ),
+	.audio_r 	 ( audio_r	  ),
 
 	// interface to the lcd
 	.lcd_clkena  ( lcd_clkena ),

files.qip

@@ -17,3 +17,5 @@ set_global_assignment -name VERILOG_FILE rtl/link.v
 set_global_assignment -name VERILOG_FILE rtl/sgb.v
 set_global_assignment -name SDC_FILE Gameboy.sdc
 set_global_assignment -name SYSTEMVERILOG_FILE Gameboy.sv
+set_global_assignment -name VERILOG_FILE jt49_dcrm2.v
+

jt49_dcrm2.v

@@ -0,0 +1,62 @@
+/*  This file is part of JT49.
+
+    JT49 is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    JT49 is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with JT49.  If not, see <http://www.gnu.org/licenses/>.
+    
+    Author: Jose Tejada Gomez. Twitter: @topapate
+    Version: 1.0
+    Date: 15-Jan-2019
+    
+    */
+
+// DC removal filter
+// input is unsigned
+// output is signed
+
+module jt49_dcrm2 #(parameter sw=8) (
+    input                clk,
+    input                cen,
+    input                rst,
+    input         [sw-1:0]  din,
+    output signed [sw-1:0]  dout
+);
+
+localparam dw=10; // widht of the decimal portion
+
+reg  signed [sw+dw:0] integ, exact, error;
+//reg  signed [2*(9+dw)-1:0] mult;
+// wire signed [sw+dw:0] plus1 = { {sw+dw{1'b0}},1'b1};
+reg  signed [sw:0] pre_dout;
+// reg signed [sw+dw:0] dout_ext;
+reg signed [sw:0] q;
+
+always @(*) begin
+    exact = integ+error;
+    q = exact[sw+dw:dw];
+    pre_dout  = { 1'b0, din } - q;
+    //dout_ext = { pre_dout, {dw{1'b0}} };    
+    //mult  = dout_ext;
+end
+
+assign dout = pre_dout[sw-1:0];
+
+always @(posedge clk)
+    if( rst ) begin
+        integ <= {sw+dw+1{1'b0}};
+        error <= {sw+dw+1{1'b0}};
+    end else if( cen ) begin
+        integ <= integ + pre_dout; //mult[sw+dw*2:dw];
+        error <= exact-{q, {dw{1'b0}}};
+    end
+
+endmodule