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https://github.com/MiSTer-devel/Genesis_MiSTer.git
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dd205b8f54
Genesis Model 1 has a global 6db/octave low-pass adio filter. An IIR filter was computed to match the measured frequency response. The Model 2 Genesis has a different 6db/octave low-pass filter for all audio and a separate 12db/octave low-pass for fm synthesis only. These are also implemented. The filter can be chosen in the OSD.
102 lines
3.1 KiB
Verilog
102 lines
3.1 KiB
Verilog
/*MIT License
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Copyright (c) 2019 Gregory Hogan (Soltan_G42)
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Permission is hereby granted, free of charge, to any person obtaining a copy
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of this software and associated documentation files (the "Software"), to deal
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in the Software without restriction, including without limitation the rights
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to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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copies of the Software, and to permit persons to whom the Software is
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furnished to do so, subject to the following conditions:
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The above copyright notice and this permission notice shall be included in all
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copies or substantial portions of the Software.
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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SOFTWARE.*/
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// This module is intended to be used from system.sv to low pass Genesis audio.
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// Both Model 1 and Model 2 Genesis models have a global 1st order low-pass
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// audio filter. The model 2 has an addition 2nd order low-pass for FM audio
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module genesis_lpf(
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input clk,
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input reset,
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input [1:0] lpf_mode,
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input signed [15:0] in,
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output signed [15:0] out);
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reg [9:0] div = 504; //For genesis we'll sample at 53.69mhz/504 = 106528 Hz
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//Coefficients computed with Octave/Matlab/Online filter calculators.
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//or with scipy.signal.bessel or similar tools
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reg signed [17:0] A2;
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reg signed [17:0] B2;
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reg signed [17:0] B1;
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wire signed [15:0] audio_post_lpf1;
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always @ (*) begin
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case(lpf_mode[1:0])
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2'b00 : begin //Model 1 Low Pass at 3.1khz with adjusted slope
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A2 = -18'd27504;
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B1 = 18'd10528;
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B2 = -18'd5264;
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end
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2'b01 : begin //model 2 is measured around 3.96khz
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A2 = -18'sd26328;
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B1 = 18'sd12888;
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B2 = -18'sd6440;
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end
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2'b10 : begin //8.5khz low pass as a "Minimal" filter.
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A2 = -18'd19088;
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B1 = 18'd6840;
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B2 = 18'd6840;
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end
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default: begin //Space for a 4th filter. This filter is not used
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A2 = -18'sd32768;
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B1 = 18'sd0;
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B2 = 18'sd0;
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end
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endcase
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end
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iir_1st_order lpf6db(.clk(clk),
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.reset(reset),
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.div(div),
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.A2(A2),
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.B1(B1),
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.B2(B2),
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.in(in),
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.out(audio_post_lpf1));
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assign out = ( lpf_mode[1:0] == 2'b11 ) ? in : audio_post_lpf1;
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endmodule //genesis_lpf
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//This module is intended to be used from system.sv to filter the FM sound before it's mixed with PSG.
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//The model 2 genesis has such a filter. The model 1 has only a global low-pass
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module genesis_fm_lpf(
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input clk,
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input reset,
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input signed [15:0] in,
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output signed [15:0] out);
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wire signed [15:0] middle;
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iir_2nd_order fm2(.clk(clk),
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.reset(reset),
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.div(10'd504), //Divider for 106528hz
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.A2(-18'sd24704),
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.A3(18'sd10728),
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.B1(18'sd1083),
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.B2(18'sd1204),
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.B3(18'sd121),
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.in(in),
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.out(out));
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endmodule //genesis_fm_lpf
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