Actual sound engine

1 files changed, 328 insertions, 0 deletions

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+// $Id$
+//
+// Sound controller for ABC80
+//
+// This attempts to simulate the SN76477 sound generator
+// *as used in ABC80*.
+//
+// ABC80 has the following (approximate) parameters:
+//
+// SLF          = 2.9 Hz
+// VCOmin       = 640 Hz
+// Noise cutoff =  10 kHz
+// Attack       =  22 ms
+// Decay        = 470 ms
+
+
+// Model the 76477 SLF.  This returns a "sawtooth" value
+// between (approximately) [1024,12288) which gives about
+// the 10:1 range needed by the VCO.
+`define vco_min  14'd1024
+`define vco_max 14'd12288
+
+module slf(
+	   input clk,		// 25 MHz
+	   input clk_en,	// One pulse every 16 us (62.5 kHz)
+	   output slf,		// SLF squarewave
+	   output [13:0] saw	// Sawtooth magnitude
+	   );
+   reg 			 up  = 1;
+   reg [13:0] 		 ctr = `vco_min;
+
+   assign      slf = up;
+   assign      saw = ctr;
+
+   always @(posedge clk)
+     if ( clk_en )
+       begin
+	  if ( ctr[13:10] == 4'b1100 )
+	    up <= 0;
+	  else if ( ctr[13:10] == 4'b0000 )
+	    up <= 1;
+	  
+	  if ( up )
+	    ctr <= ctr + 1;
+	  else
+	    ctr <= ctr - 1;
+       end // if ( clk_en )
+endmodule // slf
+
+//
+// The VCO.  The output frequency = clk/pitch/2.
+//
+module vco(
+	   input clk,		// 25 MHz
+	   input [13:0] pitch,	// Pitch control
+	   output vco,		// VCO squarewave output
+	   output vco2		// VCO output with every other pulse suppressed
+	   );
+   reg [14:0] 	  ctr = 0;
+   reg [1:0] 	  cycle;
+
+   assign 	  vco  = cycle[0];
+   assign 	  vco2 = cycle[0] & cycle[1];
+   
+   always @(posedge clk)
+     begin
+	if ( ctr == 0 )
+	  begin
+	     ctr <= { pitch, 1'b0 };
+	     cycle <= cycle + 1;
+	  end
+	else
+	  ctr <= ctr - 1;
+     end // always @ (posedge clk)
+endmodule // vco
+
+//
+// Noise (e.g. random number) generator./
+// 
+module noise(
+	     input clk,    // 25 MHz
+	     input clk_en, // One pulse every 16 us (62.5 kHz)
+	     output noise
+	     );
+   reg [15:0] 	    lfsr = ~16'h0; // Must be nonzero
+
+   assign 	    noise = lfsr[15];
+
+   wire 	    lfsr_zero = (lfsr == 0);
+   
+   always @(posedge clk)
+     if ( clk_en )
+       lfsr <= { lfsr[14:0], lfsr_zero } ^ (lfsr[15] ? 16'h54b9 : 16'h0);
+endmodule // noise
+
+//
+// Mixer
+//
+module mixer(
+	     input slf,
+	     input vco,
+	     input noise,
+	     input [2:0] mixer_ctl,
+	     output mixer_out
+	     );
+   reg 		    out;
+   
+   assign 	    mixer_out = out;
+   
+   always @(*)
+     case ( mixer_ctl )
+       3'b000:
+	 out <= vco;
+       3'b001:
+	 out <= slf;
+       3'b010:
+	 out <= noise;
+       3'b011:
+	 out <= vco & noise;
+       3'b100:
+	 out <= slf & noise;
+       3'b101:
+	 out <= slf & vco & noise;
+       3'b110:
+	 out <= slf & vco;
+       3'b111:
+	 out <= 1;
+     endcase // case( mixer_ctl )
+endmodule // mixer
+
+//
+// Envelope generator, consisting of one-shot generator,
+// envelope select, and envelope generation (attack/decay.)
+// Output is parallel digital.
+//
+module oneshot(
+	       input clk,    // 25 MHz
+	       input clk_en, // One pulse every 16 us (62.5 kHz)
+	       input inhibit,
+	       output oneshot
+	       );
+   reg 		      out = 0;
+   reg 		      inhibit1 = 0;
+   reg [10:0] 	      ctr = 0;
+
+   reg 		      oneshot;
+
+   wire 	      ctr_or = |ctr;
+      
+   always @(posedge clk)
+     if ( clk_en )
+       begin
+	  inhibit1 <= inhibit;
+	  oneshot  <= ctr_or;
+	  
+	  if ( ~inhibit & inhibit1 )
+	    ctr <= 11'd1624;	// ~26 ms
+	  else if ( ctr_or )
+	    ctr <= ctr - 1;
+       end
+endmodule // oneshot
+
+module envelope_select(
+		       input [1:0] envsel,
+		       input oneshot,
+		       input vco,
+		       input vco2,
+		       output envelope
+		       );
+   reg 			      envelope;
+   
+   always @(*)
+     begin
+	case ( envsel )
+	  2'b00:
+	    envelope <= vco;
+	  2'b01:
+	    envelope <= 1;
+	  2'b10:
+	    envelope <= oneshot;
+	  2'b11:
+	    envelope <= vco2;
+	endcase // case( envsel )
+     end // always @ (*)
+endmodule // envelope_select
+
+module envelope_shape(
+		      input clk,    // 25 MHz
+		      input clk_en, // One pulse every 16 us (62.5 kHz)
+		      input envelope,
+		      output [13:0] env_mag
+		      );
+   reg [13:0] 			    env_mag = 0;
+
+   always @(posedge clk)
+     if ( clk_en )
+       begin
+	  if ( envelope )
+	    begin
+	       if ( env_mag[13:11] != 3'b111 )
+		 env_mag <= env_mag + 20;
+	    end
+	  else
+	    begin
+	       if ( |env_mag )
+		 env_mag <= env_mag - 1;
+	    end
+       end // if ( clk_en )
+endmodule // envelope_shape
+
+//
+// Delta-sigma DAC for the final output
+//
+module dac(
+	   input [13:0] magnitude,
+	   input clk,		// The higher the better
+	   output signal
+	   );
+   reg [13:0] 	  mag_latch = 0;
+   reg [15:0] 	  sigma_latch = 0;
+
+   assign 	  signal = sigma_latch[15];
+   
+   wire [15:0] 	  delta_add = { sigma_latch[15], sigma_latch[15], mag_latch };
+   wire [15:0] 	  sigma_add = delta_add + sigma_latch;
+   
+   
+   always @(posedge clk)
+     begin
+	mag_latch   <= magnitude;
+	sigma_latch <= sigma_add;
+     end
+endmodule // dac
+
+//
+// Putting it all together...
+//
+module sound_generator(
+		       input ctr_16us,
+		       input clk_fast,
+		       input clk_25MHz,
+
+		       input [2:0] mixer_ctl,
+		       input vco_sel,
+		       input vco_pitch,
+		       input [1:0] envsel,
+		       input inhibit,
+
+		       output signal,
+		       output [7:0] debug
+		       );
+   wire        w_slf;
+   wire [13:0] saw;
+   wire [13:0] vco_level;
+   wire        w_vco;
+   wire        w_vco2;
+   wire        w_envelope;
+   wire        w_noise;
+   wire        w_oneshot;
+   wire        w_mixer_out;
+   
+   wire [13:0] env_mag;
+   wire        signal_on;
+   wire [13:0] magnitude;
+
+   reg 	       ctr_16us_1;
+   reg 	       clk_en;
+
+   // ctr_16us should be synchronous with clk_25MHz.
+   // We set clk_en to 1 for one cycle every positive edge of ctr_16us.
+
+   always @(posedge clk_25MHz)
+     begin
+	ctr_16us_1 <= ctr_16us;
+	clk_en <= ctr_16us & ~ctr_16us_1;
+     end
+
+   slf slf ( .clk (clk_25MHz),
+	     .clk_en (clk_en),
+	     .saw (saw),
+	     .slf (w_slf) );
+
+   assign vco_level = vco_sel ? saw : vco_pitch ? `vco_max : `vco_min;
+   
+   vco vco ( .clk (clk_25MHz),
+	     .pitch (vco_level),
+	     .vco (w_vco),
+	     .vco2 (w_vco2) );
+
+   noise noise ( .clk (clk_25MHz),
+		 .clk_en (clk_en),
+		 .noise (w_noise) );
+
+   
+   mixer mixer ( .slf (w_slf),
+		 .vco (w_vco),
+		 .noise (w_noise),
+		 .mixer_ctl (mixer_ctl),
+		 .mixer_out (w_mixer_out) );
+
+   
+   oneshot oneshot ( .clk (clk_25MHz),
+		     .clk_en (clk_en),
+		     .inhibit (inhibit),
+		     .oneshot (w_oneshot) );
+
+   envelope_select envelope_select ( .envsel (envsel),
+				     .oneshot (w_oneshot),
+				     .vco (w_vco),
+				     .vco2 (w_vco2),
+				     .envelope (w_envelope) );
+
+   envelope_shape envelope_shape ( .clk (clk_25MHz),
+				   .clk_en (clk_en),
+				   .envelope (w_envelope),
+				   .env_mag (env_mag) );
+
+
+   assign signal_on = ~inhibit & w_mixer_out;
+   assign magnitude = signal_on ? env_mag : 0;
+
+   dac dac ( .magnitude (magnitude),
+	     .clk (clk_fast),
+	     .signal (signal) );
+
+   assign debug = { w_noise, w_slf, w_vco, w_vco2, w_oneshot, w_envelope, w_mixer_out, signal_on };
+   
+endmodule // sound_generator