add comments for audio implementations on a2n20v2-SDRAM and a2n9

Author: edanuff

boards/a2n20v2-SDRAM/hdl/top.sv

@@ -625,6 +625,10 @@ module top #(
 
     // HDMI
 
+    // TODO - Needs to incorporate the audio filter code from a2n20v2
+    // IMPORTANT - the Ensoniq module outputs signed 16-bit audio, so we need to
+    // properly mix it with the apple audio and the mockingboard audio
+
     localparam AUDIO_RATE = 44100;
     localparam AUDIO_BIT_WIDTH = 16;
     localparam AUDIO_CLK_COUNT = (CLOCK_SPEED_HZ / 2) / AUDIO_RATE;

boards/a2n9/hdl/top.sv

@@ -338,6 +338,10 @@ module top #(
 
     // HDMI
 
+    // TODO - Adapt the IIR filter im A2N20, but GW1NR-9C FPGA does not
+    // have sufficient DSP resources to run IIR filter code.  This means
+    // A2N9 audio will have high frequency noise.
+
     localparam AUDIO_RATE = 44100;
     localparam AUDIO_BIT_WIDTH = 16;
     localparam AUDIO_CLK_COUNT = (CLOCK_SPEED_HZ / 2) / AUDIO_RATE;