Adds a low-pass filter for PWM audio

Improves the audio quality of PWM audio with a simple low-pass filter.
This uses R3-C3 and R4-C4 (one filter for each channel).

Author: ScrapComputing

.gitignore

@@ -0,0 +1,4 @@
+/kicad/*-cache 
+/kicad/*-cache.lib 
+/kicad/*-bak
+/kicad/gerbers/*

README.md

@@ -16,6 +16,7 @@ For controlling the mt32-pi from DOS, please use [mt32-pi-control](https://githu
 
 - It currently supports two modes of audio: using the raspberry pi's PWM audio out (which is noisy and sounds pretty bad), and an external Hi-Fi PCM5102A based DAC.
 - You can also attach a 0.91 OLED SSD1306 I2C 128x32 display.
+- Starting from rev 0.3 PWM audio is filtered by a simple low-pass filter (R3-C3 and R4-C4).
 
 # I want to build one myself!
 - Please note that the current version of the WavetablePi board is quite tall (76mm x 40mm) so please check if it fits your sound card, especially if it needs to be placed perpendicularly to the ISA slots.
@@ -26,16 +27,18 @@ For controlling the mt32-pi from DOS, please use [mt32-pi-control](https://githu
 
 # Bill of materials
 
-Item                                     | ##  | Description
+Item                                     | #   | Description
 -----------------------------------------|-----|-----------------------------------------
 Raspberry Pi Zero 2 W                    | 1   | The older RPi Zero is not fast enough
 40-pin 2x20 male header 2.54mm pitch     | 1   | For attaching the RPi to the board.
 26-pin 2x13 female header 2.54mm pitch   | 1   | For attaching the board to the wavetable header
 6-pin 2x3 pin male header 2.54mm pitch   | 1   | For the audio select jumpers
 Jumper 2.54mm pitch                      | 2   | Jumpers for the audio selection header
-10uF through-hole ceramic capacitors     | 2   |
+Capacitor 10uF through-hole ceramic      | 2   |
+Capacitor 100nF through-hole ceramic     | 2   | C3 and C4 which form the PWM audio low-pass filter along with R3 and R4. 100nF is for a 1.6KHz cutoff frequency. Use lower values for higher cutoff
 Resistor 22K Ohm                         | 1   |
 Resistor 10K Ohm                         | 1   |
+Resistor 1K Ohm                          | 2   | R3 and R4, which along with C3 and C4 form a low-pass filter of the PWM audio
 0.91 OLED SSD1306 I2C 128x32 module      | 1   | (Optional)
 PCM5102A I2S DAC Module                  | 1   | (Optional but highly recommended) Please check the PCB for the supported dimensions and form factor!