Playing PCM sound over RS232, RS422, RS485 or UART TX pin using pulse width modulation with noise shaping

play.sh

#! /bin/bash

# FT232RL:
# baud=500000 idle=0
baud=1000000 idle=0
# baud=2000000 idle=0
# baud=3000000 idle=0

# CP210X:
# baud=500000 idle=1
# baud=1000000 idle=2
# baud=2000000 idle=4

# PL2303:
# baud=500000 idle=0
# baud=1000000 idle=0
# baud=1000000 idle=1
# baud=2000000 idle=1
# baud=3000000 idle=1
# baud=4000000 idle=1

order=2
device=/dev/ttyUSB0
stty -F $device raw $baud
sox blah.ogg -t s16 -c 1 -r $((baud/(1+8+1+idle))) - | ./pwm $order > $device

pwm.c

#include <stdio.h>
#include <stdlib.h>

int sigma_delta_modulation(int x, int order)
{
	static int sum;
	if (!order)
		sum = x;
	int y = sum >> 13;
	if (y < 0)
		y = 0;
	if (y > 8)
		y = 8;
	int e = y << 13;
	if (order >= 2) {
		static int sum2;
		sum2 += x - e;
		x = sum2;
	}
	sum += x - e;
	return y;
}

int main(int argc, char **argv)
{
	int order = 1;
	if (argc == 2)
		order = atoi(argv[1]);

	for (short input; fread_unlocked(&input, 2, 1, stdin) == 1;)
		if (EOF == putchar_unlocked(-256 >> sigma_delta_modulation(input+32768, order)))
			return 1;

	return 0;
}