Shairport and alsa configuration to make a Raspberry Pi able to receive AirPlay sound streams, equalize the sound with ALSA equal plugin and play them either on HDMI audio interface or Raspberry PWM analog output

asound.conf

# File: /etc/asound.conf
# .alsaequal.bin is automatically created tuning the equalizer with "alsamixer -D equal"

ctl.equal {
  type equal;
  controls "/home/pi/.alsaequal.bin"
}

pcm.plugequal {
  type equal;
  # Change plughw:0,0 accordingly with the physical audio device (type "alsa -L" for a list of devices availables). On Raspberry Pi, the card 0 internal audio DAC, both on HDMI digital mode and PWM analog audio (launch "amixer cset numid=3 2" to enable HDMI audio, "amixer cset numid=3 1" to switch to analog)
  slave.pcm "plughw:0,0";
  controls "/home/pi/.alsaequal.bin"
}

pcm.equal {
  type plug;
  slave.pcm plugequal;
}

shairport

#!/bin/bash
#
# File: /etc/init.d/shairport
# This starts and stops shairport on Raspberry, using the libasound2-plugin-equal as output ALSA device.
#
### BEGIN INIT INFO
# Provides:          shairport
# Required-Start:    $network
# Required-Stop:
# Short-Description: shairport - Airtunes emulator!
# Description:       Airtunes emulator!
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
### END INIT INFO


# Source function library.
. /lib/lsb/init-functions

NAME=shairport
AODEV="equal"
DAEMON="/usr/local/bin/shairport.pl"
PIDFILE=/var/run/$NAME.pid
DAEMON_ARGS="-w $PIDFILE -a RaspberryPi --ao_devicename=$AODEV"

[ -x $binary ] || exit 0

RETVAL=0

start() {
    echo -n "Starting shairport: "
    start-stop-daemon --start --quiet --pidfile "$PIDFILE" \
                      --exec "$DAEMON" -b --oknodo -- $DAEMON_ARGS
    log_end_msg $?
}

stop() {
    echo -n "Shutting down shairport: "
    start-stop-daemon --stop --quiet --pidfile "$PIDFILE" \
                --retry 1 --oknodo
    log_end_msg $?
}

restart() {
    stop
    sleep 1
    start
}

case "$1" in
    start)
        start
    ;;
    stop)
        stop
    ;;
    status)
        status shairport
    ;;
    restart)
        restart
    ;;
    *)
        echo "Usage: $0 {start|stop|status|restart}"
    ;;
esac

exit 0

Based on http://amiexp.blogspot.it/2013/01/respberry-pi-with-shairport-and-usb.html, https://jeffskinnerbox.wordpress.com/2012/11/15/getting-audio-out-working-on-the-raspberry-pi/, http://lifehacker.com/5978594/turn-a-raspberry-pi-into-an-airplay-receiver-for-streaming-music-in-your-living-room