#!/bin/sh # Thanks to Reynir H. Stefánsson for the original version of this script. # In marine radio, a Mayday emergency call is transmitted preceded by a# 30-second alert sound.  The alert sound comprises two audio tones at# 1300Hz and 2100Hz alternating at a rate of 4Hz.  This script shows how SoX# can be used to construct an audio file containing the alert sound. # This is a `Bourne shell' (sh) script but it should be simple to translate# this to another scripting language if you do not have access to sh. # If you run this script, you may want to hit Ctrl-C fairly soon after the# alert tone starts playing---it's not a pleasant sound! # The synth effect is used to generate each of the tones; "-e mu-law -r 8000"# selects u-law 8kHz sampling-rate audio (i.e. relatively low fidelity,# suitable for the marine radio transmission channel); each tone is# generated at a length of 0.25 seconds to give the required 4Hz# alternation. # Note the use of `raw' as the intermediary file format; a self-describing# (header) format would just get in the way here.  The self-describing# header is added only at the final stage. # --------------------------------------------------------------------------- SOX=../src/sox rm -f 2tones.ul    # Make sure we append to a file that's initially empty for freq in 1300 2100; do  $SOX -e mu-law -r 8000 -n -t raw - synth 0.25 sine $freq gain -3 >> 2tones.uldone $SOX -c 1 -r 8000 2tones.ul alert.au repeat - trim 0 30                   # Repeat to fill 30 seconds and add a file headerrm 2tones.ul       # Tidy up intermediate file $SOX alert.au -d