make
The current status of this entry is:
STATUS: INABIAF - please **DO NOT** fix
For more detailed information see 2020 giles bugs.
./prog < dtmf.wav
./prog [digits] > digits.wav
where dtmf.wav
is a WAV file of the dialling of a number being dialled with
DTMF (Dual-tone multi
frequency)
(more commonly known as Touch-Tone) and [digits]
is a sequence of digits that
will be converted to the DTMF sound!
./try.sh
./prog < pi.wav
# See
./prog 867-5309 > jenny.wav
If the script try.sh did not play a WAV file, it means you do not have
SoX installed; see FAQ 3.10: How do I compile and use an IOCCC entry that
requires sound?). In this case install it and try it again or
play the files jenny.wav
and ioccc28.wav
in a program that can play WAV
files.
NOTE: the phone number of jenny.wav
comes from
https://en.wikipedia.org/wiki/867-5309/Jenny.
A cross platform utility for both encoding and decoding touch tones.
This program encodes and decodes dual-tone multiple frequency signals used in telephone systems, more commonly known as Touch-Tones.
If the program is executed with no arguments, it will read a WAV file from standard input, decode the touch-tones in the file, and output the corresponding digits to standard output. This program only supports WAV files that have exactly 16 bits per sample, but it allows any sample rate and any number of audio channels.
$ ./prog < pi.wav
31415926
If the program is executed with a command-line argument, it will generate the tones corresponding to the specified characters, writing them to standard output as a WAV file.
$ ./prog 867-5309 | aplay
gcc -pedantic -Wall -Wextra
there are no compiler
warnings.libm
.memcpy(3)
so there are no strict-aliasing
violations.CHAR_BIT == 8
.int16_t
, uint16_t
, and uint32_t
exist.int16_t
is 2’s-complement.double
is an IEEE 754 binary64 floating-point type.sizeof(double) == 8
.aa
is both a variable and a macro for error
handling, and memcpy(3)
is both a library function and a oto
label.f
macro is used to unroll some of the loops.l
. This array is usually accessed at index z
plus a constant.
Accesses of this type have been rewritten from, e.g. l[z+17]
to 17[l+z]
.
[l+z]
ends up occurring quite often, so I created the macro a
as a
shorthand.Each tone in a DTMF signal is a combination of two frequencies. The lower frequency determines which row the digit is in, and the higher frequency determines the column. There are 4 possible row frequencies and 4 possible column frequencies.
| | **1209 Hz** | **1336 Hz** | **1477 Hz** | **1633 Hz** |
|------------|-------------|-------------|-------------|-------------|
| **697 Hz** | 1 | 2 abc | 3 def | A |
| **770 Hz** | 4 ghi | 5 jkl | 6 mno | B |
| **852 Hz** | 7 pqrs | 8 tuv | 9 wxyz | C |
| **941 Hz** | * | 0 | # | D |
This program determines which frequencies are present in the input by passing it through a set of 8 virtual resonators, each tuned to one of the frequencies used for DTMF. These can be implemented using only basic arithmetic, and they are able to ‘select’ a specific frequency from the input sound, blocking all other frequencies from reaching the output. The loudness of the sound output from each resonator can therefore be used as a measure of how much of a particular frequency was present in the input. The program then decides, over the course of one tone, which row frequency and which column frequency were most prominent.
Tones are generated using those same resonators by providing an impulse as input. An impulse can be thought of as consisting of sine-waves at all frequencies, and when it is put through one of the resonators, the result is a single pure sine wave.