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506 lines
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<?xml version="1.0" encoding="utf-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" lang="fr" xml:lang="fr">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<meta name="keywords" content="wav, C, format, programming">
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<link rel="alternate" type="application/rss+xml" title="RSS" href="http://feeds.feedburner.com/yannespositocomen"/>
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<link rel="alternate" lang="fr" xml:lang="fr" title="S'amuser avec un .wav" type="text/html" hreflang="fr" href="/Scratch/fr/blog/2010-10-14-Fun-with-wav/" />
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<link rel="alternate" lang="en" xml:lang="en" title="Fun with wav" type="text/html" hreflang="en" href="/Scratch/en/blog/2010-10-14-Fun-with-wav/" />
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<title>Fun with wav</title>
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<body lang="en" class="article">
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<div class="return"><a href="#entete">↓ Menu ↓</a></div>
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<a href="/Scratch/fr/blog/2010-10-14-Fun-with-wav/" onclick="setLanguage('fr')">en Français</a>
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<div id="titre">
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<h1>
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Fun with wav
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</h1>
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</div>
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<div class="flush"></div>
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<div id="afterheader">
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<div class="corps">
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<div class="intro">
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<p><span class="sc"><abbr title="Too long; didn't read">tl;dr</abbr>: </span> Played to process a <code>wav</code> file. <code>C</code> was easier and cleaner than Ruby.</p>
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<p>edit: I wanted this program to work only on one specific machine (a x86 on a 32 bit Ubuntu). Therefore I didn’t had any portability consideration. This is only a <em>hack</em>.</p>
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</div>
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<p>I had to compute the sum of the absolute values of data of a <code>.wav</code> file.
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For efficiency (and fun) reasons, I had chosen <code>C</code> language.</p>
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<p>I didn’t programmed in <code>C</code> for a long time.
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From my memory it was a pain to read and write to files.
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But in the end I was really impressed by the code I get.
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It was really clean.
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This is even more impressive knowing I used mostly low level functions.</p>
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<p>A <code>wav</code> file has an header containing many metadata.
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This header was optimized to take as few space as possible.
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The header is then a block of packed bytes.</p>
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<ul>
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<li>The 4<sup>th</sup> first bytes must contains <code>RIFF</code> in ASCII,</li>
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<li>the following 4<sup>th</sup> Bytes is an 32 bits integer giving the size of the file minus 8, etc…</li>
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</ul>
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<p>Surprisingly, I believe that reading this kind of file is easier in <code>C</code> than in most higher level language.
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Proof: I only have to search on the web the complete header format and write it in a struct.</p>
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<pre class="twilight">
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struct wavfile
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{
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char <span class="Storage">id</span>[4]; // should always contain "RIFF"
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int totallength; // total file length minus 8
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char wavefmt[8]; // should be "WAVEfmt "
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int format; // 16 for PCM format
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short pcm; // 1 for PCM format
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short channels; // channels
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int frequency; // sampling frequency
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int bytes_per_second;
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short bytes_by_capture;
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short bits_per_sample;
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char data[4]; // should always contain "data"
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int bytes_in_data;
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};
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</pre>
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<p>To read this kind of data in Ruby, I certainly had to write a block of code for each element in the struct.
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But in <code>C</code> I simply written:</p>
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<pre class="twilight">
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fread(&header,sizeof(header),1,wav)
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</pre>
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<p>Only one step to fill my data structure. Magic!</p>
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<p>Then, get an int value coded on two Bytes is also not a natural operation for high level language.
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In <code>C</code>, to read a sequence of 2 Bytes numbers I only had to write:</p>
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<pre class="twilight">
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short value=0;
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while( fread(&value,sizeof(value),1,wav) ) {
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// do something with value
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}
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</pre>
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<p>Finally I ended with the following code. Remark I know the wav format (16 bit / 48000Hz):</p>
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<div class="code"><div class="file"><a href="/Scratch/en/blog/2010-10-14-Fun-with-wav/code/wavsum.c"> ➥ wavsum.c </a></div><div class="withfile">
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<pre class="twilight">
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#include <stdio.h>
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#include <stdlib.h>
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#include <stdint.h>
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struct wavfile
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{
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char <span class="Storage">id</span>[4]; // should always contain "RIFF"
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int totallength; // total file length minus 8
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char wavefmt[8]; // should be "WAVEfmt "
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int format; // 16 for PCM format
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short pcm; // 1 for PCM format
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short channels; // channels
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int frequency; // sampling frequency
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int bytes_per_second;
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short bytes_by_capture;
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short bits_per_sample;
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char data[4]; // should always contain "data"
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int bytes_in_data;
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};
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int main(int argc, char *argv[]) {
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char *filename=argv[1];
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FILE *wav = fopen(filename,"rb");
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struct wavfile header;
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if ( wav == NULL ) {
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fprintf(stderr,"Can't open input file %s", filename);
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exit(1);
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}
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// read header
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if ( fread(&header,sizeof(header),1,wav) < 1 )
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{
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fprintf(stderr,"Can't read file header\n");
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exit(1);
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}
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if ( header.<span class="Storage">id</span>[0] != 'R'
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|| header.<span class="Storage">id</span>[1] != 'I'
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|| header.<span class="Storage">id</span>[2] != 'F'
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|| header.<span class="Storage">id</span>[3] != 'F' ) {
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fprintf(stderr,"ERROR: Not wav format\n");
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exit(1);
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}
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fprintf(stderr,"wav format\n");
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// read data
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long sum=0;
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short value=0;
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while( fread(&value,sizeof(value),1,wav) ) {
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// fprintf(stderr,"%d\n", value);
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if (value<0) { value=-value; }
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sum += value;
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}
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printf("%ld\n",sum);
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exit(0);
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}
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</pre>
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</div></div>
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<p>Of course it is only a hack.
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But we can see how easy and clean it should be to improve.
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As I say often: the right tool for your need instead of the same tool for all your needs.
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Because here <code>C</code> is clearly far superior than Ruby to handle this simple tasks.</p>
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<p>I am curious to know if somebody know a nice way to do this with Ruby or Python.</p>
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<p><em>edit: for compatibility reasons (64bit machines) used <code>int16_t</code> instead of <code>short</code> and <code>int</code> instead of <code>int</code>.</em></p>
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<div class="intro">
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<p>Edit (2): after most consideration about portability I made an <em>hopefully</em> more portable version.
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But I must confess this task was a bit tedious.
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The code remain as readable as before.
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But I had to use some compiler specific declaration to force the structure to be packed:</p>
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<pre class="twilight">
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__attribute__((__packed__))
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</pre>
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<p>Therefore this implementation should for big and little endian architecture.
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However, it must be compiled with <code>gcc</code>.
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The new code make more tests but still don’t use <code>mmap</code>.
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Here it is:</p>
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</div>
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<div class="code"><div class="file"><a href="/Scratch/en/blog/2010-10-14-Fun-with-wav/code/wavsum2.c"> ➥ wavsum2.c </a></div><div class="withfile">
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<pre class="twilight">
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h> // for memcmp
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#include <stdint.h> // for int16_t and int32_t
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struct wavfile
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{
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char <span class="Storage">id</span>[4]; // should always contain "RIFF"
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int32_t totallength; // total file length minus 8
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char wavefmt[8]; // should be "WAVEfmt "
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int32_t format; // 16 for PCM format
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int16_t pcm; // 1 for PCM format
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int16_t channels; // channels
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int32_t frequency; // sampling frequency
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int32_t bytes_per_second;
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int16_t bytes_by_capture;
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int16_t bits_per_sample;
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char data[4]; // should always contain "data"
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int32_t bytes_in_data;
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} __attribute__((__packed__));
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int is_big_endian(void) {
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union {
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uint32_t i;
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char c[4];
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} bint = {0x01000000};
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return bint.c[0]==1;
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}
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int main(int argc, char *argv[]) {
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char *filename=argv[1];
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FILE *wav = fopen(filename,"rb");
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struct wavfile header;
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if ( wav == NULL ) {
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fprintf(stderr,"Can't open input file %s\n", filename);
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exit(1);
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}
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// read header
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if ( fread(&header,sizeof(header),1,wav) < 1 ) {
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fprintf(stderr,"Can't read input file header %s\n", filename);
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exit(1);
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}
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// if wav file isn't the same endianness than the current environment
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// we quit
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if ( is_big_endian() ) {
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if ( memcmp( header.<span class="Storage">id</span>,"RIFX", 4) != 0 ) {
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fprintf(stderr,"ERROR: %s is not a big endian wav file\n", filename);
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exit(1);
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}
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} else {
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if ( memcmp( header.<span class="Storage">id</span>,"RIFF", 4) != 0 ) {
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fprintf(stderr,"ERROR: %s is not a little endian wav file\n", filename);
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exit(1);
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}
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}
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if ( memcmp( header.wavefmt, "WAVEfmt ", 8) != 0
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|| memcmp( header.data, "data", 4) != 0
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) {
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fprintf(stderr,"ERROR: Not wav format\n");
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exit(1);
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}
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if (header.format != 16) {
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fprintf(stderr,"\nERROR: not 16 bit wav format.");
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exit(1);
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}
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fprintf(stderr,"format: %d bits", header.format);
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if (header.format == 16) {
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fprintf(stderr,", PCM");
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} else {
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fprintf(stderr,", not PCM (%d)", header.format);
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}
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if (header.pcm == 1) {
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fprintf(stderr, " uncompressed" );
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} else {
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fprintf(stderr, " compressed" );
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}
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fprintf(stderr,", channel %d", header.pcm);
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fprintf(stderr,", freq %d", header.frequency );
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fprintf(stderr,", %d bytes per sec", header.bytes_per_second );
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fprintf(stderr,", %d bytes by capture", header.bytes_by_capture );
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fprintf(stderr,", %d bits per sample", header.bytes_by_capture );
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fprintf(stderr,"\n" );
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if ( memcmp( header.data, "data", 4) != 0 ) {
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fprintf(stderr,"ERROR: Prrroblem?\n");
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exit(1);
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}
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fprintf(stderr,"wav format\n");
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// read data
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long long sum=0;
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int16_t value;
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int i=0;
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fprintf(stderr,"---\n", value);
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while( fread(&value,sizeof(value),1,wav) ) {
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if (value<0) { value=-value; }
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sum += value;
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}
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printf("%lld\n",sum);
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exit(0);
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}
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</pre>
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</div></div>
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<p><em>Edit(3)</em>:
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On <a href="http://reddit.com">reddit</a>
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<a href="http://www.reddit.com/user/Bogdanp">Bogdanp</a>
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proposed a Python version:</p>
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<div class="code"><div class="file"><a href="/Scratch/en/blog/2010-10-14-Fun-with-wav/code/wavsum.py"> ➥ wavsum.py </a></div><div class="withfile">
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<pre class="twilight">
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<span class="Comment"><span class="Comment">#</span>!/usr/bin/env python</span>
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<span class="Keyword">from</span> struct <span class="Keyword">import</span> calcsize, unpack
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<span class="Keyword">from</span> sys <span class="Keyword">import</span> argv, exit
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<span class="Storage">def</span> <span class="Entity">word_iter</span>(<span class="Variable">f</span>):
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<span class="Keyword">while</span> <span class="Constant">True</span>:
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_bytes <span class="Keyword">=</span> f.read(<span class="Constant">2</span>)
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<span class="Keyword">if</span> <span class="SupportFunction">len</span>(_bytes) <span class="Keyword">!=</span> <span class="Constant">2</span>:
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<span class="Keyword">raise</span> <span class="Support">StopIteration</span>
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<span class="Keyword">yield</span> unpack(<span class="String"><span class="String">"</span>=h<span class="String">"</span></span>, _bytes)[<span class="Constant">0</span>]
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<span class="Keyword">try</span>:
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<span class="Keyword">with</span> <span class="Support">open</span>(argv[<span class="Constant">1</span>], <span class="String"><span class="String">"</span>rb<span class="String">"</span></span>) <span class="Keyword">as</span> f:
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wav <span class="Keyword">=</span> <span class="String"><span class="String">"</span>=4ci8cihhiihh4ci<span class="String">"</span></span>
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wav_size <span class="Keyword">=</span> calcsize(wav)
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metadata <span class="Keyword">=</span> unpack(wav, f.read(wav_size))
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<span class="Keyword">if</span> <span class="String"><span class="String">"</span><span class="String"><span class="String">"</span></span></span>.join(metadata[:<span class="Constant">4</span>]) <span class="Keyword">!=</span> <span class="String"><span class="String">"</span>RIFF<span class="String">"</span></span>:
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<span class="Keyword">print</span> <span class="String"><span class="String">"</span>error: not wav file.<span class="String">"</span></span>
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exit(<span class="Constant">1</span>)
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<span class="Keyword">print</span> <span class="SupportFunction">sum</span>(<span class="SupportFunction">abs</span>(word) <span class="Keyword">for</span> word <span class="Keyword">in</span> word_iter(f))
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<span class="Keyword">except</span> <span class="Support">IOError</span>:
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<span class="Keyword">print</span> <span class="String"><span class="String">"</span>error: can't open input file '<span class="StringConstant">%s</span>'.<span class="String">"</span></span> <span class="Keyword">%</span> argv[<span class="Constant">1</span>]
|
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exit(<span class="Constant">1</span>)
|
||
</pre>
|
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</div></div>
|
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|
||
<p>and <a href="http://www.reddit.com/user/luikore">luikore</a>
|
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proposed an impressive Ruby version:</p>
|
||
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<div class="code"><div class="file"><a href="/Scratch/en/blog/2010-10-14-Fun-with-wav/code/wavsum.rb"> ➥ wavsum.rb </a></div><div class="withfile">
|
||
<pre class="twilight">
|
||
data <span class="Keyword">=</span> <span class="Variable">ARGF</span>.<span class="Entity">read</span>
|
||
keys <span class="Keyword">=</span> <span class="String"><span class="String">%w[</span>id totallength wavefmt format</span>
|
||
<span class="String"> pcm channels frequency bytes_per_second</span>
|
||
<span class="String"> bytes_by_capture bits_per_sample</span>
|
||
<span class="String"> data bytes_in_data sum</span>
|
||
<span class="String"> <span class="String">]</span></span>
|
||
values <span class="Keyword">=</span> data.<span class="Entity">unpack</span> <span class="String"><span class="String">'</span>Z4 i Z8 i s s i i s s Z4 i s*<span class="String">'</span></span>
|
||
sum <span class="Keyword">=</span> values.<span class="Entity">drop</span>(<span class="Constant">12</span>).<span class="Entity">map</span>(<span class="Keyword">&</span><span class="Constant"><span class="Constant">:</span>abs</span>).<span class="Entity">inject</span>(<span class="Constant"><span class="Constant">:</span>+</span>)
|
||
keys.<span class="Entity">zip</span>(values.<span class="Entity">take</span>(<span class="Constant">12</span>) <span class="Keyword"><<</span> sum) {|<span class="Variable">k</span>, <span class="Variable">v</span>|
|
||
puts <span class="String"><span class="String">"</span><span class="StringEmbeddedSource"><span class="StringEmbeddedSource">#{</span>k<span class="StringEmbeddedSource"><span class="StringEmbeddedSource">.</span><span class="Entity">ljust</span></span> <span class="StringConstant">17</span><span class="StringEmbeddedSource">}</span></span>: <span class="StringEmbeddedSource"><span class="StringEmbeddedSource">#{</span>v<span class="StringEmbeddedSource">}</span></span><span class="String">"</span></span>
|
||
}
|
||
</pre>
|
||
</div></div>
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<a rel="license" href="http://creativecommons.org/licenses/by-sa/3.0/">Copyright ©, Yann Esposito</a>
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Created: 10/14/2010
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Modified: 11/16/2011
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Entirely done with
|
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<a href="http://www.vim.org">Vim</a>
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and
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<a href="http://jigsaw.w3.org/css-validator/check/referer?profile=css3"> [css] </a>
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