432 lines
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30 KiB
HTML
432 lines
No EOL
30 KiB
HTML
<?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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<link rel="shortcut icon" type="image/x-icon" href="/Scratch/img/favicon.ico" />
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<link rel="stylesheet" type="text/css" href="/Scratch/assets/css/main.css" />
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<link rel="stylesheet" type="text/css" href="/Scratch/css/twilight.css" />
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<link rel="stylesheet" type="text/css" href="/Scratch/css/idc.css" />
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<link rel="alternate" type="application/rss+xml" title="RSS" href="http://feeds.feedburner.com/yannespositocomfr"/>
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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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<script type="text/javascript" src="/Scratch/js/jquery-1.3.1.min.js"></script>
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<script type="text/javascript" src="/Scratch/js/jquery.cookie.js"></script>
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<script type="text/javascript" src="/Scratch/js/index.js"></script>
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<title>S'amuser avec un .wav</title>
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</head>
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<body lang="fr">
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<script type="text/javascript">// <![CDATA[
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document.write('<div id="blackpage"><img src="/Scratch/img/loading.gif" alt="Chargement en cours..."/></div>');
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// ]]>
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</script>
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<div id="content">
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<div id="choix">
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<div class="return"><a href="#entete">↓ Menu ↓</a></div>
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<div id="choixlang">
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<a href="/Scratch/en/blog/2010-10-14-Fun-with-wav/" onclick="setLanguage('en')">in English</a>
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</div>
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</div>
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<img src="/Scratch/img/presentation.png" alt="Presentation drawing"/>
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<div id="titre">
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<h1>
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S'amuser avec un .wav
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</h1>
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</div>
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<div class="flush"></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="Trop long à lire">tlàl</abbr> : </span> Je me suis amusé à lire un fichier <code>wav</code>. Le <code>C</code> fut le langage le mieux adapté à ce traitement. Bien meilleur que Ruby par exemple.</p>
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<p>edit: Je voulais que ce programme fonctionne sur une machine spécifique. En aucun cas je ne pensais publier ce code pour une utilisation autre que celle-ci.</p>
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</div>
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<p>J’ai eu besoin de calculer la somme des valeurs absolue des données d’un fichier <code>wav</code>.
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Pour des raison d’efficacité (et aussi de fun), j’ai fait le programme en <code>C</code>.</p>
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<p>Celà faisait longtemps que je n’avais pas programmé en <code>C</code>.
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De mémoire il était peu aisé de manipuler des fichiers.
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Mais je dois concéder que j’ai été étonné de la clarté du code que j’ai obtenu.</p>
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<p>Tout d’abord, un fichier <code>wav</code> se compose d’un entête qui contient pas mal de meta données.
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Cet entête a été optimisé pour prendre peu de place.
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Donc on discute de l’entête avec des nombres d’octets :</p>
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<ul>
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<li>Les 4 premiers octets doivent contenir <code>RIFF</code> en ASCII ;</li>
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<li>les 4 octects suivant correspondent à un entier codé sur 32 bits qui donne la taille du fichier moins 8 octets. etc..</li>
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</ul>
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<p>Etonnamment je pense que lire ce type de fichier avec un langage de haut niveau aurait été plus pénible qu’en C.
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La preuve, il m’a suffit de chercher sur le net le format complet de l’entête et de l’écrire dans un struct.</p>
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<pre class="twilight">
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<span class="Storage">struct</span> wavfile
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{
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<span class="Storage">char</span> id[<span class="Constant">4</span>]; <span class="Comment"><span class="Comment">//</span> should always contain "RIFF"</span>
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<span class="Storage">int</span> totallength; <span class="Comment"><span class="Comment">//</span> total file length minus 8</span>
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<span class="Storage">char</span> wavefmt[<span class="Constant">8</span>]; <span class="Comment"><span class="Comment">//</span> should be "WAVEfmt "</span>
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<span class="Storage">int</span> format; <span class="Comment"><span class="Comment">//</span> 16 for PCM format</span>
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<span class="Storage">short</span> pcm; <span class="Comment"><span class="Comment">//</span> 1 for PCM format</span>
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<span class="Storage">short</span> channels; <span class="Comment"><span class="Comment">//</span> channels</span>
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<span class="Storage">int</span> frequency; <span class="Comment"><span class="Comment">//</span> sampling frequency</span>
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<span class="Storage">int</span> bytes_per_second;
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<span class="Storage">short</span> bytes_by_capture;
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<span class="Storage">short</span> bits_per_sample;
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<span class="Storage">char</span> data[<span class="Constant">4</span>]; <span class="Comment"><span class="Comment">//</span> should always contain "data"</span>
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<span class="Storage">int</span> bytes_in_data;
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};
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</pre>
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<p>Si j’avais eu à faire ça en Ruby, je pense qu’il m’aurait fallu pour chaque bloc de l’entête écrire un bout de code de lecture du bon nombre d’octets.
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Alors qu’en <code>C</code> il m’a suffit d’écrire: </p>
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<pre class="twilight">
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<span class="SupportFunction">fread</span>(&header,<span class="Keyword">sizeof</span>(header),<span class="Constant">1</span>,wav)
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</pre>
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<p>Et en une seule étape ma structure de donnée a été remplie avec les valeurs souhaitées. Magique !</p>
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<p>Ensuite, récupérer un entier à partir de deux octets n’est pas non plus une opération naturelle dans les nouveaux langages de programmation.
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Alors qu’en <code>C</code>. Pour récupérer un entier codé sur 16 bits il suffit d’écrire :</p>
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<pre class="twilight">
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<span class="Storage">short</span> value=<span class="Constant">0</span>;
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<span class="Keyword">while</span>( <span class="SupportFunction">fread</span>(&value,<span class="Keyword">sizeof</span>(value),<span class="Constant">1</span>,wav) ) {
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<span class="Comment"><span class="Comment">//</span> do something with value</span>
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}
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</pre>
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<p>Finallement je suis arrivé au code suivant, sachant que le format de wav était connu, avec notamment échantillonage sur 16 bits en 48000Hz :</p>
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<div class="code"><div class="file"><a href="/Scratch/fr/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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<span class="CCCPreprocessorLine">#<span class="CCCPreprocessorDirective">include</span> <span class="String"><span class="String"><</span>stdio.h<span class="String">></span></span></span>
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<span class="CCCPreprocessorLine">#<span class="CCCPreprocessorDirective">include</span> <span class="String"><span class="String"><</span>stdlib.h<span class="String">></span></span></span>
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<span class="CCCPreprocessorLine">#<span class="CCCPreprocessorDirective">include</span> <span class="String"><span class="String"><</span>stdint.h<span class="String">></span></span></span>
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<span class="Storage">struct</span> wavfile
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{
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<span class="Storage">char</span> id[<span class="Constant">4</span>]; <span class="Comment"><span class="Comment">//</span> should always contain "RIFF"</span>
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<span class="Storage">int</span> totallength; <span class="Comment"><span class="Comment">//</span> total file length minus 8</span>
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<span class="Storage">char</span> wavefmt[<span class="Constant">8</span>]; <span class="Comment"><span class="Comment">//</span> should be "WAVEfmt "</span>
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<span class="Storage">int</span> format; <span class="Comment"><span class="Comment">//</span> 16 for PCM format</span>
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<span class="Storage">short</span> pcm; <span class="Comment"><span class="Comment">//</span> 1 for PCM format</span>
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<span class="Storage">short</span> channels; <span class="Comment"><span class="Comment">//</span> channels</span>
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<span class="Storage">int</span> frequency; <span class="Comment"><span class="Comment">//</span> sampling frequency</span>
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<span class="Storage">int</span> bytes_per_second;
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<span class="Storage">short</span> bytes_by_capture;
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<span class="Storage">short</span> bits_per_sample;
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<span class="Storage">char</span> data[<span class="Constant">4</span>]; <span class="Comment"><span class="Comment">//</span> should always contain "data"</span>
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<span class="Storage">int</span> bytes_in_data;
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};
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<span class="Storage">int</span> <span class="Entity">ma<span class="Entity">in</span></span>(<span class="Storage">int</span> argc, <span class="Storage">char</span> *argv[]) {
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<span class="Storage">char</span> *filename=argv[<span class="Constant">1</span>];
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FILE *wav = <span class="SupportFunction">fopen</span>(filename,<span class="String"><span class="String">"</span>rb<span class="String">"</span></span>);
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<span class="Storage">struct</span> wavfile header;
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<span class="Keyword">if</span> ( wav == <span class="Constant">NULL</span> ) {
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<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>Can't open input file <span class="StringConstant">%s</span><span class="String">"</span></span>, filename);
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<span class="SupportFunction">exit</span>(<span class="Constant">1</span>);
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}
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<span class="Comment"><span class="Comment">//</span> read header</span>
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<span class="Keyword">if</span> ( <span class="SupportFunction">fread</span>(&header,<span class="Keyword">sizeof</span>(header),<span class="Constant">1</span>,wav) < <span class="Constant">1</span> )
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{
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<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>Can't read file header<span class="StringConstant">\n</span><span class="String">"</span></span>);
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<span class="SupportFunction">exit</span>(<span class="Constant">1</span>);
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}
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<span class="Keyword">if</span> ( header.id[<span class="Constant">0</span>] != <span class="String"><span class="String">'</span>R<span class="String">'</span></span>
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|| header.id[<span class="Constant">1</span>] != <span class="String"><span class="String">'</span>I<span class="String">'</span></span>
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|| header.id[<span class="Constant">2</span>] != <span class="String"><span class="String">'</span>F<span class="String">'</span></span>
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|| header.id[<span class="Constant">3</span>] != <span class="String"><span class="String">'</span>F<span class="String">'</span></span> ) {
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<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>ERROR: Not wav format<span class="StringConstant">\n</span><span class="String">"</span></span>);
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<span class="SupportFunction">exit</span>(<span class="Constant">1</span>);
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}
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<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>wav format<span class="StringConstant">\n</span><span class="String">"</span></span>);
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<span class="Comment"><span class="Comment">//</span> read data</span>
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<span class="Storage">long</span> sum=<span class="Constant">0</span>;
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<span class="Storage">short</span> value=<span class="Constant">0</span>;
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<span class="Keyword">while</span>( <span class="SupportFunction">fread</span>(&value,<span class="Keyword">sizeof</span>(value),<span class="Constant">1</span>,wav) ) {
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<span class="Comment"><span class="Comment">//</span> fprintf(stderr,"%d\n", value);</span>
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<span class="Keyword">if</span> (value<<span class="Constant">0</span>) { value=-value; }
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sum += value;
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}
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<span class="SupportFunction">printf</span>(<span class="String"><span class="String">"</span><span class="StringConstant">%ld</span><span class="StringConstant">\n</span><span class="String">"</span></span>,sum);
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<span class="SupportFunction">exit</span>(<span class="Constant">0</span>);
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}
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</pre>
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</div></div>
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<p>Bien entendu ce code n’est qu’un <em>hack</em>.
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Mais on voit bien comment on peut facilement améliorer ce code, ajouter des cas possibles par exemple.
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Comme je dis souvent : le bon outil pour la bonne tâche.
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On voit en effet que pour cette tâche <code>C</code> est bien supérieur à Ruby par exemple.</p>
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<p>_màj : pour des raisons de compatibilité (machines 64 bits) j’ai utilisé <code>int16_t</code> au lieu de <code>short</code> et <code>int</code> au lieu de <code>int</code>.</p>
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<p>Je serai curieux de savoir s’il existe un manière plus propre en Ruby que je ne connais pas.
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Certainement qu’en Python ça doit être la cas.</p>
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<div class="intro">
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<p>màj (2) : après toutes les remarques concernant la portabilité.
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J’ai fait une nouvelle version qui devrait être plus portable.
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Elle fait aussi plus de test pour vérifier le fichier.
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Cependant j’utilise une assertion spécifique à <code>gcc</code> pour être certain que la structure de donnée n’ai pas de “trou” :</p>
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<pre class="twilight">
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__attribute__((__packed__))
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</pre>
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<p>Le nouveau code n’utilise pas mmap et devrait être plus compatible.<br />
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Voici le dernier résultat :</p>
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</div>
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<div class="code"><div class="file"><a href="/Scratch/fr/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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<span class="CCCPreprocessorLine">#<span class="CCCPreprocessorDirective">include</span> <span class="String"><span class="String"><</span>stdio.h<span class="String">></span></span></span>
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<span class="CCCPreprocessorLine">#<span class="CCCPreprocessorDirective">include</span> <span class="String"><span class="String"><</span>stdlib.h<span class="String">></span></span></span>
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<span class="CCCPreprocessorLine">#<span class="CCCPreprocessorDirective">include</span> <span class="String"><span class="String"><</span>string.h<span class="String">></span></span> </span><span class="Comment"><span class="Comment">//</span> for memcmp</span>
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<span class="CCCPreprocessorLine">#<span class="CCCPreprocessorDirective">include</span> <span class="String"><span class="String"><</span>stdint.h<span class="String">></span></span> </span><span class="Comment"><span class="Comment">//</span> for int16_t and int32_t</span>
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<span class="Storage">struct</span> wavfile
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{
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<span class="Storage">char</span> id[<span class="Constant">4</span>]; <span class="Comment"><span class="Comment">//</span> should always contain "RIFF"</span>
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<span class="Support">int32_t</span> totallength; <span class="Comment"><span class="Comment">//</span> total file length minus 8</span>
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<span class="Storage">char</span> wavefmt[<span class="Constant">8</span>]; <span class="Comment"><span class="Comment">//</span> should be "WAVEfmt "</span>
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<span class="Support">int32_t</span> format; <span class="Comment"><span class="Comment">//</span> 16 for PCM format</span>
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<span class="Support">int16_t</span> pcm; <span class="Comment"><span class="Comment">//</span> 1 for PCM format</span>
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<span class="Support">int16_t</span> channels; <span class="Comment"><span class="Comment">//</span> channels</span>
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<span class="Support">int32_t</span> frequency; <span class="Comment"><span class="Comment">//</span> sampling frequency</span>
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<span class="Support">int32_t</span> bytes_per_second;
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<span class="Support">int16_t</span> bytes_by_capture;
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<span class="Support">int16_t</span> bits_per_sample;
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<span class="Storage">char</span> data[<span class="Constant">4</span>]; <span class="Comment"><span class="Comment">//</span> should always contain "data"</span>
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<span class="Support">int32_t</span> bytes_in_data;
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} __attribute__((__packed__));
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<span class="Storage">int</span> is_big_endian(<span class="Storage">void</span>) {
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<span class="Storage">union</span> {
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<span class="Support">uint32_t</span> i;
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<span class="Storage">char</span> c[<span class="Constant">4</span>];
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} bint = {<span class="Constant">0x01000000</span>};
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<span class="Keyword">return</span> bint.c[<span class="Constant">0</span>]==<span class="Constant">1</span>;
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}
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<span class="Storage">int</span> <span class="Entity">ma<span class="Entity">in</span></span>(<span class="Storage">int</span> argc, <span class="Storage">char</span> *argv[]) {
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<span class="Storage">char</span> *filename=argv[<span class="Constant">1</span>];
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FILE *wav = <span class="SupportFunction">fopen</span>(filename,<span class="String"><span class="String">"</span>rb<span class="String">"</span></span>);
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<span class="Storage">struct</span> wavfile header;
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<span class="Keyword">if</span> ( wav == <span class="Constant">NULL</span> ) {
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<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>Can't open input file <span class="StringConstant">%s</span><span class="StringConstant">\n</span><span class="String">"</span></span>, filename);
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<span class="SupportFunction">exit</span>(<span class="Constant">1</span>);
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}
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<span class="Comment"><span class="Comment">//</span> read header</span>
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<span class="Keyword">if</span> ( <span class="SupportFunction">fread</span>(&header,<span class="Keyword">sizeof</span>(header),<span class="Constant">1</span>,wav) < <span class="Constant">1</span> ) {
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<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>Can't read input file header <span class="StringConstant">%s</span><span class="StringConstant">\n</span><span class="String">"</span></span>, filename);
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<span class="SupportFunction">exit</span>(<span class="Constant">1</span>);
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}
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<span class="Comment"><span class="Comment">//</span> if wav file isn't the same endianness than the current environment</span>
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<span class="Comment"><span class="Comment">//</span> we quit</span>
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<span class="Keyword">if</span> ( is_big_endian() ) {
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<span class="Keyword">if</span> ( <span class="SupportFunction">memcmp</span>( header.id,<span class="String"><span class="String">"</span>RIFX<span class="String">"</span></span>, <span class="Constant">4</span>) != <span class="Constant">0</span> ) {
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<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>ERROR: <span class="StringConstant">%s</span> is not a big endian wav file<span class="StringConstant">\n</span><span class="String">"</span></span>, filename);
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<span class="SupportFunction">exit</span>(<span class="Constant">1</span>);
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}
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} <span class="Keyword">else</span> {
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<span class="Keyword">if</span> ( <span class="SupportFunction">memcmp</span>( header.id,<span class="String"><span class="String">"</span>RIFF<span class="String">"</span></span>, <span class="Constant">4</span>) != <span class="Constant">0</span> ) {
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<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>ERROR: <span class="StringConstant">%s</span> is not a little endian wav file<span class="StringConstant">\n</span><span class="String">"</span></span>, filename);
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<span class="SupportFunction">exit</span>(<span class="Constant">1</span>);
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}
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}
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<span class="Keyword">if</span> ( <span class="SupportFunction">memcmp</span>( header.wavefmt, <span class="String"><span class="String">"</span>WAVEfmt <span class="String">"</span></span>, <span class="Constant">8</span>) != <span class="Constant">0</span>
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|| <span class="SupportFunction">memcmp</span>( header.data, <span class="String"><span class="String">"</span>data<span class="String">"</span></span>, <span class="Constant">4</span>) != <span class="Constant">0</span>
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) {
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<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>ERROR: Not wav format<span class="StringConstant">\n</span><span class="String">"</span></span>);
|
|
<span class="SupportFunction">exit</span>(<span class="Constant">1</span>);
|
|
}
|
|
<span class="Keyword">if</span> (header.format != <span class="Constant">16</span>) {
|
|
<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span><span class="StringConstant">\n</span>ERROR: not 16 bit wav format.<span class="String">"</span></span>);
|
|
<span class="SupportFunction">exit</span>(<span class="Constant">1</span>);
|
|
}
|
|
<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>format: <span class="StringConstant">%d</span> bits<span class="String">"</span></span>, header.format);
|
|
<span class="Keyword">if</span> (header.format == <span class="Constant">16</span>) {
|
|
<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>, PCM<span class="String">"</span></span>);
|
|
} <span class="Keyword">else</span> {
|
|
<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>, not PCM (<span class="StringConstant">%d</span>)<span class="String">"</span></span>, header.format);
|
|
}
|
|
<span class="Keyword">if</span> (header.pcm == <span class="Constant">1</span>) {
|
|
<span class="SupportFunction">fprintf</span>(stderr, <span class="String"><span class="String">"</span> uncompressed<span class="String">"</span></span> );
|
|
} <span class="Keyword">else</span> {
|
|
<span class="SupportFunction">fprintf</span>(stderr, <span class="String"><span class="String">"</span> compressed<span class="String">"</span></span> );
|
|
}
|
|
<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>, channel <span class="StringConstant">%d</span><span class="String">"</span></span>, header.pcm);
|
|
<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>, freq <span class="StringConstant">%d</span><span class="String">"</span></span>, header.frequency );
|
|
<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>, <span class="StringConstant">%d</span> bytes per sec<span class="String">"</span></span>, header.bytes_per_second );
|
|
<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>, <span class="StringConstant">%d</span> bytes by capture<span class="String">"</span></span>, header.bytes_by_capture );
|
|
<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>, <span class="StringConstant">%d</span> bits per sample<span class="String">"</span></span>, header.bytes_by_capture );
|
|
<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span><span class="StringConstant">\n</span><span class="String">"</span></span> );
|
|
|
|
<span class="Keyword">if</span> ( <span class="SupportFunction">memcmp</span>( header.data, <span class="String"><span class="String">"</span>data<span class="String">"</span></span>, <span class="Constant">4</span>) != <span class="Constant">0</span> ) {
|
|
<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>ERROR: Prrroblem?<span class="StringConstant">\n</span><span class="String">"</span></span>);
|
|
<span class="SupportFunction">exit</span>(<span class="Constant">1</span>);
|
|
}
|
|
<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>wav format<span class="StringConstant">\n</span><span class="String">"</span></span>);
|
|
|
|
<span class="Comment"><span class="Comment">//</span> read data</span>
|
|
<span class="Storage">long</span> <span class="Storage">long</span> sum=<span class="Constant">0</span>;
|
|
<span class="Support">int16_t</span> value;
|
|
<span class="Storage">int</span> i=<span class="Constant">0</span>;
|
|
<span class="SupportFunction">fprintf</span>(stderr,<span class="String"><span class="String">"</span>---<span class="StringConstant">\n</span><span class="String">"</span></span>, value);
|
|
<span class="Keyword">while</span>( <span class="SupportFunction">fread</span>(&value,<span class="Keyword">sizeof</span>(value),<span class="Constant">1</span>,wav) ) {
|
|
<span class="Keyword">if</span> (value<<span class="Constant">0</span>) { value=-value; }
|
|
sum += value;
|
|
}
|
|
<span class="SupportFunction">printf</span>(<span class="String"><span class="String">"</span><span class="StringConstant">%lld</span><span class="StringConstant">\n</span><span class="String">"</span></span>,sum);
|
|
<span class="SupportFunction">exit</span>(<span class="Constant">0</span>);
|
|
}
|
|
</pre>
|
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<a rel="license" href="http://creativecommons.org/licenses/by-sa/3.0/deed.fr">Droits de reproduction ©, Yann Esposito</a>
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<div id="lastmod">
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Écrit le : 14/10/2010
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modifié le : 15/10/2010
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