151 lines
4.2 KiB
Markdown
151 lines
4.2 KiB
Markdown
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isHidden: false
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menupriority: 1
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kind: article
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created_at: 2010-10-14T11:04:58+02:00
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title: Fun with wav
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author_name: Yann Esposito
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author_uri: yannesposito.com
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# tags:
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-----
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begindiv(intro)
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<%= tldr %> Playing to process a `wav` file. `C` was easier and cleaner than Ruby.
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enddiv
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I had to calculate the sum of the absolute value of datas of a `.wav` file.
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For efficiency (and fun) reasons, I had chosen `C` language.
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It was a long time I didn't used `C`.
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From my memory it was a pain to read and write to files.
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But I was really impressed by how clean the code is.
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And it is even more impressive knowing I used mostly low level functions.
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A `wav` file has an header containing many meta-datas.
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This header was optimized to take the less space possible.
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Therefore, header is thinked with Bytes.
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- The 4th first Bytes must contains `RIFF` in ASCII,
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- the following 4th Bytes is an 32 bits integer giving the size of the file minus 8, etc...
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Surprisingly, I believe read this kind of file with a higher level language would have more difficult than in `C`.
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Proof: I only have to search on the web the complete header format and write it in a struct.
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<code class="c">
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struct wavfile
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{
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char id[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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</code>
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If I had to read it in Ruby (for example), I believe I'd had to write a read entry for each bloc value.
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But in `C` I simply written:
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<code class="c">
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fread(&header,sizeof(header),1,wav);
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</code>
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Only one step to fill my data structure. Magic!
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Then, get an int value coded on two Bytes is also not a natural operation for high level language.
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In `C`, to read a sequence of 2 Bytes numbers I only had to write:
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<code class="c">
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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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</code>
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Finally I ended with the following code. Remark I know the wav format (16 bit / 48000Hz):
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<code class="c" file="wavsum.c">
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#include <stdio.h>
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#include <stdlib.h>
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struct wavfile
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{
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char id[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) < sizeof(header) )
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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.id[0] != 'R'
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|| header.id[1] != 'I'
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|| header.id[2] != 'F'
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|| header.id[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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</code>
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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 `C` is clearly far superior than Ruby to handle this simple tasks.
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I am curious to know if somebody know a nice way to do this with Ruby or Python.
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