4.2 KiB
isHidden | menupriority | kind | created_at | title | author_name | author_uri |
---|---|---|---|---|---|---|
false | 1 | article | 2010-10-14T11:04:58+02:00 | Fun with wav | Yann Esposito | yannesposito.com |
begindiv(intro)
<%= tldr %> Playing to process a wav
file. C
was easier and cleaner than Ruby.
enddiv
I had to calculate the sum of the absolute value of datas of a .wav
file.
For efficiency (and fun) reasons, I had chosen C
language.
It was a long time I didn't used C
.
From my memory it was a pain to read and write to files.
But I was really impressed by how clean the code is.
And it is even more impressive knowing I used mostly low level functions.
A wav
file has an header containing many meta-datas.
This header was optimized to take the less space possible.
Therefore, header is thinked with Bytes.
- The 4th first Bytes must contains
RIFF
in ASCII, - the following 4th Bytes is an 32 bits integer giving the size of the file minus 8, etc...
Surprisingly, I believe read this kind of file with a higher level language would have more difficult than in C
.
Proof: I only have to search on the web the complete header format and write it in a struct.
struct wavfile
{
char id[4]; // should always contain "RIFF"
int totallength; // total file length minus 8
char wavefmt[8]; // should be "WAVEfmt "
int format; // 16 for PCM format
short pcm; // 1 for PCM format
short channels; // channels
int frequency; // sampling frequency
int bytes_per_second;
short bytes_by_capture;
short bits_per_sample;
char data[4]; // should always contain "data"
int bytes_in_data;
};
If I had to read it in Ruby (for example), I believe I'd had to write a read entry for each bloc value.
But in C
I simply written:
fread(&header,sizeof(header),1,wav);
Only one step to fill my data structure. Magic!
Then, get an int value coded on two Bytes is also not a natural operation for high level language.
In C
, to read a sequence of 2 Bytes numbers I only had to write:
short value=0;
while( fread(&value,sizeof(value),1,wav) ) {
// do something with value
}
Finally I ended with the following code. Remark I know the wav format (16 bit / 48000Hz):
#include
#include
struct wavfile
{
char id[4]; // should always contain "RIFF"
int totallength; // total file length minus 8
char wavefmt[8]; // should be "WAVEfmt "
int format; // 16 for PCM format
short pcm; // 1 for PCM format
short channels; // channels
int frequency; // sampling frequency
int bytes_per_second;
short bytes_by_capture;
short bits_per_sample;
char data[4]; // should always contain "data"
int bytes_in_data;
};
int main(int argc, char *argv[]) {
char *filename=argv[1];
FILE *wav = fopen(filename,"rb");
struct wavfile header;
if ( wav == NULL ) {
fprintf(stderr,"Can't open input file %s", filename);
exit(1);
}
// read header
if ( fread(&header,sizeof(header),1,wav) < sizeof(header) )
{
fprintf(stderr,"Can't read file header\n");
exit(1);
}
if ( header.id[0] != 'R'
|| header.id[1] != 'I'
|| header.id[2] != 'F'
|| header.id[3] != 'F' ) {
fprintf(stderr,"ERROR: Not wav format\n");
exit(1);
}
fprintf(stderr,"wav format\n");
// read data
long sum=0;
short value=0;
while( fread(&value,sizeof(value),1,wav) ) {
// fprintf(stderr,"%d\n", value);
if (value<0) { value=-value; }
sum += value;
}
printf("%ld\n",sum);
exit(0);
}
Of course it is only a hack.
But we can see how easy and clean it should be to improve.
As I say often: the right tool for your need instead of the same tool for all your needs.
Because here C
is clearly far superior than Ruby to handle this simple tasks.
I am curious to know if somebody know a nice way to do this with Ruby or Python.