Cutting amplitude of a signal from .wav file - c

Question

I'm currently working on a C program that is opening a .wav file , copy the .wav header in the output file and after the header it adds the processed data that is obtained from the input file. The process consist of multiplying each sample with 0.5 in order to reduce the amplitude of the signal.
Here is my code:
program.c
#include <stdio.h>
#include <stdlib.h>
struct header_file
{
char chunk_id[4];
int chunk_size;
char format[4];
char subchunk1_id[4];
int subchunk1_size;
short int audio_format;
short int num_channels;
int sample_rate;
int byte_rate;
short int block_align;
short int bits_per_sample;
char subchunk2_id[4];
int subchunk2_size;
};
struct header_file header;
int main()
{
FILE * inputfile = fopen("maneskin.wav","rb"); // deschidere fisier .wav, modul de citire
FILE * outputfile = fopen("Output.wav","wb"); // creeaza fisierul .wav, modul de scriere
int BUFFSIZE = 10000; // lungimea buffer-ului
int count = 0; // numara cate frame-uri sunt in fisier
short int inputBuffer[BUFFSIZE];
short int outputBuffer[BUFFSIZE];
/* header_p meta = (header_p)malloc(sizeof(header)); */
int nb; // numarul de bytes returnati
int temp;
if (inputfile)
{
fread(&header, sizeof(header),1, inputfile);
fwrite(&header,sizeof(header),1, outputfile);
short int ch2[BUFFSIZE];
for(int i=0; i<BUFFSIZE; i++) \
ch2[i] = 1;
while (!feof(inputfile))//cat timp se citeste fisierul
{
for(int i=0; i<BUFFSIZE && !feof(inputfile); i++)
fread((char*)&inputBuffer[i],sizeof(short int),1, inputfile); // citirea datelor
count++;// incrementarea numarului de frame-uri
for(int z=0; z<BUFFSIZE; z++){
outputBuffer[z]=0.5*inputBuffer[z];
}
for(int k=0; k<BUFFSIZE; k++)
{
fwrite((char*)&outputBuffer[k], sizeof(short int), 1, outputfile);// scriere in fisier
}
}
printf("Numarul de cadre din fisierul wav:%d\n", count);
}
return 0;
}
The problemm that I have is when I'm trying to open the output file with VLC or any other media player it doens't open and gives me an error. I think it might be a problem with the header writing in the output file but I'm not sure. I tried importing the file as raw data in Audacity and played the audio file just fine but if I'm trying to import it as wav file it doesn't work.What could be the problem?

Answer 1

I don't know if this is your only problem, but it's definitely a problem...
In C, generally when you define a structure, the members of that structure do not sit right next to each other in memory. The compiler arranges them to fall on boundaries that are comfortable for the processor, usually in increments of the word size of the processor. So an element of type char surrounded by two ints, is not nestled right between them, but actually has perhaps 7 bytes of space between it and the next element.
This is done for execution efficiency, and usually makes sense.
But in situations like this, you want to pack the structure elements tightly together and for this there is a standard way to tell the compiler that this is what you want:
/* pack in 1 byte increments */
#pragma pack(1)
struct header_file
{
char chunk_id[4];
int chunk_size;
char format[4];
char subchunk1_id[4];
int subchunk1_size;
short int audio_format;
short int num_channels;
int sample_rate;
int byte_rate;
short int block_align;
short int bits_per_sample;
char subchunk2_id[4];
int subchunk2_size;
};
/* reset to default */
#pragma pack()
I would also suggest using size-specific types, for example uint16_t instead of short int, because you do not know the size of int on every platform where your code might be compiled.
In your code, try printing the sizeof header to see if it's what you expect, eg:
printf("sizeof(header)=%d\n", sizeof(header));
If the number does not match what it should be for a WAV header then you have got a problem with the size of one or more elements, or a problem with the packing.