Writing to WAV file C++ - c++

Question

I have a homework about WAV files and FIR filters for a Digital Signal Processing class.
My program must read a WAV file, apply a filter to the data and write the output data to another WAV file again.
I have completed reading and applying filters but I can't write the WAV file. The program doesn't give any errors while compiling but the WAV file doesn't play.
If I write "temp" to the WAV, it runs properly. But if I write "data", it doesn't.
How can I write a WAV file properly?
#define _CRT_SECURE_NO_WARNINGS
#define PI 3.14f
#define WAV_HEADER_LENGTH 44
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <iostream>
#include <fstream>
char* read_wav(const char* filename, short*, short*, int*);
void write_wav(const char* filename, const char*, int);
using namespace std;
int main()
{
short nchannel, ssample;
int csample;
//Reading WAV file and returning the data.
char* temp = read_wav("sum.wav", &nchannel, &ssample, &csample);
short* data = (short*)&temp[WAV_HEADER_LENGTH];
cout << "How many coefficients are there in filter ?" << endl;
int N;
cin >> N ;
float filter[N];
cout << "Type coefficients in filter." << endl;
for(int i=0; i<N;i++){
cin >> filter[i];
}
short* output = (short*)&temp[WAV_HEADER_LENGTH];
for(int i=0; i < csample; i++){
double sum = 0;
for(int j=0; j < N; j++){
if((i - j) >= 0)
sum += filter[j] * data[i-j];
}
output[i] = (short) sum;
}
write_wav("test.wav", out, csample * ssample + WAV_HEADER_LENGTH);
}
char* read_wav(const char* filename, short* nchannel, short* ssample, int* csample) {
//Reading the file.
FILE* fp = fopen(filename, "rb");
if (!fp) {
fprintf(stderr, "Couldn't open the file \"%s\"\n", filename);
exit(0);
}
fseek(fp, 0, SEEK_END);
int file_size = ftell(fp);
fseek(fp, 0, SEEK_SET);
printf("The file \"%s\" has %d bytes\n\n", filename, file_size);
char* buffer = (char*)malloc(sizeof(char) * file_size);
fread(buffer, file_size, 1, fp);
// Dump the buffer info.
*nchannel = *(short*)&buffer[22];
*ssample = *(short*)&buffer[34] / 8;
*csample = *(int*)&buffer[40] / *ssample;
printf("ChunkSize :\t %u\n", *(int*)&buffer[4]);
printf("Format :\t %u\n", *(short*)&buffer[20]);
printf("NumChannels :\t %u\n", *(short*)&buffer[22]);
printf("SampleRate :\t %u\n", *(int*)&buffer[24]); // number of samples per second
printf("ByteRate :\t %u\n", *(int*)&buffer[28]); // number of bytes per second
printf("BitsPerSample :\t %u\n", *(short*)&buffer[34]);
printf("Subchunk2ID :\t \"%c%c%c%c\"\n", buffer[36], buffer[37], buffer[38], buffer[39]); // marks beginning of the data section
printf("Subchunk2Size :\t %u\n", *(int*)&buffer[40]); // size of data (byte)
printf("Duration :\t %fs\n\n", (float)(*(int*)&buffer[40]) / *(int*)&buffer[28]);
fclose(fp);
return buffer;
}
void write_wav(const char* filename, const char* data, int len) {
FILE* fp = fopen(filename, "wb");
if (!fp) {
fprintf(stderr, "Couldn't open the file \"%s\"\n", filename);
exit(0);
}
fwrite(data, len, 1, fp);
fclose(fp);
}

Answer 1

This works for me:
int main()
{
short nchannel, ssample;
int csample;
// Reading WAV file and returning the data.
char* temp = read_wav("sum.wav", &nchannel, &ssample, &csample);
short* data = (short*)&temp[WAV_HEADER_LENGTH];
// cout << "How many coefficients are there in filter ?" << endl;
const int N = 2;
// cin >> N;
float filter[N] = {0.5, 0.75};
// cout << "Type coefficients in filter." << endl;
// for (int i = 0; i < N; i++)
// {
// cin >> filter[i];
// }
short* output = (short*)&temp[WAV_HEADER_LENGTH];
for (int i = 0; i < csample; i++)
{
double sum = 0;
for (int j = 0; j < N; j++)
{
if ((i - j) >= 0) sum += filter[j] * data[i - j];
}
output[i] = (short)sum;
}
write_wav("test.wav", (char*)temp, csample * ssample + WAV_HEADER_LENGTH);
}
My changes:
The major change is to use the full buffer, with extremely misleading name: temp, instead of your out that does not compile, as the argument of write_wav.
I applied "my" filter coefficients (the sound from the output file is really distorted),
I applied my favorite indentation
If the code is to be portable, you need to check the endiannes and act accordingly.
I would expect the input and output files to be of the same length, but they're not. Please check it yourself why this is not the case.
Example:
-rw-r--r-- 1 zkoza zkoza 787306 06-23 14:09 sum.wav
-rw-r--r-- 1 zkoza zkoza 787176 06-23 14:16 test.wav
It looks like 130 bytes are missing in the output file.
Your float filter[N] with N not known at compile time is a C++ extension: please use std::vector in your final code instead.
Next time please provide also a link for any input files. For my tests, I used https://freewavesamples.com/alesis-fusion-clean-guitar-c3 , but all these little things, like finding an input file (WAV format has several flavors, I could have missed the correct one), guessing filter parameters etc. take time and effort.
Your condition if ((i - j) >= 0) can be written in a way easier to understand; preferably by changing the inner loop "header".