Mono WAV playing on both channels - c

Question

I created a random mono WAV, but when I play it, I can hear the audio through both channels (left & right). Here's my code:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>
struct wav{
char ChunkID[4];
unsigned int ChunkSize;
char Format[4];
char Subchunk1ID[4];
unsigned int Subchunk1Size;
unsigned short int AudioFormat;
unsigned short int NumChannels;
unsigned int SampleRate;
unsigned int ByteRate;
unsigned short int BlockAlign;
unsigned short int BitsPerSample;
char SubChunk2ID[4];
unsigned int Subchunk2Size;
};
int main(){
struct wav wavHdr;
FILE *fp;
fp = fopen("MonoSound.wav", "wb");
strcpy(wavHdr.ChunkID, "RIFF");
strcpy(wavHdr.Format, "WAVE");
strcpy(wavHdr.Subchunk1ID, "fmt ");
wavHdr.Subchunk1Size = 16;
wavHdr.AudioFormat = 1;
wavHdr.NumChannels = 1;
wavHdr.SampleRate = 220505;
wavHdr.ByteRate = 441010; //(SampleRate*NumChannels*BitsPerSample/8)
wavHdr.BlockAlign = 2; //(NumChannels*BitsPerSample/8)
wavHdr.BitsPerSample = 16;
strcpy(wavHdr.SubChunk2ID, "data");
/* multiplied by 5 because there's 5 seconds of audio */
wavHdr.Subchunk2Size = (5 * wavHdr.ByteRate);
wavHdr.ChunkSize = (wavHdr.Subchunk2Size + 36);
fwrite(&wavHdr, 44, 1, fp);
int i, randVal;
unsigned int audio;
float freq = 50.0;
int amp = 32600;
float w;
srand(time(NULL));
for(i = 0; i < (5 * wavHdr.SampleRate); i++){
randVal = (rand() % 1) + 1;
amp += randVal;
w = 2.0 * 3.141592 * freq;
audio = amp * sin(w * i / 220505.0);
fwrite(&audio, 2, 1, fp);
}
return 0;
}
What have I done wrong here? The audio should only come out through one of the speakers. Thanks in advance for the help.

Answer 1

"The audio should only come out through one of the speakers"
Not really. When you have mono file i.e. you had one microphone when you were recording the audio, you will get same data on both output channels. If you want to hear audio only from one channel make 2 channel wav, with one channel all zeros

Answer 2

The audio should only come out through one of the speakers
Why do you you think so? Probably the audio driver tries to be smart and plays mono signals through both speakers (like all other consumer audio hardware does).
If you want to be sure that a signal is played on the left channel only, you have to create a stereo signal with the right channel set to silence (all zeros).

Answer 3

To achieve you goal you can either trick you audio card (which is by default playing the mono file into both speakers channels), or you can create a stereo file with one empty channel.
In order to do so you have to change the number of channels (set to 2 using wavHdr.NumChannels) and you have to write a the empty channel alternating with the good one (see the second command fwrite at the end of the code.)
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>
#include <cstring>
struct wav{
char ChunkID[4];
unsigned int ChunkSize;
char Format[4];
char Subchunk1ID[4];
unsigned int Subchunk1Size;
unsigned short int AudioFormat;
unsigned short int NumChannels;
unsigned int SampleRate;
unsigned int ByteRate;
unsigned short int BlockAlign;
unsigned short int BitsPerSample;
char SubChunk2ID[4];
unsigned int Subchunk2Size;
};
int main(){
struct wav wavHdr;
FILE *fp;
fp = fopen("MonoSound.wav", "wb");
strcpy(wavHdr.ChunkID, "RIFF");
strcpy(wavHdr.Format, "WAVE");
strcpy(wavHdr.Subchunk1ID, "fmt ");
wavHdr.Subchunk1Size = 16;
wavHdr.AudioFormat = 1;
wavHdr.NumChannels = 2;
wavHdr.BitsPerSample = 16;
wavHdr.SampleRate = 220505;
wavHdr.ByteRate = wavHdr.SampleRate * wavHdr.NumChannels * wavHdr.BitsPerSample/8;
wavHdr.BlockAlign = wavHdr.NumChannels * wavHdr.BitsPerSample/8;
strcpy(wavHdr.SubChunk2ID, "data");
/* multiplied by 5 because there's 5 seconds of audio */
wavHdr.Subchunk2Size = (5 * wavHdr.ByteRate);
wavHdr.ChunkSize = (wavHdr.Subchunk2Size + 36);
fwrite(&wavHdr, 44, 1, fp);
int i, randVal;
unsigned int audio, empty=0;
float freq = 50.0;
int amp = 32600;
float w;
srand(time(NULL));
for(i = 0; i < (5 * wavHdr.SampleRate); i++){
randVal = (rand() % 1) + 1;
amp += randVal;
w = 2.0 * 3.141592 * freq;
audio = amp * sin(w * i / 220505.0);
// write LEFT channel
fwrite(&audio, 2, 1, fp);
// write RIGHT channel
fwrite(&empty, 2, 1, fp);
}
return 0;
}
The order in which you write the two channel matters. If you want the empty channel to be the left one, you have to invert the two fwrite commands.
Moreover, you have to change ByteRate and BlockAlign to take into account the new channel.