[wpfilebase tag=file id=91 tpl=supplement /]
In Chapters 2 and 3, we showed you how to generate sinusoidal waveforms as arrays of values that could be sent directly to the sound device and played. But what if you want to work with more complicated sounds and music in C++? In this case, you’ll need to be able to read audio files into your programs in order to experiment with audio processing.
In C++, raw audio data in 8-bit or 16-bit format can be read into an array of characters (char) or short integers (short), respectively, assuming that characters are stored in 8 bits and short integers are 16 bits on your system. This is demonstrated in Program 5.1. You can use this in the programming exercise on dithering and mu-law encoding.
//This program runs under OSS
//The program expects an 8-bit raw sound file.
//You can alter it to read a 16-bit file into short ints
#include <sys/ioctl.h> //for ioctl()
#include <math.h> //sin(), floor()
#include <stdio.h> //perror
#include <fcntl.h> //open
#include <linux/soundcard.h> //SOUND_PCM*
#include <stdlib.h>
#include <unistd.h>
using namespace std;
#define TYPE char
#define RATE 44100 //sampling rate
#define SIZE sizeof(TYPE) //size of sample, in bytes
#define CHANNELS 1 //number of stereo channels
#define PI 3.14159
#define SAMPLE_MAX (pow(2,SIZE*8 - 1) - 1)
void writeToSoundDevice(TYPE* buf, int deviceID, int buffSize) {
int status;
status = write(deviceID, buf, buffSize);
if (status != buffSize)
perror("Wrote wrong number of bytes\n");
status = ioctl(deviceID, SOUND_PCM_SYNC, 0);
if (status == -1)
perror("SOUND_PCM_SYNC failed\n");
}
int main(int argc, char* argv[])
{
int deviceID, arg, status, i, numSamples;
numSamples = atoi(argv[1]);
TYPE* samples = (TYPE *) malloc((size_t) numSamples * sizeof(TYPE)* CHANNELS);
FILE *inFile = fopen(argv[2], "rb");
fread(samples, (size_t)sizeof(TYPE), numSamples*CHANNELS, inFile);
fclose(inFile);
deviceID = open("/dev/dsp", O_WRONLY, 0);
if (deviceID < 0)
perror("Opening /dev/dsp failed\n");
arg = SIZE * 8;
status = ioctl(deviceID, SOUND_PCM_WRITE_BITS, &arg);
if (status == -1)
perror("Unable to set sample size\n");
arg = CHANNELS;
status = ioctl(deviceID, SOUND_PCM_WRITE_CHANNELS, &arg);
if (status == -1)
perror("Unable to set number of channels\n");
arg = RATE;
status = ioctl(deviceID, SOUND_PCM_WRITE_RATE, &arg);
if (status == -1)
perror("Unable to set number of bits\n");
writeToSoundDevice(samples, deviceID, numSamples * CHANNELS);
FILE *outFile = fopen(argv[3], "wb");
fwrite(samples, 1, numSamples*CHANNELS, outFile);
fclose(outFile);
close(deviceID);
}
Program 5.1 Reading raw data in a C++ program, OSS library
When you write a low-level sound program such as this, you need to use a sound library such as OSS or ALSA. There’s a lot of documentation online that will guide you through installation of the sound libraries into the Linux environment and show you how to program with the libraries. (See the websites cited in the references at the end of the chapter.) The ALSA version of Program 5.1 is given in Program 5.2.
/*This program demonstrates how to read in a raw
data file and write it to the sound device to be played.
The program uses the ALSA library.
Use option -lasound on compile line.*/
#include </usr/include/alsa/asoundlib.h>
#include <math.h>
#include <iostream>
using namespace std;
static char *device = "default"; /*default playback device */
int main(int argc, char* argv[])
{
int err, numSamples;
snd_pcm_t *handle;
snd_pcm_sframes_t frames;
numSamples = atoi(argv[1]);
char* samples = (char*) malloc((size_t) numSamples);
FILE *inFile = fopen(argv[2], "rb");
int numRead = fread(samples, 1, numSamples, inFile);
fclose(inFile);
if ((err=snd_pcm_open(&handle, device, SND_PCM_STREAM_PLAYBACK, 0)) < 0){
printf("Playback open error: %s\n", snd_strerror(err));
exit(EXIT_FAILURE);
}
if ((err =
snd_pcm_set_params(handle,SND_PCM_FORMAT_U8,
SND_PCM_ACCESS_RW_INTERLEAVED,1,44100, 1, 100000) ) < 0 ){
printf("Playback open error: %s\n", snd_strerror(err));
exit(EXIT_FAILURE);
}
frames = snd_pcm_writei(handle, samples, numSamples);
if (frames < 0)
frames = snd_pcm_recover(handle, frames, 0);
if (frames < 0) {
printf("snd_pcm_writei failed: %s\n", snd_strerror(err));
}
snd_pcm_close(handle);
return 0;
}
Program 5.2 Reading raw data in a C++ program, ALSA library