Why is there latency in this C++ ALSA (Linux audio) program?

Question

I am exploring sound generation using C++ in Ubuntu Linux. Here is my code:

#include <iostream>
#include <cmath>
#include <stdint.h>
#include <ncurses.h>

//to compile: make [file_name] && ./[file_name]|aplay


int main()
{
    initscr();

    cbreak();
    noecho();
    nodelay(stdscr, TRUE);

    scrollok(stdscr, TRUE);
    timeout(0);

   for ( int t=0;; t++ )
      {

     int ch = getch();
     if (ch == 'q')
     {
         break;
     }

      uint8_t temp = t;
      std::cout<<temp;

    }
}

When this code is run, I want it to generate sound until I press "q" on my keyboard, after which I want the program to quit. This works fine; however, there is a noticeable delay between pressing the keyboard and the program quitting. This is not due to a delay with ncurses, as when I run the program without std::cout<<temp; (i.e. no sound generated), there is no latency

Is there a way to amend this? If not, how are real-time responsive audio programs written?

Edits and suggestions to the question are welcome. I am a novice to ALSA, so I am not sure if any additional details are required to replicate the bug.

Answer 1

The latency in the above loop is most likely due to delays introduced by the ncurses getch function.

Typically for realtime audio you will want to have a realtime audio thread running and a non-realtime user control thread running. The user control thread can alter the memory space of the real time audio thread which forces the real time audio loop to adjust synthesis as required.

In this gtkIOStream example, a full duplex audio class is created. The process method in the class can have your synthesis computation compiled in. This will handle the playback of your sound using ALSA.

To get user input, one possibility is to add a threaded method to the class by inheriting the FullDuplexTest class, like so :

class UIALSA : public FullDuplexTest, public ThreadedMethod {
    void *threadMain(void){
        while (1){
          // use getchar here to block and wait for user input
          // change the memory in FullDuplexTest to indicate a change in variables
        }
        return NULL;
    }
public:
    UIALSA(const char*devName, int latency) : FullDuplexTest(devName, latency), ThreadedMethod() {};
};

Then change all references to FullDuplexTest to UIALSA in the original test file (you will probably have to fix some compile time errors) :

UIALSA fullDuplex(deviceName, latency);

Also you will need to call UIALSA::run() to make sure the UI thread is running and listening for user input. You can add the call before you call "go" :

fullDuplex.run(); // start the UI thread
res=fullDuplex.go(); // start the full duplex read/write/process going.