[SOLVED] Question on the output of audio decoding programs.

giuliom_95 · 10-06-2015, 10:10 AM

Hi all,
I've noticed that a lot of audio decoding command-line programs (such as flac, mpg123 or ffmpeg) have the possibility to "print" the decoded sound to stdout. What is the format of this output and where can I find more information about it?

rtmistler · 10-06-2015, 01:43 PM

I think that depends on the particular decoder you're using. And when you decode something or convert it, such as MP4 to MP3, then the "output" is MP3. Be that binary or Ascii it doesn't matter. "Seeing" it on stdout may be useless. For one, it'll just zip by, especially if it's large. Better to redirect via > to a file. That may be the intention, something like "convert input.mp4 > output.mp3"

giuliom_95 · 10-06-2015, 02:53 PM

Thanks for answer,
Actually I need that stdout output because a program need it...
I think that is better explain what I'm trying to achieve.
There is this program, pifm, that turns a Raspberry PI board in a FM transmitter: it normally broadcasts wav files but it can also "read" audio from stdin. Now I want to write a program that, thanks to a ncurses GUI, let me do some basic operations like broadcasting the DJ's voice while a song starts in the background. In order to do that I need to know how to overlap the voice with the music and print the result to stdout...

schneidz · 10-06-2015, 03:22 PM

i didnt know pifm could read from a stdin.

maybe using fifo's will help ?

giuliom_95 · 10-06-2015, 03:51 PM

I'm almost a newbie so don't kill me if I don't know what fifo is... Do you mean first-in first-out structures? And if that so, how can I use them in this case?

schneidz · 10-06-2015, 05:56 PM

yes. using the mkfifo command you can set one program to write to a file and another to read from it at the same time.

i am curious, how you are calling pifm from within your script.

giuliom_95 · 10-07-2015, 03:40 AM

Can you make an example on usage of mkfifo and fifo commands?

However, I'm planning to call pifm with something like:

Code:

$ ./radiogui | sudo ./pifm - 98 96000

and radiogui is a C-written program which prints continuously to stdout the outputs of commands like "flac -dc music/01.flac", "mpg123 -s music/02.mp3" or "arecord -d0 -c2 -f S16_LE -r 96000 -twav -D copy", depending on the user input.
Now, I want to enhance this program by adding the possibility to print the overlapped output of two commands (probably flac and arecord) while turning down the volume of one of these two streams (if we can call them this way).

giuliom_95 · 10-07-2015, 04:27 AM

Ok I've found the solution. I can simply arithmetically add one output to one other in order to broadcast from two files and I can turn down the volume by subtracting an integer to the output. Here's an example code:

Code:

#include <stdio.h>
#include <stdlib.h>


int main( int argc, char *argv[] ) {

  FILE *fp1, *fp2;

  fp1 = popen("/usr/bin/flac -dc 1.flac", "r");
  fp2 = popen("/usr/bin/mpg123 -s 2.mp3", "r");
  
  if( fp1 == NULL || fp2 == NULL ) {
    printf("Failed to run command\n" );
    exit(1);
  }

  //Reads the output until one of two streams ends.
  while( feof( fp1 ) == 0 || feof( fp2 ) == 0 ) {
  
  	//"Prints" the flac almost silenced and the mp3 at full volume. 
    printf("c", ( fgetc( fp1 ) - 30 ) + fgetc( fp2 ));
  }

  pclose(fp1);
  pclose(fp2);
  
  return 0;
}