[ecasound] Ecasound for Beatmatching

Hello list!
                I am a first time poster trying to get help using
ecasound for beatmatching (http://en.wikipedia.org/wiki/Beatmatching).
When I am finished I would like to be able to control how fast a track
is playing in real time by the frequency of a key press. Basically, I
want to sync a key press to the downbeat of a song: the faster I press
the key the faster the song plays. Then I will be able to fade into a
different song. I know that time stretching in real time is very
difficult computationally. What I plan to do is pre-generate many
files at different speeds using
rubberband(http://www.breakfastquay.com/rubberband/). When I "speed
the song up" I will just switch to a faster version of the track and
set the position to the same place in the song. I currently have a
mock up of the program which only has two speeds which you can toggle
by pressing enter. It is working however there is a slight pause after
the switch. Reading the list makes me think that I need
"sample-accurate looping"
(http://www.eca.cx/ecasound-list/2005/09/0028.html), however the file
type is wav which i think has this.

My questions for the list:
1. Is beatmatching going to be possible?
2. Is there a better approach?
3. Any ideas about how to remove this small pause.
4. Suggestions on my code are appreciated.

Thanks for your time!

   -Mason Fischer

//the slower track can be generated with rubberband -T140:120
dylan.wav dylan_140.wav
#include <stdio.h>
#include <unistd.h>
#include <ecasoundc.h>
void set_speed(float old_bpm,float bpm);

int main(int argc, char *argv[])
{
  int ch;
  int speed=1;

  eci_init();
  eci_command("cs-add play_chainsetup");

  eci_command("c-add 1st_chain");
  eci_command("ai-add /home/masonf/eca/dylan.wav");

  // Load a version of the track at 140 bpm and mute it
  eci_command("c-add 2nd_chain");
  eci_command("ai-add /home/masonf/eca/dylan_140.wav");
  eci_command("c-mute");

  eci_command("c-select 1st_chain,2nd_chain");
  eci_command("ao-add /dev/dsp");
  eci_command("start");

  while ((ch = getchar()) != EOF){
    // When the user types a key we toggle the speed
    if(speed){
      set_speed(120.0f,140.0f);
      speed=0;
    }else{
      set_speed(140.0f,120.0f);
      speed=1;
    }
  }

  //Cleanup
  eci_command("stop");
  eci_command("cs-disconnect");
  eci_cleanup();
  return(0);
}

// Toggles which track is playing
// And sets the position accordingly

void set_speed(float old_bpm,float bpm){
  float pos=0;
  char command[100];

  eci_command("c-select 1st_chain");
  eci_command("cs-get-position");
  pos = eci_last_float();
  eci_command("c-select 2nd_chain");

  // Set the posisiton proportional to the chnage in bpm
  // For example when bpm is slowed the postiton should be later

  sprintf(command,"cs-set-position %3f",pos*(bpm/old_bpm));
  eci_command(command);

  // toggle chains
  eci_command("c-mute");
  eci_command("c-select 1st_chain");
  eci_command("c-mute");
}

/* Local Variables: */
/* compile-command:"gcc -o eca ./eca.c `libecasoundc-config --cflags
--libs`&& ./eca" */
/* End: */

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Received on Sun Oct 4 00:15:02 2009