Re: a joint letter on low latency and Linux

On Fri, Jun 30, 2000 at 07:40:52PM +0100, Dale Amon wrote:
> Where it all matters is in two cases that I can think of:
> 
> 	* The mixing desk.
> 	* Performance.
> 
> A mixing desk has to be able to take multiple streams from
> multiple sources, line them up and generate a glitch free
> master. It has to be realtime because in the music biz this
> is mostly done by ear (although there is also some queing
> done as well, ie things like Cakewalk and other systems that
> are already digital).

I don't see how Linux can ever do this  - without expensive peripheral
devices.   

> Instrumental performance is the most demanding of them all. If
> I am onstage in front of a live audience and playing an
> improvisation with other musicians, I wonder if even
> 5ms is fast enough. Buffering is useless because if I
> strike a note, the attack must be nearly instantaneous.
> If you have never done live improv with really good musicians
> you won't believe how tight this can be. [And if you've ever 
> been on stage with bad sound trying to do this you know what
> pure hell is.]

That's where the pedal to the metal, shave every microsecond response
of something like RTLinux is all you can do.

BTW: despite the impression one sometimes gets, programming in RTLinux
does not require mastery of the Babylonian alphabet presented in octal.
The programming interface is pretty much POSIX threads with some 
simplifications to make periodic tasks easier to define. 

module_init(){
           pthread_create(f,stacksize,priority,0);
           }

/* don't write code like this without error checks */
void f(void *data){
         fd_r = open("/dev/rtf0", O_READ);
         fd_w = open("/dev/rtf1", O_WRITE);
         fd_d = open("/dev/sound", O_READ);
         while(1){
                  read(fd_d,buf,BUFSZ);
                  write(fd_w, buf,BUFSZ);
                  if(read(fd_r,command,COMMANDSIZE) >0)
                      process_command_from_linux_process(command);
                  pthread_wait_np(); /* wait for next period */
                }
  }

         

The user process looks like
main(){
         fd_data = open("/dev/rtf1",O_READ);
         fd_command = open("/dev/rtf0",O_WRITE);

         while(1){ 
                   read(fd_data,buf,BUFSZ);
                   process_data(buf);
                   if(need_to_change_something)write(fd_command,command);
                  }

}

-- 
---------------------------------------------------------
Victor Yodaiken 
FSMLabs:  www.fsmlabs.com  www.rtlinux.com
FSMLabs is a servicemark and a service of 
VJY Associates L.L.C, New Mexico.