Question

In order to write a MIDI sequencer I need a steady pulse that calls a timing routine that has absolute priority over anything else in the program and preferrably over anything in the computer. I do this by using TimeSetEvent like this:

TimeSetEvent (FInterval, 0, TimerUpdate, uInt32 (Self), TIME_PERIODIC);

where TimerUpdate is a callback that resumes a separate thread with priority tpTimeCritical and that one calls a routine (FOnTimer) in which to handle all MIDI events.

procedure TThreaded_Timer.Execute;
begin
   if Assigned (FOnTimer) then
   begin
      while not Terminated do
      begin
         FOnTimer (Self);
         if not Terminated then Suspend;
      end; // while
   end; // if
   Terminate;
end; // Execute //

Although this construction is much better than some things I tried before it is still very sensitive to some events. To my surprise it stutters at each display of a hint. Why can a simple hint cause such an interruption of a time critical thread? Of course I can switch it off but which nasty surprises are still waiting for me?

Answer 1

The accuracy of the multimedia timers is not that great.Here is an article that explains why.

Instead of relying on a timer to wake up your thread why don’t you manage your sleep and wake times all within the thread itself?

Maybe something like this (in pseudo-code, sorry I don’t know Delphi):

my_critical_thread()
{
    while (true) {
        time = get_current_time()
        do_work();
        time = interval - (get_current_time() - time)
        if (time > 0)
            sleep(time)
    }
}

This should get you very close to your target interval if the thread is set to critical priority, assuming the work you do on each iteration leaves time for your other threads and the rest of the system to do their thing.

Good luck.