[linux-audio-dev] XAP: Some thoughts on control ramping

[David Olofson]

On Tuesday 21 January 2003 00.41, David Olofson wrote:
[...]
> > > So, alternative 1; RAMP events only:
[...]
> > No branch is needed,
>
> Excellent! :-)

I just realized something that might be worth pointing out.

If you send multiple RAMP events for the same control at the same 
timestamp, only the last one will have any effect. This is because 
RAMP events change only the delta, and it takes sample frame loops to 
actually change the value.

The problem is that if you send multiple RAMP events that are so 
close together they end up getting the same timestamp, results won't 
be what you want.

For example, setting the initial level of an envelope and then 
setting up the delay or attack phase won't work, unless you wait for 
one frame before sending the RAMP event for the delay or attack 
phase. Similarly, an attack duration of 0 would result in the decay 
starting at the current value, instead of the attack peak level.

Note that waiting for one frame after saying "ramp to y over 0 
frames" is actually illegal by the definition! You're one frame past 
the aim point when the next event arrives, so results are undefined.

So, it seems like we'll need that 0 test anyway. It should obviously 
apply the target value instantly, so that later events will work even 
if they land at the same timestamp. It doesn't have to set the delta 
at all, as sending a 0 duration means you *have* to send another 
event with the same timestamp anyway.

Resulting Audiality code (how unusual! ;-):

	if(ev->arg2)
	{
		v->ic[ev->index].dv = ev->arg1 << RAMP_BITS;
		v->ic[ev->index].dv -= v->ic[ev->index].v;
		v->ic[ev->index].dv /= ev->arg2 + 1;
	}
	else
		v->ic[ev->index].v = ev->arg1 << RAMP_BITS;


//David Olofson - Programmer, Composer, Open Source Advocate

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