[linux-audio-dev] PTAF link and comments

[David Olofson]

On Monday 03 February 2003 15.12, Sami P Perttu wrote:
> On Mon, 3 Feb 2003, David Olofson wrote:
> > > As a plugin author, having to implement
> > > both is a pain. I use code generation to make both functions.
> >
> > You don't necessarily *have* to implement both. Even the
> > primitive FX plugin API of Audiality have these variants:
> >
> > 	void (*process)(struct ADY_plugin *p,
> > 			int *buf, unsigned frames);
> > 	void (*process_r)(struct ADY_plugin *p,
> > 			int *in, int *out, unsigned frames);
> > 	void (*process_m)(struct ADY_plugin *p,
> > 			int *in, int *out, unsigned frames);
> >
> > ...and you only *have* to provide *one* - any variant will do. If
> > you don't provide all of them, the host "SDK" will emulate the
> > others using the ones that are provided.
>
> I think this is bad.

It's a primitive performance hack. Once upon a time, these things 
actually mattered... ;-)


> There should be just one process() function,
> which could be given two gain values, one for previous output and
> another for the plugin's own output. Plugins would do
>
> out[i] = previous_gain * out[i] + gain * myoutput;
>
> When previous gain is zero this is almost equivalent to replace
> except that NANs and maybe some other values will screw you.
> Solution: forbid NANs as output and maybe cleanse buffers
> periodically. And initially, of course.

Well, that cover's all the cases, and you *can* still optimize with 
alternative inner loops if you like. Just check for 0.0 gains - but 
remember that the gain controls will most probably be normal *ramped* 
controls, so you can't just check the initial value.

Looks like a great idea.

As to NANs, I think you're pretty screwed if you get them in an audio 
stream anyway. Plugins that generate NANs are broken, as are hosts 
that allow plugins to operate on garbage buffers.


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

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