[linux-audio-dev] Fixed vs Floating point comparison.

[Juan Linietsky]

On Friday 17 October 2003 12:20, Paul Davis wrote:
>
> if you're writing software where you do this conversion more than once
> per interrupt, i think it probably needs a redesign. if the cost of
> the conversion is an appreciable fraction of the over
> cycles/interrupt, then and only then does it seem to make sense to me
> to consider *not* using float.
>
> --p

This is the case of a resampler in a polyphonic sampler (and other algorithms
such as multivoiced chorus and time domain pitchshifters), or synthesizers 
wich relies on prerendered oscillators. it ocurs N times per interrupt, being 
N the amount of voices active. A resampler needs a fractional increment which 
is calculated according to the voice pitch.
When you need to get the actual sample position (and/or the surrounding
ones if you are interpolating), a fractional -> integer conversion is needed.
I've seen this problem happening on existing software, which greatly limits
the CPU availability. Some examples of this are fluidsynth, csound and 
zynaddsubfx. If you compare them with software of this kind written
in fixed point (cheesetracker, legasynth, timidity), the later can easily
reach many times more the amount of virtual voices.

I have to add something in here, besides the float->int conversion,
there is another very common source of slowdowns in this kind of
software, which is using conditions inside the critical loops. These,
at minimum, will flush the processor pipeline (And I think something else
too but I coudlnt tell) until decoded.

Juan Linietsky