Hi Ivano,
> I'm trying to include a time delay line in a tool dedicated
> to audio in real time built with PortAudio.
> I'm not sure I should have to try with
> Fl::add_timeout. Could someone give some suggestions about that?
> Thank you in advance
How much of a delay do you want to introduce?
I don't think ::add_timeout() is the way to go.
If we are just talking about fairly short delays, e.g. for chorus /
flange effects, or perhaps even quite long delays for echo effects, then
I'd proceed as follows...
- I'd be driving the PA (PortAudio) stream from a large circular buffer.
- Normally, we'd read out of that buffer at just one point (the "head"
of the buffer) and feed that into the PA output.
- To introduce a delay, we instead read out of the buffer at 2 points,
the "head" and some point that was already passed. We then add these two
values together before writing them to the PA output.
- Assuming we are sampling at 44.1kHz (safe guess!) then to introduce a
100ms delay we'd sample from the two positions "head" and "head - 4410"
in the buffer. That should produce a nice "chorus" effect.
- If we assume the buffer has 64K samples, then we are good for any
delay up to about 1.5 seconds, so we can get a nice echo effect by
simply doing the above with a bigger offset.
- For more subtle effects, we can read out from more than 2 points, and
scale each by a small factor before summing them all together. We can
get a nice "room ambience" by reading out from 5 or 6 points, then
summing them like:
(head * 1.0) + (p2 * 0.4) + (p3 * 0.3) + (p4 * 0.2) + (p5 * 0.1)
- If we do the above, but slowly vary the offset from head to the
sub-points P2 to P5, we can get a nice flanging effect. Vary the offsets
more quickly (and by differing amounts) and you can get some nice spacey
phase effects...
- If you can cope with a lower quality, then a lower sampling rate means
a smaller buffer can be used quite safely. The "CD" quality 44.1kHz is
probably higher than is really needed for simple effects processing...
Hope that helps,
--
Ian
SELEX Galileo Ltd
Registered Office: Sigma House, Christopher Martin Road, Basildon, Essex SS14
3EL
A company registered in England & Wales. Company no. 02426132
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