foo_convolve - Impulse response convolver, 'Mother of all effects' plugin Options

[Garf]

Download for foobar2000 0.8.x (250K)
Please see the official additional components page for the foobar2000 0.9.x compatible version.

This DSP component takes impulse response and does a
fast convolution of the sound data with that impulse
response.

Basically, it allows quick computation of any combination of
linear effects, such as equalization, echo, flange, reverb,
phase shift and so on. It can transform the sound to exactly
the image you would get as where the impulse response was
recorded, allowing you to get an accurate reproduction of
a big theather or stadium.

This effect is available as 'Acoustic Mirror' in Sound Forge.
A restricted version is available as 'Digital Convolution'
in Cool Edit.

A small manual is included in the readme.

Some discussion about convolutions:

http://www.hydrogenaudio.org/show.php/act/...t/ST/f/1/t/9116

Edit: Added download instructions for foobar2000 0.9.x compatible version.

This post has been edited by foosion: Sep 13 2007, 10:56

[DickD]

Can you add a cross channel feature? 

I don't think this is worth the hassle at all, and I can't see many if any practical situations in which this would be useful.

You can crossmix after the convolver BTW. If there's a need for a crossmixer I'll make one.

Garf,

I'm not saying there's much need for crossmixing, but I do have an application. In a previous post I suggested a simple stereo ambience effect (which allows adding stereo ambience to a mono recording as well as to mono) that could ideally be implemented with cross-channel convolution.

The special case (theta = 90° per pass) has two factors which I've tried rather laboriously with Cool Edit (Delay and Amplify/Crossmix/Invert features), but it does work rather nicely with just 4 passes:

1. Delay between passes (e.g. 1152 samples at 44.1 kHz = 26.112 ms)
2. Attenuation Coefficient per pass (e.g. -18 dB)

Assume you have L and R original sample values, you end up adding a delayed and attenuated version with channels swapped and one inverted to the input signal.

You can then pass this delayed signal through the same transformation/attenuation and add that too. (By this stage it's -L and -R but delayed and attenuated)

As an example, if you have a delta function on the left channel only, delay by 1152 and attenuate by 6dB per pass (rather "wet" sounding), the output pulse switches channel with each pass of the delay and inverts when going from right to left:

L(0) = 1.0000

R(1152) = 0.5000

L(2304) = -0.2500

R(3456) = -0.1250

L(4608) = 0.0625

My previous post showed this on an argand diagram (complex number) representation, including options to use non-90° steps per pass, which could also be represented using separate left and right convolution functions.

That defines the left channel's stereo convolution function for 4 passes (in practice -18 dB is a subtler, less wet effect, and sounds rather natural). I must get round to posting some samples.

The right channel's stereo convolution function for 4 passes of -6 dB per pass, 1152 samples delay per pass would be:

R(0) = 1.0000

L(1152) = -0.5000

R(2304) = -0.2500

L(3456) = 0.1250

R(4608) = 0.0625

When used subtly, the effect is a rather polished sheen and space around recordings that can be quiet pleasant when used sparingly yet doesn't colour the sound's frequency response like some measured room effects or comb-filter stereo effects.

Compared to the Cool Edit approach I've used a couple of times now, I could actually save some steps in achieving this effect by generating the same-channel file with one convolution using your foo_convolve as it stands, then mix-pasting with a file created by convolving a cross-mixed version with a different convolution function (and auto gain off).

Further applications of crossmixed FIR convolution include playback of binaural material over loudspeakers (probably requires rather exact listener location, so probably not very useful) and probably some kinds of cancellation of room-interaction with loudspeakers (you need to generate the inverse convolution function to cancel out the effects). I suspect the frequency-flattening part of the latter doesn't need cross-mix, and cross mix might not add much value to it.

By the way, Garf, I note in the Readme.txt that you wrote it 18000 years in the future:

Copyright © 20003 Gian-Carlo Pascutto

Just thought I'd point it out.

DickD