VST Code generation issues with element-wise multiplication

The following code produces audible discontinuities when compiled to a VST:

gains = [0.5, 0.725];
out = in .* gains;

But this works fine

gains = [0.5, 0.725];
out = [in(:,1) * gains(1), in(:,2) * gains(2)];

Is this a known issue with code generation?

Answer 1

[VBBV]

gains = [0.5, 0.725];
in = rand(8)

in = 8×8

0.8849 0.9486 0.2943 0.5330 0.5551 0.2483 0.9058 0.8506 0.6508 0.8914 0.3171 0.4493 0.6775 0.0840 0.3678 0.6322 0.2078 0.4190 0.9428 0.2469 0.3487 0.5543 0.0761 0.7525 0.8023 0.4535 0.7250 0.9433 0.3312 0.5522 0.5628 0.8056 0.2804 0.6671 0.6911 0.2097 0.8895 0.6512 0.3580 0.4607 0.2275 0.3705 0.9019 0.3368 0.4651 0.0667 0.7093 0.3143 0.8744 0.2188 0.9651 0.2883 0.7966 0.2366 0.2595 0.6238 0.6302 0.3316 0.5598 0.3719 0.0362 0.0559 0.2016 0.5517

out = [in(:,1) * gains(1), in(:,2) * gains(2)]

out = 8×2

0.4425 0.6877 0.3254 0.6463 0.1039 0.3038 0.4011 0.3288 0.1402 0.4837 0.1137 0.2686 0.4372 0.1587 0.3151 0.2404

out = in .* gains  % check the matrix multiplication rule 

Arrays have incompatible sizes for this operation.

[VBBV]

when the cols of 1st matrix is equal to rows of 2nd matrix then it works fine (not exceeding the maximum array size value)

[Robert]

Thanks for the quick response. I agree with this, but the default VST is 2 in, 2 out. This code should work if that were true:

gains = [0.5, 0.75];
in = [ones(5,1), ones(5,1)*2];
out = in .* gains

out = 5×2

0.5000 1.5000 0.5000 1.5000 0.5000 1.5000 0.5000 1.5000 0.5000 1.5000

Is it possible that the VST doesn't always default to stereo like the documentation says?

"By default, both the input to and output from the process method have two channels (columns). The number of input rows (frame size) passed to process is determined by the environment in which it is run. The output must have the same number of rows as the input.", found here:

https://www.mathworks.com/help/audio/gs/audio-plugins-in-matlab.html

[Jimmy Lapierre]

I was able to reproduce the issue. I will investigate further.

>> p=loadAudioPlugin('GainTest.dll')
p = 
VST plugin 'GainTest'  2 in, 2 out

>> process(p,ones(4,2))
ans =
0.5000    0.7250
1.0000    1.0000
1.0000    1.0000
1.0000    1.0000

Please use the workaround in the meantime.

p.s. modifying the input in-place is allowed, so it could look like this:

function x = process(~,x)
    gains = [0.5, 0.725];
    x(:,1) = gains(1)*x(:,1);
    x(:,2) = gains(2)*x(:,2);
end