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Order of access to DSP signals important?

August 13, 2008 | 12:23 pm

Hey all-

I noticed the following. If I have a 2-input, 1-output MSP external,
and I access the vector array inside the DSP loop like so:

buf[r_indx] = *++in1;
tmp = *++in2;
[...]
*++out = buf[w_indx];

Everything is fine. But if I access the second input out of order like
so:

buf[r_indx] = *++in1;
[...]
*++out = buf[w_indx];
tmp = *++in2;

tmp seems to never take the values of the second input signal, it’s
always zero.

Anyone else noticed this, or am I crazy/mistaken? Can anyone amplify on
the situation?

Thanks, Charles


August 13, 2008 | 1:50 pm

On 2008 Aug 13, at 7:23 AM, Charles Turner wrote:

> Hey all-
>
> I noticed the following. If I have a 2-input, 1-output MSP external,
> and I access the vector array inside the DSP loop like so:
>
> buf[r_indx] = *++in1;
> tmp = *++in2;
> [...]
> *++out = buf[w_indx];
>
> Everything is fine. But if I access the second input out of order like
> so:
>
> buf[r_indx] = *++in1;
> [...]
> *++out = buf[w_indx];
> tmp = *++in2;
>
> tmp seems to never take the values of the second input signal, it’s
> always zero.
>
> Anyone else noticed this, or am I crazy/mistaken? Can anyone amplify
> on
> the situation?

There is an optimization in MSP to share and re-use signal vectors,
and it sounds like this is causing your trouble. Try setting the
Z_NO_INPLACE flag to turn the optimization off. Something like this
in your new method:
x->x_ob.z_misc = Z_NO_INPLACE;

best,
Tim


August 13, 2008 | 2:12 pm

On Wed, 13 Aug 2008 08:50:42 -0500, Timothy Place wrote:
> There is an optimization in MSP to share and re-use signal vectors,
> and it sounds like this is causing your trouble. Try setting the
> Z_NO_INPLACE flag to turn the optimization off. Something like this
> in your new method:
> x->x_ob.z_misc = Z_NO_INPLACE;

Thanks for the info, Tim. The order isn’t critical for me, so it was
easily addressed. Did a few hours of head scratching before I figured
out what I was doing wrong, though!

Best, Charles


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