Hi,

I have discovered a subtle bug in ftconv during a comparison with a
convolver that I'm writing from scratch in lisp.

The last two bins of the output buffer in `multiply_fft_buffers'
(ftconv.c:52) are always incremented (never zeroed) because

    memset(outBuf, 0, sizeof(MYFLT)*(partSize - 2));
                                    ^^^^^^^^^^^^^^

but at the end of the function, after the while loop:

    ...
    while (outBufPtr < outBufEndPm2) {
      ...
    }
    outBufPtr[0] += re;
    outBufPtr[1] += im;
    ...

The fix is trivial, the correct size for memset is

    memset(outBuf, 0, sizeof(MYFLT)*partSize);

Here is an example to "hear" the bug:

<CsoundSynthesizer>
<CsOptions>
-odac
</CsOptions>
<CsInstruments>

sr     = 48000
ksmps  = 64
nchnls = 1
0dbfs  = 1

instr 1

iplen = 32
idel = iplen/sr

;; Input
ain rand 1

;; Direct convolution
adirect dconv ain, 64, 1
adirect delay adirect, idel

;; Partitioned convolution
apart ftconv ain, 1, iplen

;; Silence after the fix
out (adirect - apart)*40

endin

</CsInstruments>
<CsScore>
f1 0 64 21 6
i1 0 $INF
e
</CsScore>
</CsoundSynthesizer>


tito

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