C5515 eZdsp Buffer Switch problem

Hi,

i'm using c5515 eZdsp in order to acquire a dsbsc modulated signal. My goal is to demodulate the signal.

The signal acquisition is made using ping pong buffer (only input left channel), taken from C5515_Audio_Filter demo code.

My algo is: FFT, frequency ***, IFFT.

The demodulation succeeds, but in the transition from an input buffer from another i have a bad spike that distances my output signal:

in main.c:

while(1)
        if(CurrentRxL_DMAChannel == 2)
        {
                buff_copy(RcvL1,BufferL1,XMIT_BUFF_SIZE);
                dsb_sc_demod(BufferL1);
                buff_copy(BufferL1,&FilterOutL1[0], XMIT_BUFF_SIZE);
        }
        else if(CurrentRxL_DMAChannel == 1)
        {
                buff_copy(RcvL2,BufferL2,XMIT_BUFF_SIZE);
                dsb_sc_demod(BufferL2);
                buff_copy(BufferL2,&FilterOutL2[0], XMIT_BUFF_SIZE);
        }
}

dsb_sc_demod():

void dsb_sc_demod(Int16 *input){
    Uint8 i;
    Uint8 idx;
    const Uint8 iMAX = FFT_LENGTH/2;
    if(freq == FREQ_32)
        idx = CARRIER_32_IDX;
    else
        idx = CARRIER_42_IDX;
    do_fft(input, realL, imagL, 1);
    for(i = 0 ; i < iMAX; i++){
        *(realL + i) = *(realL + i + idx);
        *(imagL + i) = *(imagL + i + idx);
    }
    for (i = 1; i < FFT_LENGTH/2; i++) {
        *(realL + FFT_LENGTH - i) = *(realL + i );
        *(imagL + FFT_LENGTH - i) = -*(imagL + i );
    }
    do_ifft(realL, imagL, input, 0);
}

In the following image you will see a spike:

Do you have any suggestion to avoid this?

Thanks

Paolo