dsp system objects: demod (QPSKDemodulator) output must be negated as input to quantizer (dsp.Scala​rQuantizer​Encoder) for correct results

Hi,

Working with DSP system objects for first time and integrated a demod with a quantizer. To get correct results, I had to negate the demod output into the quantizer.

Can someone explain why this was needed? It was not at all documented nor seemingly intuitive.

Thanks,

Jerry

Code snippet

Demod     = comm.QPSKDemodulator('BitOutput', true, ...
          'DecisionMethod', 'Log-likelihood ratio', ...
          'VarianceSource', 'Input port');
Quantizer = dsp.ScalarQuantizerEncoder('Partitioning', 'Unbounded', ...
          'BoundaryPoints', -bound:bound, 'OutputIndexDataType', 'uint8');

      .
      .
      .
 dmd_sig = Demod.step(rx_sig, noise_var); % demod
        
 % need to negate the dmd_signal???
 llr = Quantizer.step(-dmd_sig);