PERCEPTUAL NOISE WEIGHTING

• Noise-weighting: shaping the spectrum to hide noise introduced by imperfect analysis and modeling techniques (essential in speech coding).
  
  
• Humans are sensitive to noise introduced in low-energy areas of the spectrum.
  
  
• Humans tolerate more additive noise when it falls under high energy areas the spectrum. The amount of noise tolerated is greater if it is spectrally shaped to match perception.
  
  
• We can simulate this phenomena using "bandwidth-broadening":
  
  
  
• Simple Z-Transform interpretation:
  
  
  which can be implemented by evaluating the Z-Transform around a contour closer to the origin in the z-plane: H_nw(z) = H(az).

Key points:

• Used in many speech compression systems (Code Excited Linear Prediction).
  
  
• Analysis performed on bandwidth-broadened speech; synthesis performed using normal speech. Effectively shapes noise to fall under the formants.