Systems and methods are described that apply a noise feedback coding (NFC) technique at the encoder of a delta modulation codec, such as a Continuously Variable Slope Delta Modulation (CVSD) codec, so as to shape the spectrum of the coding noise produced thereby in such a way that the speech quality of the delta modulation decoder output is enhanced. The techniques described herein are not limited to delta modulation codecs and may also be applied to any sample-by-sample codec, including a G.711 μ-law codec, a linear pulse code modulation (LPCM), or any other of a wide variety of sample-by-sample codecs, to improve the audio quality of the decoder output thereof.