mixture of voiced and unvoiced excitation sound more natural and smooth, three different modes of voiced speech (Mixed Voiced-1, Mixed Voiced-2, Full Voiced) are differentiated. The degree of voicing is determined by the value of the normalized autocorrelation function at a shift of one pitch period. Depending on the chosen mode, different amounts of band-pass Gaussian noise are added to the synthesized harmonic signal by the decoder. Voiceless (VXC) Residual Coding Unvoiced segments are encoded according to the CELP scheme, which is also referred to as vector excitation coding (VXC). The CELP coding in HVXQ is performed using only a stochastic codebook. In other CELP codecs, a dynamic codebook is used additionally to perform long-term prediction of voiced segments. However, since HVXC does not use CELP for voiced segments, the dynamic codebook is omitted from the design. See also Opus (audio format) References MPEG-4 Speech codecs