ANSI SCTE 24-21 pdf free download
ANSI SCTE 24-21 pdf free download.BV16 Speech Codec Specification for Voice over IP Applications in Cable Telephony.
In Figure 1 above, the outer layer (including the two short-term predictors and the short-term noise feedback filter) follows the structure of the conventional NFC codec. The TSNFC structure in Figure 1 is obtained by replacing the simple scalar quantizer in the conventional (single-stage) NFC structure by a “predictive quantizer” that employs long-term prediction and long-term noise spectral shaping. This “predictive quantizer” is represented by the inner feedback ioop in Figure 1, including the long-term predictor and long-term noise feedback filter. This inner feedback loop uses an alternative but equivalent conventional NFC structure, where N,(z) represents the filter whose frequency response is the desired noise shape for long-term noise spectral shaping. In the outer layer, the short-term noise feedback filter F(z) is usually chosen as a bandwidth-expanded version of the short-term predictor I (z). The choice of different NFC structures in the outer and inner layers is based on complexity consideration. By combining two stages of NFC in a nested ioop, the TSNFC in Figure 1 can reap the benefits of both short-term and long-term prediction and also achieve short-term and long-term noise spectral shaping at the same time.
It is natural and straightforward to use a scalar quantizer in Figure 1. However, to achieve better coding efficiency, a vector quantizer is used in BVI6. In the Vector Quantization (VQ) codebook search. the ii(n) vector cannot be generated before the VQ codebook search starts. Due to the feedback structure in Figure 1, the elements of zi(n) from the second element on will depend on the vector-quantized version of earlier elements. Therefore, the VQ codebook search is performed by trying out each of the candidate codevectors in the VQ codebook (i.e. fixing a candidate uq(n) vector first), calculating the corresponding u(n) vector and the corresponding VQ error q(n) = u(n) — uq(iz). The VQ codevector that minimizes the energy of q(n) within the current vector time span is chosen as the winning codevector, and the corresponding codebook index becomes part of the encoder output bit stream for the current speech frame.The TSNFC decoder structure is simply a quantizer decoder followed by the two feedback filter structures involving the long-term predictor and the short-term predictor, respectively, shown on the right half of Figure 1. Thus, the TSNFC decoder is similar to the decoders of other predictive coding techniques such as Adaptive Predictive Coding (APC), Multi-Pulse Linear Predictive Coding (MPLPC), and Code-Excited Linear Prediction (CELP). If the alternative NFC structure in the inner feedback loop of Figure 1 is also used in the outer feedback loop, an alternative TSNFC codec structure is obtained, as shown in Figure 2 below. Here N, (z) represents a short-term filter whose frequency response is the desired noise shape for short-term noise spectral shaping. The codec structure in Figure 2 is mathematically equivalent to the structure in Figure 1, but it allows direct specification of the short-term noise spectral shape as defined by N,(z). This can be an advantage in some applications.ANSI SCTE 24-21 pdf download.
