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a total algorithmic delay of 375 milliseconds Of course, other small delays will take place within the coder itself as a result of the processing effort involved Each frame is passed through a high-pass filter to remove any DC component and then is divided into 4 subframes of 60 samples each Various operations are performed on these subframes in order to determine the appropriate filter coefficients Algebraic Code-Excited Linear Prediction (ACELP) is used in the case of the lower bit rate of 53 Kbps and Multipulse Maximum Likelihood Quantization (MP-MLQ) in the case of the higher rate of 63 Kbps The information transmitted to the far end includes linear prediction coefficients, gain parameters, and excitation codebook index values The information transmitted comprises 24-octet frames in the case of transmission at 63 Kbps and 20-octet frames in the case of transmission at 53 Kbps Normal conversation involves significant periods of silence (or at least silence from one of the parties) During such periods of silence, it is desirable not to consume significant bandwidth by transmitting the silence at the same rate as speech is transmitted For this reason, G7231 Annex A specifies a mechanism for silence suppression whereby Silence Insertion Description (SID) frames can also be used These are only 4 octets in length, which means that transmission of silence occupies about 1 Kbps This is significantly better than G711 where silence is still transmitted at 64 Kbps Therefore, three different types of frame can be transmitted by using G7231: one for 63 Kbps, one for 53 Kbps, and an SID frame Within each frame, the two least significant bits of the first octet indicate the frame size and the codec version in use (as shown in Table 3-1) G7231 has an MOS of about 38, which is good considering the vastly reduced bandwidth that it uses G7231 does, however, have the disadvantage of a minimum 375-millisecond delay at the encoder Although this delay is well within the bounds of what is acceptable for good-quality
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Table 3-1 G7231 frame size and codec version
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Bits 00 01 10 11
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Meaning High-rate speech (63 Kbps) Low-rate Speech (53 Kbps) SID frame
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Octets/frame 24 20 4 N/A
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speech, we must remember that it is round-trip delay that is important, not just one-way delay Moreover, there will be various other delays in the network, including processing delays and queuing delays such as at routers in a VoIP network, for example
G729
The basic ITU-T Recommendation G729 specifies a speech coder that operates at 8 Kbps This coder uses input frames of 10 milliseconds, corresponding to 80 samples at a sampling rate of 8,000 Hz G729 also includes a 5-millisecond look-ahead, resulting in an algorithmic delay of 15 milliseconds (significantly better than G7231) From each input frame, the coder determines linear prediction coefficients, excitation codebook indices, and gain parameters These pieces of information are transmitted to the far end in 80-bit frames Given that the input signal corresponds to 10 milliseconds of speech and results in a transmission of 80 bits, the transmitted bit rate is 8 Kbps G729 offers an MOS of about 40 Figure 3-9 shows a high-level block diagram of the G729 encoder G729 Annex A G729 is a complex codec In order to reduce the complexity in the algorithm, a number of simplifications were introduced in Annex A to G729 These include simplified codebook search routines and a simplification to the postfilter at the decoder among other things G729A uses exactly the same transmitted frame structure as G729 and therefore uses the same bandwidth In other words, the encoder may be operating according to G729, while the decoder may operate using G729A or vice versa Note that G729A can result in slightly lower quality than G729 G729A provides a MOS of about 37 G729 Annex B Annex B to G729 is a recommendation for voice activity detection (VAD), discontinuous transmission (DTX), and comfort noise generation (CNG) VAD is simply the decision as to whether voice or noise is present at the input The decision is based on an analysis of several parameters of the input signal Note that the determination is not done simply on the basis of one frame; rather, the determination is made on the basis of the current frame, plus the preceding two frames This mechanism ensures that transmission occurs for at least two frames after a person has stopped speaking The next decision is whether to send nothing at all or to send a SID frame The SID frame contains some information to enable the decoder to
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