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Table 2-7 Silence Compression Algorithms
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Voice activity detector (VAD)
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Determines if the input signal is speech or background noise If the signal is declared speech, it is coded at the full fixed bit rate; if the signal is declared noise, it is coded at a lower bit rate As appropriate, no bits are transmitted Mechanism invoked at the receiver end to reconstruct the main characteristic of the background noise
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Comfort noise generation (CNG)
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The CNG mechanism must be designed in such a way that the encoder and decoder remain synchronized, even when there are no bits transmitted during some interval This enables smooth transitions between active and nonactive speech segments
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Delay The delay of a speech-coding system usually consists of three major
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Frame delay Speech processing delay Bridging delay
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Typically, low-bit-rate vocoders process a frame of speech data at a time, so that the speech parameters can be updated and transmitted for every frame Hence, before the speech can be analyzed, it is necessary to buffer a frame s worth of speech samples The resulting delay is called algorithmic delay It is sometimes necessary to analyze the signal beyond the frame boundary (this is referred to as look-ahead); here, additional speech samples need to be buffered, with additional concomitant delay Note that this is the only implementation-independent delay (Other delay components depend on the specific implementation, for example, the power that the processor used to run the algorithm, the kind of RAM used, and so on) Algorithmic delays are unavoidable; hence, they need to be considered as part of the delay budget by the planner The second major component of the delay originates from the processing time it takes the encoder to analyze the speech and the processing time required by the decoder to reconstruct the speech This processing delay depends on the speed of the hardware used to implement the vocoder The combination of algorithmic and processing delays is called the one-way system delay The maximum tolerable value for the one-way system delay is 400 ms if there are no echoes, but for ease and efficiency of communication, it is preferable to have the one-way delay below 200 ms If there are echoes, the tolerable one-way delay is 20 to 25 ms; therefore, the use of echo cancellation is often necessary In applications such as teleconferencing, it may be necessary to bridge several callers using a multipoint control unit (MCU) to enable each person to communicate This requires decoding each bit stream, summing the decoded signals, and then re-encoding the combined signal This process doubles the delay and at the same time it reduces the speech quality because of the multiple (tandem) encodings Given the previous observation, a bridged system can tolerate a maximum one-way delay of 100 ms because the bridging will result in the doubling of the one-way system delay to 200 ms
Technologies for Packet-Based Voice Applications
Algorithm s Complexity Vocoders are often implemented on DSP hardware Complexity can be measured in terms of computing speed in MIPS, random access memory (RAM), and read-only memory (ROM) Complexity determines cost; hence, in selecting a vocoder for an application, the developer must make an appropriate choice When the vocoder shares a processor with other applications, the developer must decide how much of these resources to allocate to the vocoder Vocoders utilizing less than 15 MIPS are considered low complexity; those using 30 MIPS or more are considered high complexity As noted, increased complexity results in higher costs and greater power usage Power usage is an important consideration in portable applications since greater power usage implies reduced time between battery recharges or using larger batteries, which in turn means more expense and weight Quality The measure of speech quality used in comparisons is how well the
speech sounds for ideal conditions, namely, clean speech, no transmission errors, and only one encoding (Note, however, that in the real world, these ideal conditions are often not met because there could be large amounts of background noise such as street noise, office noise, air-conditioning noise, and so on) Table 2-8 shows the quality for the major coding schemes being utilized in voice over data networks How well the vocoder performs under adverse conditions (for example, what happens when there are channel errors or the loss of entire frames; how does the vocoder sound when the speech is encoded and decoded in tandem, as is the case in a bridging application; how does it sound when transcoding with another standard vocoder; how does it sound for a variety of languages) is the question that the standards bodies try to answer during the testing phase of the standards drafting and generation process The accepted measure of quality is MOS With MOS, the scores of multiple listeners are averaged to obtain a single figure of merit Table 2-9
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