ssrs barcode image Speech interpolation and prediction in Software

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14.7.10 Speech interpolation and prediction
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Because of the intermittent nature of speech, a speech transmission channel lies inactive for a considerable fraction of the time it is in use. A number of factors contribute to this. The talk-listen nature of a normal two-way telephone conversation means that transmission in any one direction occurs only about 50 percent of the time. In addition, the pauses between words and phrases may further decrease this to about 33 percent. If further allowance is made for end party delays such as the time required for a party to answer a call, the average fraction of the total connect time may drop to as low as 25 percent. The fraction of time a transmission channel is active is known as the telephone load activity factor, and for system design studies, the value of 0.25 is recommended by Comit Consutatif Internationale T l graphique et T l phonique (CCITT), although higher values are also used (Pratt and Bostian, 1986). The point is that for a significant fraction of the time the channel is available for other transmissions, and advantage is taken of this in the form of demand assignment known as digital speech interpolation.
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Digital speech interpolation may be implemented in one of two ways, these being digital time assignment speech interpolation (digital TASI) and speech predictive encoded communications (SPEC).
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Digital TASI.
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The traffic-burst format for an INTELSAT burst carrying demand-assigned channels and preassigned channels is shown in Fig. 14.21. As mentioned previously, the demand-assigned channels utilize digital TASI, or what is referred to in the INTELSAT nomenclature as DSI, for digital speech interpolation. These are shown by the block labeled interpolated in Fig. 14.21. The first satellite channel (channel 0) in this block is an assignment channel, labeled DSI-AC. No traffic is carried in the assignment channel; it is used to transmit channel assignment information as will be described shortly. Figure 14.22 shows in outline the DSI system. Basically, the system allows N terrestrial channels to be carried by M satellite channels, where N M. For example, in the INTELSAT arrangement, N 240 and M 127. On each incoming terrestrial channel, a speech detector senses when speech is present, the intermittent speech signals being referred to as speech spurts. A speech spurt lasts on average about 1.5 s (Miya, 1981). A control signal from the speech detector is sent to the channel assignment unit, which searches for an empty TDMA buffer. Assuming that one is found, the terrestrial channel is assigned to this satellite channel, and the speech spurt is stored in the buffer, ready for transmission in the DSI subburst. A delay is inserted in the speech circuit, as shown in Fig. 14.22, to allow some time for the assignment process to
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Intelsat traffic burst structure. (Courtesy of Intelsat, 1983. With permission.)
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Digital speech interpolation. DSI digital noninterpolation.
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digital speech interpolation; DNI
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be completed. However, this delay cannot exactly compensate for the assignment delay, and the initial part of the speech spurt may be lost. This is termed a connect clip. In the INTELSAT system an intermediate step occurs where the terrestrial channels are renamed international channels before being assigned to a satellite channel (Pratt and Bostian, 1986). For clarity, this step is not shown in Fig. 14.22. At the same time as an assignment is made, an assignment message is stored in the assignment channel buffer, which informs the receive stations which terrestrial channel is assigned to which satellite channel. Once an assignment is made, it is not interrupted, even during pauses between spurts, unless the pause times are required for another DSI
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channel. This reduces the amount of information needed to be transmitted over the assignment channel. At the receive side, the traffic messages are stored in their respective satellite-channel buffers. The assignment information ensures that the correct buffer is read out to the corresponding terrestrial channel during its sampling time slot. During speech pauses when the channel has been reassigned, a low-level noise signal is introduced at the receiver to simulate a continuous connection. It has been assumed that a free satellite channel will be found for any incoming speech spurt, but of course there is a finite probability that all channels will be occupied and the speech spurt lost. Losing a speech spurt in this manner is referred to as freeze-out, and the freeze-out fraction is the ratio of the time the speech is lost to the average spurt duration. It is found that a design objective of 0.5 percent for a freeze-out fraction is satisfactory in practice. This means that the probability of a freeze-out occurring is 0.005. Another source of signal mutilation is the connect clip mentioned earlier. Again, it is found in practice that clips longer than about 50 ms are very annoying to the listener. An acceptable design objective is to limit the fraction of clips which are equal to or greater than 50 ms to a maximum of 2 percent of the total clips. In other words, the probability of encountering a clip that exceeds 50 ms is 0.02. The DSI gain is the ratio of the number of terrestrial channels to number of satellite channels, or N/M. The DSI gain depends on the number of satellite channels provided as well as the design objectives stated earlier. Typically, DSI gains somewhat greater than 2 can be achieved in practice. The block diagram for the SPEC system is shown in Fig. 14.23 (Sciulli and Campanella, 1973). In this method, the incoming speech signals are converted to a PCM multiplexed signal using 8 bits per sample quantization. With 64 input lines and sampling at 125 s, the output bit rate from the multiplexer is 8 64/125 4.096 Mb/s. The digital voice switch following the PCM multiplexer is timeshared between the input signals. It is voice-activated to prevent transmission of noise during silent intervals. When the zero-order predictor receives a new sample, it compares it with the previous sample for that voice channel, which it has stored, and transmits the new sample only if it differs from the preceding one by a predetermined amount. These new samples are labeled unpredictable PCM samples in Fig. 14.23a. For the 64 channels a 64-bit assignment word is also sent. A logic 1 in the channel for the assignment word means that a new sample was sent
Speech predictive encoded communications (SPEC).
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