Pulse or digital communications – Systems using alternating or pulsating current – Antinoise or distortion
Reexamination Certificate
1999-12-03
2003-09-23
Phu, Phoung (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Antinoise or distortion
C375S222000, C375S254000, C455S067700
Reexamination Certificate
active
06625226
ABSTRACT:
The present invention relates generally to the communication of digital information, such as speech data communicated in a cellular, or other radio, communication system. More particularly, the present invention relates to a variable bit rate coder, and an associated method, by which to encode the digital information at a selected bit rate. Selection of the coding rate is made responsive to indicia of actual coding performance, subsequent to encoding of the information at more than one coding rate.
BACKGROUND OF THE INVENTION
Advancements in communication technologies have permitted the introduction of, and popularization of, new types of, and improvements in existing, communication systems. Increasingly large amounts of data are permitted to be communicated at increasing thruput rates through the use of such new, or improved, communication systems. As a result of such improvements, new types of communications, requiring high data thruput rates, are possible. Digital communication techniques, for instance, are increasingly utilized in communication systems to communicate efficiently via digital data, and the use of such techniques has facilitated the increase of data thruput rates.
When digital communication techniques are used, information which is to be communicated is digitized. For example, when the information is formed of speech, such as that generated by a user using a mobile station of a cellular communication system, the speech is digitized, then signal processing operations are performed upon the digitized speech, and, then, quantization operations are performed upon the digitized speech. The result forms a compressed bit stream, referred to as speech data.
Conventionally, the speech initially in the form of a speech waveform, is first partitioned into a sequence of successive frames of constant length. Then, the operations noted above are performed to form the compressed bit stream which is sometimes formatted into packets of data. Such packets typically also include groups of bits which specify parameters used, at a receiving station to reconstruct the speech.
In a conventional analysis-by-syntheses (“AbS”) coding of speech, the speech waveform is partitioned into a sequence of successive frames and each frame has a fixed length and is partitioned into an integer number of equal length subframes. The encoder generates an excitation signal by a trial and error search process whereby each candidate excitation for a subframe is applied to a synthesis filter and the resulting segment of synthesized speech is compared with a corresponding segment of target speech. A measure of distortion is computed and a search mechanism identifies the best (or nearly-best) choice of excitation of each subframe among an allowed set of candidates. The candidates are sometimes stored as vectors in a codebook; in this case, the coding method is called CELP (code excited linear prediction). At other times, the candidates are generated as they are needed for the search by a predetermined generating mechanism; this case includes in particular multipulse linear predictive coding (MP-LPC) or algebraic code excited linear prediction (ACELP). The bits needed to specify the chosen excitation subframe are part of the package of data that is transmitted to a receiving station in each frame. Usually the excitation is formed in two stages, where the first approximation to the excitation subframe is selected by the ab
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ve-described procedure, and then a modified target signal for the subframe is formed as the new target for a second AbS search operation Depending on the periodic or aperiodic character of the speech, different coding strategies can be employed. In order to eliminate as much redundancy as possible in coding the excitation signal for each frame, it is often desirable to classify the frames into categories. The coding method can then be tailored to each category.
In voiced speech, the energy peaks of the smoothed residual energy contour generally occur at pitch period intervals and correspond to pitch pulses. Pitch here refers to the fundamental frequency of periodicity in a segment of voiced speech and pitch period refers to the fundamental period of periodicity. In some transitional regions of the speech signal, the waveform does not have the character of being periodic or stationary random and often it contains one or more isolated energy bursts, as in plosive sounds. The unvoiced class consists of frames which are aperiodic and where the speech appears random-like in character, without strong isolated energy peaks. The silent class refers to frames where speech is absent but some background noise may be present.
In a typical implementation, the sampling rate is 8000 samples per second, the frame size is 160 samples. Each frame is classified into one of several classes, e.g., voiced, unvoiced, silence, transition. Other ways of classification include use of two voicing classes, e.g., weakly voiced, and strongly voiced voicing classes.
Coding techniques in general can be categoried according to several different manners by which to encode a frame of speech.
For instance, one category of encoding is referred to as fixed bit-rate coding. In a fixed bit-rate coding technique, every encoded frame of speech encoded by a particular fixed bit-rate coding technique is formed of the same number of bits. That is to say, an encoded frame of speech, encoded by a fixed bit-rate coding technique, is formed of a fixed number of bits.
In a discontinuous transmission (DTX) technique, a determination is made whether a frame of speech which is to be encoded is formed of active speech bits. If the frame is determined to be formed of active speech bits, a fixed bit allocation is applied to each of such frames. If a determination is made that the frame does not contain active speech bits, a reduced bit allocation is applied to such frames, such as “silent” frames.
In a dynamically-variable, bit-rate coding technique, each frame of speech is encoded using a different number of bits. In this technique, a large range of possible bit allocations of the encoded frame is possible, e.g., any integral number of bits up to some maximum value.
And, in a multi-class, variable bit-rate coding technique, each frame of speech is assigned, by way of a class selection procedure, to be one amongst a set of allowed classes. Each of such classes is associated with a particular allocation of bits for various parameters of the frame. And, all frames assigned to a single class have the same bit allocation. Class selection of a speech frame is based, for instance, upon a phonetic classification of the frame in which the major characteristics of the frame are classified according to the phonetic character of that frame of speech. More generally, a classifier is utilized to operate upon input speech applied to an encoder, once frame-formatted, or upon a linear prediction residual obtained from the input speech, to extract parameters better then combined to make a class decision. Typically, a relatively small number of classes, e.g., between three and six classes, are employed in speech coding when using a multi-class, variable bit-rate coding technique.
In some situations, different coding algorithms are applied to different classes. In some coders, two different classes may have the same total number of bits allocated for the frame but may differ in how the bits are allocated to different speech parameters of the frame. As long as all the classes do not have the same total bit allocation for the frame, a coder is considered to be a variable rate coder. In multi-class coders, each class has a different bit allocation so that any class selection mechanism controls the instantaneous bit rate of the coder. And, such a mechanism is referred to as a rate determination algorithm. The instantaneous bit rate at a particular time is merely the ratio of the number of bits allocated to the current frame divided by the time duration of the frame.
Fixed bit-rate coding techniques do not require a rate control mechanism and, therefore, are typi
Ahmadi Sassan
Cuperman Vladimir
Gersho Allen
Heidari Ryan
Linden Jan
Patel Milan I.
Phu Phoung
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