AU5549090A – Excitation pulse positioning method in a linear predictive speech coder
– Google Patents
AU5549090A – Excitation pulse positioning method in a linear predictive speech coder
– Google Patents
Excitation pulse positioning method in a linear predictive speech coder
Info
Publication number
AU5549090A
AU5549090A
AU55490/90A
AU5549090A
AU5549090A
AU 5549090 A
AU5549090 A
AU 5549090A
AU 55490/90 A
AU55490/90 A
AU 55490/90A
AU 5549090 A
AU5549090 A
AU 5549090A
AU 5549090 A
AU5549090 A
AU 5549090A
Authority
AU
Australia
Prior art keywords
phase
pulse
positions
excitation
frame
Prior art date
1989-05-11
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
AU55490/90A
Other versions
AU629637B2
(en
Inventor
Tor Bjorn Minde
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Telefonaktiebolaget LM Ericsson AB
Original Assignee
Telefonaktiebolaget LM Ericsson AB
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
1989-05-11
Filing date
1990-03-09
Publication date
1990-11-29
1990-03-09
Application filed by Telefonaktiebolaget LM Ericsson AB
filed
Critical
Telefonaktiebolaget LM Ericsson AB
1990-11-29
Publication of AU5549090A
publication
Critical
patent/AU5549090A/en
1992-10-08
Application granted
granted
Critical
1992-10-08
Publication of AU629637B2
publication
Critical
patent/AU629637B2/en
2005-10-20
Assigned to TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
reassignment
TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Request to Amend Deed and Register
Assignors: TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
2010-03-09
Anticipated expiration
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Status
Expired
legal-status
Critical
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Classifications
G—PHYSICS
G10—MUSICAL INSTRUMENTS; ACOUSTICS
G10L—SPEECH ANALYSIS OR SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING; SPEECH OR AUDIO CODING OR DECODING
G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
G10L19/04—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using predictive techniques
G10L19/08—Determination or coding of the excitation function; Determination or coding of the long-term prediction parameters
G10L19/10—Determination or coding of the excitation function; Determination or coding of the long-term prediction parameters the excitation function being a multipulse excitation
Abstract
A method for positioning excitation pulses for a linear predictive coder (LPC) operating according to the multi-pulse principle, i.e. a number of such pulses are positioned at specific time points and with specific amplitude. The time points and the amplitudes are determined from the predictive parameters (ak) and the predictive residue signal (dk), by correlation between a speech representative signal (y) and a composed synthesized signal (y/< ANd >). This can provide all possible time positions for the excitation pulses within a given frame interval. According to the proposed method, the possible time positions are divided into a number (nf) of phase positions and each phase- position is divided into a number of phases (f). These phases are vacant for the first excitation pulse. When this pulse has been positioned, the phase determined for this pulse is denied to the following excitation pulses until all pulses in a frame have been positioned.
Description
EXCITATION PULSE POSITIONING METHOD IN A LINEAR PREDICTIVE SPEECH CODER
TECHNICAL FIELD
The present invention relates to a method of positioning excita- tion pulses in a linear predictive speech coder which operates according to the multi-pulse principle. Such a speech coder may be incorporated, for instance, in a mobile telephone system, for the purpose of compressing speech signals prior to transmission from a mobile.
BACKGROUND ART
Linear predictive speech coders which operate according to the aforesaid multi-pulse principle are known to the art, from, for instance, US-PS 3,624,302, which describes linear predictive coding of speech signals, and also from US-PS 3,740,476 which teaches how predictive parameters and predictive residue signals can be formed in such a speech coder.
When forming an artifical speech signal by means of linear predictive coding, there is generated from the original signal a number of predictive parameters (a. ) which characterize the synthesized speech signal. Thus, there can be formed with the aid of these parameters a speech signal which will not include the redundancy which is normally found in natural speech and the conversion of which is unnecessary when transmitting speech between, for instance, a mobile and a base station included in a mobile radio system. From the aspect of bandwidth, it is more appropriate to transfer solely predictive parameters instead of the original speech signal, which requires a much wider band¬ width. The speech signal regenerated in a receiver and consti¬ tuting a synthetic speech signal can, however, be difficult to apprehend, due to a lack of agreement between the speech pattern of the original signal and the synthetic signal recreated with the aid of the prediction parameters. These deficiencies have been described in detail in US-PS 4,472,832 (SE-A—456618) and can be
alleviated to some extent by the introduction of so-called excitationpulses (multi-pulses) whenformingthe syntheticspeech copy. In this case, the original speech input pattern1is divided into frame intervals. Within each such interval there is formed a given number of pulses of varying amplitude and phase position (time position) , on the one hand in dependence on the prediction parameters a, , and on the other hand in dependence on the predic¬ tive residue d, between the speech input pattern and the speech copy. Each of the pulses is permitted to influence the speech pattern copy, so that the predictive residue will be as small as possible. The excitation pulses generated have a relatively low bit-rate and can therefore be coded and transmitted in a narrow band, as can also the prediction parameters. This results in an improvement in the quality of the regenerated speech signal.
DISCLOSURE OF THE INVENTION
In the case of the aforesaid known methods, the excitation pulses are generated within each frame interval of the speech input pat¬ tern, by weighting the residue signal d, and by feeding-back and weighting the generated values of the excitation pulses, each in a separate predictive filter. The output signals from the two filters are then correlated. This is followed by maximization of the correlation of a number of signal elements fromthe correlated signal, therewith forming the parameters (amplitude and phase position) of the excitation pulses. The advantage of this multi- pulse algorithm for generating excitation pulses is that various types of sound canbe generated with a small number of pulses (e.g. 8 pulses per frame interval) . The pulse searching algorithm is general with respect to the positioning of pulses in the frame. It ispossibletorecreatenon-accentuatedsounds (consonants) , which normally require randomly positioned pulses, and accentuated sounds (vowels) , which require more collected positioning of the pulses.
One drawback with the known pulse positioning method is that the coding effected subsequent to defining the pulse positions is complexwithrespect to bothcalculationand storage. Furthermore,
the method requires a large number of bits for each pulse position in the frame interval. The bits in the code words obtained from the optimal combinatorypulse-coding algorithms are also prone to bit- error. A bit-error in the code word being transmitted from trans- mitter to receiver can have a disastrous consequence with regard to pulse positioning when decoding the code word in the receiver.
The present invention is based on the fact that the number of pulse positions for the excitation pulses within a frame interval is so large as to make it possible to forego exact positioning of one or more excitation pulses within the frame and still obtain a regenerated speech signal of acceptable quality subsequent to coding and transmission.
According to the known methods, the correct phase positions are calculated for the excitation pulses within one frame and following frames of the speech signal and positioning of the pulses is effected solely in dependence on complex processing of speech signal parameters (predictive residue, residue signal and the parameters of the excitation pulses in preceding frames) .
According to the present inventive method, certain phase position limitations are introducedwhenpositioningthepulses, by denying a given number of previously determined phase positions to those pulses which follow the phase position of an excitation pulse that has already been calculated. Subsequent to calculating the position of a first pulse within the frame and subsequent to placing this pulse in the calculated phase position, said phase position is denied to following pulses within the frame. This rule will preferably apply to all pulse positions in the frame.
Accordingly, the object of the present invention is to provide a method for determining the positions of the excitation pulses within a frame interval and following frame intervals of a speech- input pattern to a linear predictive coder which requires a less complex coder and a smaller bandwidth and which will reduce the risk of bit-error in the subsequent recoding prior to trans¬ mission.
The inventivemethod is characterized by the features set forth in the characterizing clause of Claim 1.
The proposed method can be applied with a speech coder which operates according to the multi-pulse principle with correlation of an original speech signal and the impulse response of an LPC- synthesized signal. The method can also be applied, however, with a so-called RPE-speech coder in which several excitation pulses are positioned in the frame interval simultaneously.
BRIEF DESCRIPTION OF DRAWINGS
The proposed method will now be described in more detail with reference to the accompanying drawings, in which
Figure 1 is a simplified block schematic of a known LPC-speech- coder;
Figure 2 is a time diagram which covers certain signals occurring in the speech coder according to Figure 1;
Figure 3 is a diagram explaining the principle of the invention;
Figure 4a,4b aremoredetaileddiagrams illustratingtheprinciple of the invention;
Figure 5 is ablock schematic illustrating a part of a speech coder which operates in accordance with the inventive principle;
Figure 6 is a flow chart for the speech coder shown in Figure 5; and
Figure 7 is an array of blocks included in the flow chart of Figure
6.
BEST MODE OF CARRYING OUT THE INVENTION
Figure 1 is a simplified block schematic of a known LPC-speech- coder which operates according to the multi-pulse principle. One such coder is described in detail in US-PS 4,472,832 (SE-A- 456618) . An analogue speech signal from, for instance, a micro- phone occurs on the input of a prediction analyzer 110. In addition to an analogue-digital converter, the prediction ana¬ lyzer 110 also includes an LPC-computer and a residue-signal generator, which form prediction parameters a, and a residue-
signal d. respectively. Thepredictionparameters characterizethe synthesized signal, whereas the residue signal shows the error between the synthesized signal and the original speech signal across the input of the analyzer.
An excitation processor 120 receives the two signals a. and d. and operates under one of a number of mutually sequential frame inter¬ vals determined by the frame signal FC, such as to emit a given number of excitation pulses during each of said intervals. Each of said pulses is determined by its amplitude A and its time position, m within the frame. The excitation-pulse parameters A , m are led to a coder 131 and are thereafter multiplexed with the prediction parameters a. , prior to transmission from a radio transmitter for instance.
The excitation processor 120 includes two predictive filters having the same impulse response for weighting the signals d. and
A 1. , m1. in dependence on the prediction parameters a.Jv during a given computing or calculating stage p. Also included is a correlation signal generator which is operative to effect correlation between the weighted original signal (y) and the weighted synthesized signal (y) each time an excitation pulse is to be generated. For each correlation there is obtained a number q of «candidates» of pulse elements A. , m. (0
