Method and device for rating of speech quality

Abstract: The present invention refers to a method and device for deciding quality of speech. The speech to be evaluated is listened in to by a person who reproduces the speech. Stops of vowel sounds in he produced and reproduced speech respectively are appointed. The difference between the stops of the vowel sounds is registered. Out of the obtained differences an average value is created. The achieved average value indicates the quality of the produced speech. The invention can be used for evaluation of different speech producing sources such as equipments and/or machines and people's ability to comprehend the speech.

Description: TECHNICAL FIELD The present invention refers to the rating of speech quality in a given speech. The speech source which is analysed can be a synthetized speech or from different persons. STATE OF TECHNOLOGY Most methods for finding out the quality of synthetic speech at text-to-speech conversion are concentrated on the segmental realisation, by perception tests with nonsense words like for instance appa, ippi, agga etc, This method says little or nothing about how good the synthetically produced speech is and how useful it is in applications. To solve this problem one has started studying cognitive stress when listening to synthetic speech, for example by making the subject of the experiment perform different tasks at the same time as he/she is exposed to information by synthetic speech, the content of which he/she has to give an account of. In synthetic speech the non-primary parameters are to a large extent lacking and consequently in that the interacting parameters in many cases give a straight contradictory information, so that the comprehension is lower than that of natural speech. Especially in noisy environments the listener has a need of these non-primary signal parameters. Therefore, the comprehension of synthetic speech is drastically diminished in such surroundings. In patent document US 4672668 is described how a system pronounces a stored standard word with defined length, stress and rhythm. A person repeats the standard words and tries to simulate the length, stress and rhythm. The repeated words are detected and processed for determining whether certain critera concerning identity of the standard words pronounced by the system are complied. If the repeated word complies with the criteria of identity it will be stored as a reference word. In the patent document US 5282475 a technology pertaining to audiometry is described. A sequence of speech stimuli is presented to a person. Surveillance is made of at least one physiological answer from the human subjects of the experiment, which answer varies according to the subject's reception (understanding). In patent document US 5303327 is described a method according to which a verbal stimuli is presented to a person, after which the answer to the verbal stimuli is registered. The answers deal with statements and/or receptivity. DESCRIPTION OF THE INVENTION TECHNICAL PROBLEM There is a need for evaluating total quality, inclusive prosody, in for instance text-to-speech conversion. The methods used today for evaluating total quality are based on trials with a large number of persons. These persons deliver an opinion on the quality of the speech in question. There is a need to find methods which are automatic and do not need to use a number of persons participating in the evaluation. In situations where the question is to chose between different speakers, it can be of importance to find the speaker who is most easy to comprehend. Thus methods for quick evaluation of such speakers and chosing the one who probably is most easy to comprehend is desirable. Further problems are that certain groups of people have more difficulties in perceiving speech than others. Even in this situation it is desirable to find methods where a grading of the quality of a speech in relation to the capacity of the group of listener can be defined. Methods which are usable for synthetic speech and pathological speech are lacking at present. Possibilities for studying social handicap are also wanted. SOLUTION The present invention relates to a method and a device for determining speech quality as set out in the appended claims. A produced speech is being listened by a person who repeats the speech. The vowels of the produced and reproduced speech respectively are identified. Further, the points of time for the start of each vowel sound are identified. A time difference between the corresponding starts of vowel sounds are established. The obtained time differences indicate the quality of the produced speech. The reproduction of the speech is performed by a person being listening to the speech and verbally reproducing it as soon as possible. The speech is produced in a text-to-speech converter and consists of a message recorded in advance which is reproduced by for instance a tape recorder. A reference to the quality of the produced speech is achieved by calibration of the system. This is performed by reading a speech with a quality known in advance. The person who repeats the calibration message will repeat the message with some delay in relation to the original message. In this way a reference is achieved, whereby different person's repeating of the message can be compared. The calibration procedure allows to take into consideration, for instance, a person's daily form. The method further allows to assess the speech quality of a text-to-speech converter, of different persons, or of human speech recorded on, for instance, a tape recorder. The invention further refers to a device for deciding speech quality. A device, 5, is arranged to produce a speech. The produced speech is analyzed and reproduced by a function, 1. A device, 7, appoints the starts of the vowel sounds in the produced and reproduced speeches, respectively. In the device, 7, a time difference between the corresponding starts of vowel sounds in the produced and reproduced speech is registered. The time difference indicates a measure of the quality of the speech and is presentable via the device, 7. The device, 5 in figure 1, consists of a text-to-speech converter for production of a speech. Further, the function, 1, consists of a person. He/she is listening in to the produced speech which will be repeated by the person. The person, 1, shall reproduce the reproduced speech as soon as possible after he/she has listened to it. In the device, 7, is arranged a time differential analysis equipment to appoint the time difference between the start of vowels in the produced and reproduced speech. The device, 7, is further arranged to give a certificate of quality of the produced speech. The time difference equipment, 7, is further arranged to create an average value of the obtained time differences. The average value indicates the quality of the produced speech. The device, 7, is further arranged to comprise a first speech recognition equipment, 2, for appointing start of vowel sound in the produced speech. Further it comprises a second speech recognition equipment, 3, for appointing start of vowel sound in the reproduced speech. For calibration of the equipment a calibration source is used, 6, according to figure 3 and 4, which is arranged to be connected instead of device 5. The calibration source is arranged to produce a speech the quality of which is known in advance. In this way a reference is obtained in relation to the person, 1, who has been used for the reproduction of the speech. A reliable evaluation of the produced speech is thus obtained independent of the person, 1. ADVANTAGES The present invention has the advantage of measuring speech quality including prosody. In previously known methods of measuring only segmented quality has been appointed. At the production of synthetic speech from a text different text-to-speech converters can be compared. The invention can be used for evaluating social handicap in connection with pathological speech. By having a speech with a given quality as a reference a graded system for different speeches can be obtained. This is achieved by a number of reference speeches with, for instance, the grades very good, goood and poor being used. The given speech can after that at the analysis be appointed to belong to one of the mentioned categories. DESCRIPTON OF FIGURES Figure 1 shows the essential composition of the system. Figure 2 shows how the equipment, 5, is divided into one text analysis equipment 1, 50, and one speech synthetizing equipment, 51. In figure 3 is shown how a reference equipment, 6, has been connected to the system and is reproduced by a person before the equipment, 5, is connected for an analysis of the given speech. Figure 4 shows the equivalent of figure 3 where the given speech is produced by a person and the reproduction is performed by a person. Figure 5 shows the invention in the form of a flow chart diagram. DETAILED EMBODIMENT In the following the invention is described with reference to the figures and the designations therein. According to figure 1 speech is produced in a device 5. The speech is transferred in parallell to devices 1 and 7. In device 1 the speech is listened to and reproduced. The produced and reproduced speech is transferred to a device 7. Analysis of the speeches then takes place and vowel sounds in each speech is identified. For each vowel sound the start of the vowel sound is appointed. In device 7 points of time for start of vowel sounds in each speech are obtained. The points of time for the starts of the vowel sounds are analysed. The time difference between the starts of vowel sounds in the speeches is appointed. If it is supposed that the starts of the vowel sound in the produced speech are marked V1, V2, V3 etc, and the starts of the vowel sounds in the reproduced speech are marked V1', V2', V3'etc the differences can be marked X1, X2 etc, where X1 = V1' - V1, X2 = V2'- V2 etc. The average value of these differences is achieved by The grading of the produced speech is obtained by the fact that the bigger the time delay in the reproduced speech is in relation to the produced speech, the worse is the understanding of the reproduced speech. The grading of the quality of the speech can for instance be referred to different time intervals within which the reproduced speech can be reproduced. In figure 3 is furher shown how a speech is produced in a text-to-speech converter 5. The speech is transferred to the analysis equipment 2, and to a person, 1, who has the duty to, as soon as possible, verbally reproduce the speech in a microphone which is connected to the equipment 3. In the equipment 2 the starts of the vowel sounds in the produced speech are appointed. In the equipment 3 the starts of the vowel sounds in the verbally reproduced speech are appointed. In the equipment 4 a difference between the starts of the vowel sounds of the produced speech and the reproduced speech is produced. A pecularity which can occur at the reproduction of speech with a person as reproducer is that a person out of the given speech and its delivery can predict the coming speech. This means that the human being at the reproduction of the speech in certain cases can reproduce the speech at the same time or even lie ahead of the speech production device. Also in this case a difference is created between the starts of the vowel sounds in the equipment 4. At the creation of the average value is it in this case possible to obtain an average which is close to 0 which indicates that the speech is very well understandable. By making different categories of people listen to the same speech, different kinds of for instance impaired hearing can be compared. Text to speech converters can in these cases in an adequate way be adapted to the need of different person categories. For instance can persons with different kinds of impaired hearing be analysed and for those people suitable equipments be produced. For obtaining an adequate grading some form of reference system is required. In figure 3 such a system is shown where a reference equipment 6 is connected to the system. The text which in this case is read by the equipment is for instance categorized in advance by subjective measurements. Such subjective measurements are performed for instance in sound laboratories. Changing between the reference equipment and the trial equipment is made via the switch. The stored message in equipment 5 can for instance consist of messages of different quality. The analys equipment receives at the reading information about the quality of the present speech. This is notified at the reference analysis and the result is stored in a memory which is arranged in the analys equipment. A system with arbitrary division of the grading is thus achieved. The 6 stored messages in the equipment preferbly consist of messages recorded on tape or other resistant medium. What is important is that the reference messages are the same at different reference alternatives to make things comparable. The time difference between the starts of the vowels of the produced and the reproduced speech are appointed and an average is created according to the mentioned. The obtained average values at that indicate the treshhold for different grades at analysis of a speech. In figure 4 how the reference equipment 6 is connected and a person, 1, who reproduces the speech are shown. After a reference evaluation has been made, in this case a person reading a text is connected by switching the swith. The person's, 5, verbal production is listened to and reproduced by a person, 1, and the speeches are analysed as described above. By comparing the starts of the vowel sounds in each speech respectively, and making an average of these as previously described, it is possible to assess the person's, 1, ability to reproduce the person's, 5, speech. By comparing the obtained average value with the average value for the reference equipment, in equipment 4 an evaluation of the speaker's, 5, verbal production ability is obtained. Thus it is possible to, starting from a reference applicated to the reference equipment, find out whether a speaker's, 5, account can be reproduced and understandable to another person in relation to a reference. The person, 1, who repeats the speech can for instance be a person or a group of persons with different kinds of impaired hearing. With the equipment is in this case achieved a tool for selecting which person/persons shall speak to a certain kind of people. This can for instance be of crucial importance at lectures, lessons etc where persons with certain hearing handicap or other types of handicap are listener. It is in this case possible to tailor-make the lecturers/teachers. This can be of crucial importance for making a message to reach the listeners. In figure 2 is further shown how a text-to-speech converter, 5, according to the previous decriptions can be realised. In this case there occurs an analysis of the text in the equipment 50. The text is transferred to a speech synthetizing equipment 51. The speech synthetizing equipment is after that producing a speech which corresponds to the given text. Both the text analysis equipment and the speech synthetizing equipment are since previously introduced on the market. A closer description of these are not necessary since the professionals in the field well know these equipments. Referring to the flow chart in figure 5 the functionality of the invention can be described as first deciding whether calibration of the system shall be made or not. Depending on whether calibration shall be made or not, a speech with known quality is produced alternatively the speech to be analysed is produced. The produced speech is being listened in to and reproduced. The starts of vowel sounds in the produced and reproduced speech respectively are appointed. The time difference between the starts of the vowel sounds in the speeches respectively is appointed. After that the average value of mentioned differences are created. If the achieved average value creation is aiming at a calibration of the system, the obtained result is placed in a reference register, 18. After that is decided whether more references are to be placed in the system. If that is the case next speech reference is taken out and the procedure according to previous description is repeated. If all references have been gone through there is even in this case a restart. If, on the other hand, the obtained average value was directed towards an evaluation of a speech produced by an equipment or a person, a comparison with values in the reference register is after that performed. That reference value which is closest to the quality of the produced speech is appointed. The equipment after that presents the quality of the speech. After that is decided whether further evaluations is to be made or not. If no further evaluations shall be performed the procedure will be finished, otherwise the same procedure as above decribed is applied. If one arranges a person to listen in to read text and gives him/her the task to repeat the text, it turns out that the time difference between the speech repeated by the subject of the experiment and the speech that is read for him/her is not very big. Sometimes the subject of the experiment even lies ahead due to the redundancy in the sentences which makes him predict the incoming speech. The chance of predicting the continuation of the incoming speech is obviously due to how much information is received from start of the speech and up to the point of time in question. The signal parameters of the accoustic signal interact in one for the production apparatus and the human brain unique way, resulting in that the information is being multidimensionally coded. Even not primary signal parameters are important for supporting the interpretation of a statement. The prosody (intonation) of the speech in the highest degree announces synthetic structure and interpretation of a statement. Synthetic speech is to a large extent lacking the non-primary signal parameters which causes the interacting parameters in many cases to give a straight contradictory information resulting in that the comprehensibility is lower than in natural speech. Especially in noisy surroundings the listener is needing these non-primary signal parameters which results in the comprehensibility being drastically lower in such surroundings. By studying the time delay between the speech repeated by the subject of the experiment and the speech that is read to him/her by naturally produced speech and synthetic speech one can classify the speech quality of the synthetic speech. Due to the fact that the time delay will vary in time is by automatic speech analysis decided the points of time of the start of the vowel segments in the read alternative of the by the synthetizer produced speech and the speech produced by the subject of the experiment. For each vowel in the speech string the time delay is appointed and the average delay calculated. The method can also be used for comparing the quality of the speech of different speakers, and at that for instance judge the social handicap for a person with speech disturbances. Comparisons between different text-to-speech converting equipments can also straightly be made.

Claims: Method for deciding speech quality, where a speech is produced and listened to, and the speech listened to is reproduced characterized in that the points of time for the starts of vowel sounds in the produced and reproduced speeches, respectively, are appointed, the time difference between corresponding starts of vowel sounds in the produced and reproduced speeches is determined, said time difference indicating the quality of the produced speech. Method according to claim 1, characterized in that the reproduction of the speech is made by a person listening to the produced speech and verbally reproducing it. Method according to claim 1, characterized in that the speech is produced either by a text-to-speech converter, or by a person reading a text, or in that the speech consists of a message recorded in advance, said message being reproduced by for instance a tape recorder. Method according to claim 2, characterized in that a speech of known quality is produced, whereupon a calibration with regard to the manner in which a person or a device is reproducing the speech is obtained. Method according to claim 1, characterized in that an average value of time differences is created and in that the average indicates the quality of the speech. Method according to claim 1, characterized in that calibration is performed by a speech, the quality of which is defined in advance, being used for determining said time difference. Method according to claim 1, characterized in that the comprehensibility of different sources of sound to different categories of persons, with for instance impaired hearing, is definable, whereupon a categorization of different speech producing sources with regard to comprehensibility is achieved. Device for deciding quality of speech, where a first device (5) is arranged to produce a speech, and a second device (1) is arranged to analyse and reproduce the speech characterized in that a third device (7) is arranged to appoint starts of vowels in the produced and reproduced speeches, the third device (7) being arranged to register a time difference between corresponding starts of vowels in the produced and reproduced speeches, said third device on the basis of registered time differences producing a measure of the quality of the produced speech. Device according to claim 8, characterized in that the first device (5) consists of either a text-to-speech converter, or a device for reproduction of a recorded speech or a person. Device according to claim 9, characterized in that the second device (1) is a person that listens to the produced speech and reproduces it verbally. Device according to claim 9, characterized in that the third device (7) is arranged to include a time difference analysis equipment (4) which registers the time differences between the starts of the vowel sounds in the produced and reproduced speeches, and is arranged to give a quality grade of the produced speech. Device according to claim 11, characterized in that the time difference analysis equipment (4) is arranged to create an average value of the obtained time differences and that the average value indicates the quality of the produced speech.

Inventors: LYBERG BERTIL, LYBERG, BERTIL
Applicants: TELIA AB
Application year: 1996
Main classification: G10L13
IPC codes: G10L11, G09B7, G10L13