System for measuring and automatically compensating for the distorsions of a connection between a telephone apparatus and a central voice processing unit

The central unit (14) comprises a distortion sampler (16) including a spectral sound analyzer and a memory for recording the digital data of at least one reference sound. The central unit by means of the analyzer analyzes a sequence of such sounds which are received by way of the telephone line (11, 13) and makes a comparison with the recorded data. It thus produces a measurement of the frequency response of the transmission, that is to say distortion and attenuation introduced by the telephone connection, in accordance with the various frequencies. The data of that measurement are used by the central unit (14) to compensate for the signals which are successively received by way of the activated telephone connection. The system comprises a portable acoustic device (35) in which the digital data of the reference sound are recorded. That device may be connected to any telephone apparatus (12) of the network and comprises a manual control for triggering transmission to the central unit (14) of the sequence of reference sounds after the telephone connection has been established by means of the telephone apparatus (12 ). The portable device (35) may also have a recording of a code for identifying the user, for example, for use in a vocal postal system. In addition in a system in which the central unit (14) comprises a voice recognition device (17) the portable device (35) may have the recording of a series of phonetic prototypes, or phonemes, which are specific to the particular user, to be transferred to the central unit to improve the quality of recognition of the speech transmitted thereby. Alternatively the phonemes of different users may be recorded in a memory of the central unit (14) and be selected thereby on the basis of identification code.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a system for measuring and automatically 
compenating for the distortions of a connection between a telephone 
apparatus and a central voice processing unit, as defined in the 
introductory portion of claim 1. 
The invention may be applied to a telephone network in which the users may 
deposit voice messages in digital form, for example for a vocal postal 
system. 
2. Description of the related art 
Systems for the digital storage of messages are known, which may be 
selected by passing to the central unit digital codes which are generated 
for example by a terminal provided with a numeric keyboard and a display. 
The selected message is then converted into vocal form by means of a 
text-speech converter and passed to the caller by way of the telephone 
connection. 
As is known, telephone connection systems are subject to some factors which 
cause distortion and degrading of the quality of the transmission, both 
because the signal transmitted is limited in frequency, so that there is a 
loss in respect of the information available at the source, and because 
the microphone and the telephone lines have a certain frequency response 
curve which may vary from one apparatus to another and from one line to 
another, and because the connection always superimposes a certain level of 
noise on the transmitted signal. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide an apparatus capable of 
improving the quality of the response of the central unit by telephonic 
means, automatically compensating for the distortion effects which are 
introduced in the connection between the telephone apparatus and the 
central voice processing unit. 
This object is met by a system according to the invention for measuring and 
automatically compensating for the distortions of a connection between a 
telephone apparatus and a central voice processing unit for a telephone 
network comprising at least one telephone line wherein the central unit 
comprises measuring means which are activable after a telephone connection 
has been established on the said line for measuring the distortion 
introduced in the transmission by the telephone line, and compensation 
means controlled by the measuring means for compensating for the 
distortion. 
In particular the system according to the invention may comprise a portable 
acoustic device capable of being connected to any telephone apparatus of 
the network, being provided with a memory for recording a sereis of 
digital data corresponding to a sequence of reference sounds for 
calibration of the connection, and a converter for converting these data 
into the corresponding reference sounds. 
These and other features of the invention will be more clearly apparent 
from the following description of a preferred embodiment which is given by 
way of non-limiting example with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, reference numeral 10 generally indicates a telephone 
exchange, for example an exchange for private networks, commonly known as 
a PABX, which is capable of operating a plurality of telephone lines 11 
connected to other user telephone apparatuses 12. Also connected to the 
PABX 10 by means of a cable 13 for transmission of speech in analog form 
is a central voice processing unit 14 capable of storing messages in 
digital form and processing them and selecting them for re-transmission. 
The apparatuses 12 may be of the multi-tone type, that is to say they are 
such as to control the selection of the user called and other functions of 
the connection by means of a series of acoustic signals of various 
frequencies. Those acoustic signals may be generated by actuating the 
numeric keys of the usual keypad 15 or by means of codes recorded in a 
suitable memory. The central unit 14 comprises a unit for measuring and 
compensating for the distortion of the connection, being generally 
indicated by reference numeral 16 (see FIG. 2), which will be described in 
greater detail hereinafter. 
The central unit 14 may also comprise a device 17 (see FIG. 2) for phonetic 
speech recognition, for example of the type described in our published 
patent application EP-A No. 0 108 609. Also connected to the central unit 
14 is a mass storage means, for example, formed by a magnetic disk reading 
and recording unit 18. In such a case the central unit 14 comprises a 
processor or CPU 19 (see FIG. 3) capable of providing control for the 
recognition device 17 under the control of a suitable program recorded in 
a read only memory (ROM) 25. The device 17 includes a sampler 21 and a 
spectrum analyser formed by a series of digital band pass filters 20. The 
signals emitted by the filters 20 are first converted into digital values 
by an A/D converter 22 and then analysed and compared with a series of 
sample phonemes recorded in a phoneme memory 23. 
The central unit 14 may also comprise a synthesiser 26 for converting 
digital data into corresponding sounds to be transmitted by way of the 
cable 13, the PABX 10 and the line 11 to the connected telephone apparatus 
12. 
In accordance with another application the central unit 14 may be included 
in a voice postal system in which the received messages are recorded under 
the control of the CPU selectively in certain postboxes of the mass 
storage means 18. Those messages can be subsequently selected by the 
calling user, for example by means of an identification number. 
The unit 16 comprises means for measuring the distortion introduced in the 
transmission by the telephone communication. Such measuring means comprise 
a permanent memory 27 (see FIG. 3) formed by a first region of a read only 
memory or ROM 28 in which there is recorded a series of digital data 
corresponding to one or more reference sounds. Such data include an item 
of data which is indicative of the level of energy of a series of bands of 
frequencies contained in the reference sound. A second region 29 of the 
ROM 28 may be recorded with a suitable program to permit a microprocessor 
30 to carry out the above-mentioned measurement operation, the results of 
which are recorded in a memory (RAM) 34, as will be described in greater 
detail hereinafter. 
The measuring means further comprise a spectral sound analyser 31 formed by 
an analog-digital converter 32 and a bank of filters 33. Each filter 33 is 
made active by the microprocessor 30 to isolate the frequency band of a 
sound from all the other bands of frequencies and to emit a digital sound 
indicative of the energy or level of the sound which is filtered in that 
way. 
The measuring and compensating system also comprises means for transmission 
of the sequence of reference sounds for automatically transmitting said 
sounds as soon as the connection between a telephone apparatus 12 and the 
central unit 14 is established. Preferably such transmission means 
comprise a portable acoustic device 35 (see FIGS. 1 and 2), which is 
preferably of pocket size, for example an electromagnetic or acoustic 
coupler known per se which is capable of being connected or coupled to any 
telephone apparatus 12 of the network for transmission on the line 11. The 
device 35 essentially comprises a microprocessor 36 (see FIG. 4) which can 
be controlled for a transmission operation by a program recorded in a 
first portion 37 of a ROM 38. 
Another portion 39 of the ROM 38 on the other hand is recorded with a 
series of digital data corresponding to the above-mentioned reference 
sound and thus identical to the data recorded in the region 27 of the ROM 
28 (see FIG. 3) of the unit 17. The recorded reference sound comprises 
various frequencies. The reference sound is transmitted a predetermined 
number of times alternately with periods of silence. It is also possible 
to use and transmit a predetermined sequence of sounds with different 
spectral characteristics. 
The device 35 (see FIG. 4) also comprises synthesiser 41 which is capable 
of generating the corresponding reference sound for each series of data 
which are recorded in the portion 39 of the ROM 38. That sound is 
amplified by an amplifier 42 to a preset level to be transmitted by way of 
a transmitter 40 which can be coupled to the microphone 43 of the 
telephone apparatus 12 (see FIG. 1) after the connection to the central 
unit 14 has been established. 
The device 34 (see FIG. 4) further comprises a battery power supply 44 for 
operation thereof and a single manual control member formed by a key 46. 
The latter is operable to activate the microprocessor 36 for sequentially 
reading the data of the reference sound which are recorded in the portion 
39 of the ROM 36. Finally the device 35 comprises a visual indicator 47 
which is formed for example by an LED which is controlled by the 
microprocessor 36 in such a way as to remain illuminated throughout the 
operation of reading the ROM 38. 
In order to effect compensation of distortion and background noise which 
are introduced by the line 11 (FIG. 11), the user, by means of the usual 
keypad 15 of the telephone apparatus 12, first makes a telephone 
connection to the central unit 14. The user then couples the device 35 to 
the microphone 43 and actuates the key 46. The microprocessor 36 (see FIG. 
4) then carries out, in the portion 39 of the ROM 38, the operation of 
reading the data relating to the reference sound. 
Those data, by way of the synthesiser 41 and the amplifier 42, repeatedly 
generate the reference sound which is spaced by periods of silence. 
The sound which is generated in that way is transmitted by way of the line 
11, the PABX 10 and the cable 13 to the central unit 14 at which they 
arrive with a certain amount of distortion or attenuation and with the 
addition of a background noise. The microprocessor 30 of the unit 16 (see 
FIG. 3), under the control of the program from the region 29 of the ROM 
28, now activates the various filters of the array 33, producing the 
digital data in respect of the levels of energy of the noise received and 
the various frequencies of the sound. The microprocessor 30 also compares 
the data obtained in that way to the corresponding data recorded in the 
memory 27, recording the result of that comparison in the registers of the 
operational memory 34. The diagram in FIG. 5 indicates in continuous line 
the theoretical levels of energy of the noise and the sound which is 
analysed at successive time intervals. It also shows in broken lines the 
level of energy of the noise received and the sound received. 
In particular the microprocessor 30 is controlled by the program recorded 
in the region 29 of the ROM 28, being capable of producing, in dependence 
on the frequency, a line transfer function h (FIG. 6a), which is an 
unknown, and the noise spectrum R (FIG. 6b) which is introduced by the 
line 11, which is another unknown. That program comprises an algorithm 
such as to resolve the following system of equations: R.sub.r =hR.sub.s 
+R;S.sub.r =hS.sub.s +R. 
In those operations R.sub.r (FIG. 6c) and R.sub.s (FIG. 6d) respectively 
represent the energy (FIG. 5) of the noise received by the unit 16 (FIG. 
3) and that of the noise emitted by the transmitter 40 (see FIG. 4) while 
S.sub.r (FIG. 6e) and S.sub.s (FIG. 6f) respectively represent the energy 
of the spectrum of the signal of the sound received by the unit 16 (FIG. 
3) and that of the spectrum of the signal transmitted by the transmitter 
40 (FIG. 4). The microprocessor 30 repeats the program for the various 
bands of frequencies represented in the spectra of the diagrams in FIG. 6 
and can thus define in digital form the frequency response curve of the 
telephone connection and the noise generated thereby. 
The result which is defined in that way is recorded in the registers of the 
memory 34 and is subsequently used by the microprocessor 30 during 
transmission by way of the same telephone line in order to compensate for 
distortion therein. For that purpose the microprocessor 30 is connected to 
the CPU 19 whereby in the subsequent transmission phases there is a 
reduction in a cause of uncertainty in respect of automatic speech 
recognition and the quality of the service in voice recognition and/or 
vocal post applications is improved. 
In accordance with another aspect of the invention the ROM 38 (FIG. 4) of 
the portable device 35 comprises another region 48 in which there is 
recorded a distinctive identification code for the individual device 35, 
which is thus distincitive of its bearer user. The program recorded in the 
region 37 of the ROM 38 is such as to condition the microprocessor 36 to 
transmit the identification code to the central unit 14 (FIG. 3) 
immediately after the region 39 of the ROM 38 has been read. That code is 
recognised by the CPU 19 of the central unit 14 which then permits the 
other operations envisaged for the connection. 
The identification code may also be distinctive of a corresponding voice 
post box of a disk unit 18 (see FIG. 2). The central unit 14, after the 
recognition phase, may then control the disk unit 18 (see FIG. 2) in such 
a way as to transmit to the user the messages contained in the 
corresponding voice post box or to transfer same to a voice post box of 
another user by coupling it to the name corresponding to that 
identification code, as the sender. 
Reception of the new message by the central unit 14 is compensated with the 
data in respect of distortion and the noise which are received from the 
measurement operation previously carried out, for the characteristics of 
the line 11 which is active at that time, by the microprocessor 30, and 
recorded in the RAM 34 of the compensation unit 16. In that way the user 
who is the bearer of the device 35 may interrogate his voice post box by 
way of any telephone apparatus 12 (see FIG. 2) of the network and possibly 
by way of a telephone apparatus of a public network which is outside the 
PABX 10 to which the PABX 10 is connected. In that case the compensation 
effect produced by the central unit 14 also takes account of the 
distortion and the noise introduced into the transmission by the 
connection between the PABX 10 and the outside telephone apparatus. 
In a system in which the central unit 14 comprises the voice recognition 
device 17, for the purposes of improving speech recognition, the memory 23 
of the device 17 (see FIGS. 3 and 4) may contain in different sections an 
archive record of the prototypes and phonemes relating to each user. The 
CPU 19, in response to recognition of the identification code sent by the 
device 35, then selects the corresponding section of the memory 23. 
Comparison of the message to be recorded is then effected with the 
relevant prototypes of the specific user whereby voice recognition will be 
of the "speaker dependent" type and is thus more precise. 
Alternatively the recognition device 17 may be provided with a collection 
of phonetic prototypes which are recorded in the memory 23 and which 
represent with some degree of approximation a vast population of different 
voices. In that case codes suitable for identifying the user voice are 
recorded in a section 49 of the ROM 38 of the portable device 35. Those 
codes are transmitted to the central unit 14 after the user identification 
code and enable the CPU 19 to select from the memory 23 the prototypes to 
be used for recognition of the speech transmitted from that particular 
user. 
In such "speaker dependent" speech recognition situations, before the 
recognition operation the central unit 14 may compensate for reception of 
the voice message with the data in respect of distortion and noise 
introduced by the telephone connection and received following reading of 
the region 39 of the ROM 38 of the portable device 35. Alternatively the 
central unit 14 may vary the prototypes and the phonemes selected in the 
memory 23, in accordance with such data. 
In accordance an alternative embodiment, in a system comprising the voice 
recognition device 17, the section 49 of the ROM 38 of the portable device 
35 may be recorded with a series of phonetic prototypes formed by words or 
syllables or a series of simple phonemes originally pronounced by the 
specific user bearing the device 35. After the identification code for the 
section 39 has been transmitted to the central unit 14, the microprocessor 
36 causes the recording of the prototypes or the phonemes in the section 
49 of the ROM 38 to be transferred to the memory 23 of the central unit 
14. In that way the recognition device 17 can be adapted to the voice of 
the user carrying the device 35, without the necessity to have recourse to 
phonetic prototypes which pre-exist in the device 17, whereby the CPU 19 
performs comparison of the phonetic prototypes or phonemes received with 
the phonetic prototypes originating from the specific bearer user. In that 
case the central unit 14 provides a compensation effect which takes 
account only of the item of data supplied by the noise, which is used to 
establish the silence-voice separation threshold. 
Various modifications and improvements may be made in the above-described 
compensation system without thereby departing from the scope of the 
invention. For example the CPU 19 and the microprocessor 30 of the central 
unit 14 may be replaced by a single common processor and the two memories 
24 and 34 may be replaced by a single RAM. In addition the memory 23 may 
be replaced by a section of ROM. Similarly the array of filters 33 and the 
converter 32 may be part of the filters 20 and the converter 22 of the 
recognition device 17. Finally the central unit 14 may be formed by a 
personal computer in which boards are fitted to perform the functions of 
the units 16 and 17.