Abstract:
The invention relates to a method for transcoding audio signals in a communications system. In order to improve the inter-operability between units ( 2,40 ) capable of handling wideband audio signals and units ( 3,46 ) or network components ( 50 ) capable of handling narrowband audio signals, it is proposed that first, an audio signal is received in a network element ( 42 ) of a communications network via which said audio signal is transmitted. Next, it is determined in said network element ( 42 ) whether a transcoding of the received audio signal is required. In case a narrowband-to-wideband transcoding of the received signal is required, the received narrowband audio signal is transcoded into a wideband audio signal in the network element ( 1,42 ). The generated wideband audio signal is then forwarded to the receiving terminal ( 2,40 ). The invention equally relates to a corresponding communications system and its components.

Description:
FIELD OF THE INVENTION 
   The invention relates to a method for transcoding an audio signal in a communications system, which audio signal is to be transmitted to a receiving unit via a communications network. The invention equally relates to a transcoder, to a network element comprising a transcoder and to a wireless communications network and a communications system comprising a network element with a transcoder. 
   BACKGROUND OF THE INVENTION 
   In conventional telecommunications systems, the bandwidth employed for transmitting speech signals has typically been limited to a frequency range of about 300 to 3400 Hz with a sampling rate of 8 kHz. This limitation applies to normal PCM (pulse code modulation) speech, which employs a 64 kbit/s coding defined in specification ITU-T G.711, as well as to most of the low-bit rate speech coding methods used in telecommunications systems. 
   The use of such a narrow frequency rage for speech transmissions reduces naturalness and intelligibility of the speech when presented at the receiving end. Therefore, wideband speech was introduced, which provides a better speech quality to the user of the receiving terminal. Wideband speech codecs (compressor/decompressor) were standardized e.g. in ITU-T G.722, which specifications extend the bandwidth to up to 8 kHz with a sampling frequency of 16 kHz. Current wideband speech transmissions, however, requires bit-transparent end-to-end connections, e.g. by ISDN (Integrated Services Digital Network). Only tandem-free operation (TFO) or transcoder-free operation (TrFO) connections between two wideband terminals allow to fully utilize the wideband terminal capabilities. In addition, it requires special terminals equipped at both transmission ends with the same wideband codecs. These restrictions have limited to date the utilization of wideband speech. 
   In the future, the importance of wideband speech will increase, as the forthcoming adaptive multirate wideband (AMR-WB) speech codec, standardized in various 3 GPP specifications, will be taken into use for the 2G (second generation) and 3G (third generation) networks. But also AMR-WB will require bit-transparent tandem free operation and AMR-WB capable terminals. 
   To date, the majority of terminals moreover still uses narrowband speech transmissions, and each connection between a wideband terminal on the one hand and a narrowband terminal on the other hand is narrowband. In order to establish e.g. a call between an AMR-WB capable and a conventional narrowband (NB) terminal, like a PSTN (Public Switched Telephone Network) or a PLMN (Public Land Mobile Network) terminal, either the coding method needs to be negotiated in a way that the AMR-WB terminal shall use a narrow band codec, or AMR-WB speech frames need to be transcoded into narrowband speech and vice versa in the network. In both cases the user of a receiving AMR-WB terminal will experience narrowband speech. Thus there will be an annoying quality difference between AMR-WB to AMR-WB and narrowband to AMR-WB calls. This further reduces the benefits gained with wideband terminals, until such terminals become widely available. 
   The same problems are encountered also with narrowband services accessed by a user of a wideband terminal, e.g. announcements, voice mail systems, interactive voice interfaces and narrowband audio streaming applications. In case speech-based network services are to be provided for both, wideband and narrowband terminals, the storage capacity required for storing speech samples for both terminal types is moreover tripled compared to the conventional narrowband case. 
   Another problem with the transmission of wideband audio signals results from the fact that the extended audio bandwidth used by the wideband terminals requires more transmission capacity. More specifically, the transmission capacity requirements are doubled for equal speech coding schemes. In addition, the adoption of wideband speech transmission in wireless communications network is difficult due to the lack of established wideband codecs for telecommunication networks. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to improve the interoperability between units capable of handling wideband audio signals and units or network components capable of handling narrowband audio signals. 
   It is moreover an object of the invention to achieve a better audio quality when narrowband audio signals are transmitted to a receiving unit capable of handling wideband audio signals. 
   On the one hand, a method for transcoding an audio signal in a communications system is proposed, for which method the audio signal is supposed to be transmitted to a receiving unit via a communications network. The proposed method comprises as first step receiving an audio signal in a network element of the communications network. In a second step of the method, it is determined in the network element whether a transcoding of the received audio signal is required. The decision is based on the kind of the received signal and on the capabilities of the receiving unit and/or the capabilities of an interconnect network interconnecting the network element with the receiving unit. Then, a received narrowband audio signal is converted into a wideband audio signal in the network element, in case the received audio signal is a narrowband audio signal and in case it was determined that a narrowband-to-wideband transcoding of the received signal is required. Finally, the generated wideband audio signal is forwarded to the receiving unit. 
   On the other hand, a transcoder for a network element of a communications network is proposed which comprises means for converting a received narrowband audio signal into a wideband audio signal. Whether a transcoding is actually performed depends on an indication from within said network element that a narrowband-to-wideband transcoding of a received signal is required. The indication might even be generated within the transcoder itself. 
   Moreover, a network element for a communications network is proposed, which comprises in addition to the proposed transcoder processing means for determining whether a transcoding of a received audio signal is required based on the kind of the received audio signal and on the capabilities of the receiving terminal and/or the capabilities of an interconnect network interconnecting the network element with a receiving unit. Further, the network element comprises means for forwarding audio signals to a receiving unit. Equally proposed is a wireless communications network with such a network element. 
   Finally, a communications system is proposed which comprises a communications network with the proposed network element, and in addition at least one unit capable of handling narrowband audio signals and at least one further unit capable of handling wideband audio signals. The two units can be interconnect via said communications network. The device capable of transmitting narrowband audio signals can be in particular either a server presenting services to a receiving terminal or another terminal, e.g. a mobile terminal. An alternatively proposed communications network comprises at least two units capable of handling wideband audio signals, which can be interconnected via a wireless communications network comprising the proposed network element. 
   The invention proceeds from the idea that a transcoding of a narrowband audio signal into a wideband audio signal can be implemented advantageously in a network element of a communications system. By integrating all the load caused by a transcoding and also the decision whether a transcoding is needed in a network element of a communications system, the interoperability of units capable of handling wideband audio signals and units or network components capable of handling narrowband audio signals can be improved. 
   It is an advantage of the invention that an extra signaling for the transcoding during a call setup or forwarding can be avoided. Terminals and other units can moreover be more simple as they do not need to know and transcode many different codecs. Thus, any terminal or other unit can be employed in the communication according to the invention without requiring extra functionalities. Further, terminals usually also have only a restricted capacity available, and it is easier to provide the processing capacity needed in the network. 
   Preferred embodiments of the invention become apparent from the subclaims. 
   Preferably, the transcoding comprises generating an at least partially artificial wideband audio signal based on the received narrowband audio signal. This way, the frequency range missing in narrowband audio signals can be supplemented artificially when converting a narrowband audio signal into a wideband audio signal. The wideband audio signal provided to the receiving unit is thus at least partially artificial, but since it is proposed to be generated based on the received narrowband signal, it can be close to the original audio signal. Therefore, the invention allows to provide high quality audio signals, in particular high quality speech, for a wideband terminal user. 
   Since a received narrowband audio signal is usually transmitted in coded form, the coded signal is advantageously first decoded to a linear narrowband audio signal before it is converted. After conversion to a linear wideband audio signal, the signal can then be encoded again before it is forwarded to the receiving unit. 
   The generation of at least partially artificial wideband signals out of received narrowband signal can be carried out in different ways. It can be based in particular on a statistical evaluation of the received narrowband signals, e.g. by using dedicated codebooks for narrowband and wideband signals and by mapping narrowband codebook values to wideband codebook values for creating a spectrum shaping filter. Alternatively, other statistical mapping methods can be employed. The artificial wideband signal can also based on an up-sampling narrowband audio signals and a subsequent frequency shaping, or on spectral foldings of narrowband audio signals. 
   Preferably, the transcoder of the invention comprises in addition to the means for converting a received narrowband audio signal into a wideband audio signal-means for converting a received wideband audio signal into a narrowband audio signal. The latter means can then be employed in case it was determined that a wideband-to-narrowband transcoding is necessary, whenever an audio signal is to be transmitted from a wideband unit to a narrowband unit. This ensures that the transcoder can be employed bi-directionally, which further improves the interoperability between different units. Accordingly, a negotiation of a common narrowband codec in involved units during call setup or call forwarding is not needed. It also enables calls in cases in which a common narrowband codec cannot be found. The method of the invention can be extended accordingly to cover a transcoding in both directions. 
   The receiving unit employed according to the invention can therefore be in particular a wideband terminal, a narrowband terminal or a speech-based network service equipment. 
   Advantageously, also a received wideband audio signal is first decoded to a linear wideband audio signal before it is converted to a linear narrowband audio signal. The linear narrowband audio signal is then encoded before it is forwarded to the receiving narrowband unit. The conversion can comprise a lowpass filtering of the linear wideband audio signal and a down-sampling of the lowpass filtered wideband audio signal, in order to achieve an alias distortion free narrowband audio signal. 
   Alternatively, the coded wideband speech could be converted directly to a coded narrowband audio signal in the speech parameter domain, i.e. without decoding and encoding in the transcoder. However, this requires that the narrowband and the wideband codec are of the same technology family, e.g. an AMR-narrowband codec and an AMR-wideband codec. By employing a parameter domain conversion, some of the parameters within the codec need to be converted, e.g. spectrum and excitation parameters. Other parameters need no or only minor adjustments, e.g. pitch and gain parameters. This approach can be used equally for narrowband to wideband conversions and for wideband to narrowband conversions. A narrowband to wideband conversion in the parameter domain can but does not have to include an artificial bandwidth expansion. 
   The invention is particularly suited for four different situations. 
   In a first situation, a PSTN (public switched telephone network) narrowband terminal is connected via a communications network with a wideband terminal. The wideband terminal can be in particular a mobile wideband terminal, in which case the communications network is formed by an interconnect network and in addition a wireless communications network to which the wideband terminal is connected. If a narrowband audio signal addressed to the wideband terminal is transmitted by the PSTN terminal via the communications network, the narrowband audio signal is converted to an at least partially artificial wideband audio signal in a network element of the communications network. In case the wideband terminal is a mobile terminal, the network element is preferably a network element of the wireless communications network to which the wideband terminal is connected. 
   Also in the three other situations, the transcoding will be carried out advantageously in a network element of a wireless communications network, e.g. a GSM (Global System for Mobile Communications) or a UMTS (Universal Mobile Telecommunications System) network. 
   In the second situation, audio signals are to be transmitted from a narrowband terminal to a wideband terminal. By providing an artificial bandwidth expansion in a network element of the communications network via which signals are transmitted from the narrowband terminal to the wideband terminal, the user of the wideband terminal can immediately utilize the wideband capabilities of the terminal. Thus, the inter-operability between wideband and narrowband terminals is improved. For the case that the narrowband terminal is to be employed as receiving unit, the network element should further include means for converting wideband audio signals into narrowband audio signals. The transcoding in either direction is preferably carried out close to the wideband terminal in order to avoid the necessity of transmitting wideband signal on the entire transmission path. 
   In the third situation, audio signals are to be transmitted between a wideband terminal as receiving unit and a speech-based network service equipment. If an artificial bandwidth expansion is provided in a network element of the communications network employed for transmitting the audio signals to the wideband terminal as receiving unit, only narrowband speech samples need to be stored in the network service equipment. Also existing narrowband speech-based network service equipment can thereby be accessed by wideband terminals without a significant reduction of subjective speech quality. Corresponding to the second situation, for the case that the service equipment is employed during a connection exclusively or in addition as receiving unit, the network element should further include means for converting wideband audio signals into narrowband audio signals. The transcoding in either direction is again preferably carried out close to the wideband terminal in order to avoid the necessity of transmitting wideband signal on the entire transmission path. 
   In the fourth situation audio signals are to be transmitted between two wideband terminals, where it is preferred to transmit the audio signals at least in a part of the interconnecting network or networks as narrowband signal. By combining bandwidth reduction, e.g. low-pass filtering, and artificial bandwidth expansion in a network element of each of the wireless communications networks to which the respective wideband terminal is connected, a wideband audio signal can be converted into a narrowband audio signal at the beginning of the transmission path and converted back into a wideband audio signal at the end of the transmission path. Thus, the operator is able to transmit wideband speech with little transmission capacity, in particular also using existing trunking networks, without a significant reduction of the speech quality perceived by the users of the wideband terminals. 
   The network element of the invention can be any network element which has access to traffic channels of a circuit switched network or which has access to the user plane of a packet network. If it is a network element of a wireless communications network, it can be for instance a network element of a GSM BSS (Base Station Subsystem), of a RAN (Radio Access Network), or of a core network. The network element should moreover have enough signal processing capacity to carry out the transcoding according to the invention. Therefore, the network element according to the invention can be in particular, though not exclusively, a transcoder unit of 2G networks, a transcoder unit of 3G networks, a media gateway enabling the inter-working of user data between packet and circuit switched networks, a multi resource function in all-IP (Internet Protocol) networks, an announcement device in any network, an interactive voice interface device, or a streaming server in packet networks. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     In the following, the invention is explained in more detail with reference to drawings, of which 
       FIG. 1  is a high-level block diagram of an embodiment of the communications system of the invention; 
       FIG. 2  is a block diagram illustrating a first embodiment of a narrowband-to-wideband conversion according to the invention; and 
       FIG. 3  is a block diagram illustrating a second embodiment of a narrowband-to-wideband conversion according to the invention. 
       FIG. 4  illustrates a schematic diagram of a communications system according to an exemplary embodiment of the present invention; 
       FIG. 5  illustrates a schematic diagram of a communications system according to another exemplary embodiment of the present invention; 
       FIG. 6  illustrates a schematic diagram of the communications system according to another exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  schematically shows selected elements of an embodiment of the communications system according to the invention. 
   In the system, a communications network comprises in one of its network elements a transcoder  1 . Of the network, which can either be a circuit switched or a packet switched network, only the transcoder  1  is depicted. The system further comprises a wideband terminal  2  and a narrowband terminal  3 . The network element with the transcoder  1  has access to traffic channels  11 ,  12 ,  21 ,  22  established respectively between the terminals  2 ,  3  and the network for uplink and downlink transmissions. 
   The wideband terminal  2  includes a wideband encoder  13  and a wideband decoder  24 . The narrowband terminal  3  includes correspondingly a narrowband encoder  23  and a narrowband decoder  14 . The transcoder  1  includes on the one hand a wideband decoder  15  connected via a wideband-to-narrowband converter  16  to a narrowband encoder  17 . On the other hand, the transcoder  1  includes a narrowband decoder  25  connected via a narrowband-to-wideband converter  26  to a wideband encoder  27 . 
   The depicted part of the communications system functions as follows: 
   In a first situation, speech is to be transmitted from the wideband terminal  2  to the narrowband terminal  3 . The wideband terminal  2  encodes the speech in the integrated wideband encoder  13  in order to obtain an encoded wideband signal. The encoded signal is then transmitted to the transcoder  1  via the physical channel  11  established for uplink transmissions between the wideband terminal  2  and the network. 
   In the transcoder  1 , the received wideband signal is first decoded by the wideband decoder  15  in order to obtain a linear wideband signal. The linear wideband signal is subsequently converted by the wideband-to-narrowband converter  16  to a linear narrowband signal. As a last step performed in the transcoder  1 , the narrowband encoder  17  encodes the obtained linear narrowband signal. 
   The encoded narrowband signal is transmitted by the network to the narrowband terminal  3  via a downlink channel  12  established between the network and the narrowband terminal  3 . The narrowband terminal  3  now decodes the received narrowband signal with its narrowband decoder  14  for presentation to a user. 
   In a second situation, speech is to be transmitted in the opposite direction, i.e. from the narrowband terminal  3  to the wideband terminal  2 . In this case, the narrowband terminal  3  encodes the speech that is to be transmitted with the integrated narrowband encoder  23 . The resulting signal is transmitted to the network element with the transcoder  1  via the uplink channel  21  established between the narrowband encoder  23  and the network. 
   In the network element, the narrowband decoder  25  of the transcoder  1  decodes the received signal to a linear narrowband signal. Further, the narrowband-to-wideband converter  26  of the transcoder  1  converts the linear narrowband signal to a linear wideband signal. The conversion includes a generation of an artificial linear wideband signal out of the linear narrowband signal. The linear wideband signal is encoded again by the wideband encoder  27  of the transcoder  1  and forwarded to the wideband terminal  2  via downlink channel  22  established between the network and the wideband terminal  2 . 
   In the wideband terminal  2 , the wideband decoder  24  decodes the received signals to linear wideband signals for presenting them to a user. 
   Thus, the depicted system enables bi-directional transmissions of speech signals between the wideband terminal  2  and the narrowband terminal  3 . 
   Selected possibilities of generating artificial wideband signals during a narrowband-to-linear conversion will now be described with reference to  FIGS. 2 and 3 . 
     FIG. 2  is a schematic block diagram of a first embodiment of the narrowband-to-wideband converter  26  of  FIG. 1 . The converter  26  of this embodiment comprises means for up-sampling  31  and means for frequency shaping  32 . 
   In the first embodiment, the linear narrowband signal is up-sampled without low-pass filtering by the means for up-sampling  31 . This generates alias frequency components of the narrowband signal onto the upper band of the wideband signal, i.e. a mirror image of the narrowband signal in the frequency domain. This aliased wideband signal, however, contains excessive distortion which would be subjectively annoying for the user of the receiving wideband terminal  2 . Therefore the distortions are smoothed in the means for frequency shaping  32  by attenuating dynamically aliased components based on the narrowband signals. It is to be noted that in particular more attenuation is needed for aliased components with voiced phonemes than for those with unvoiced phonemes. 
   Another possibility of converting linear narrowband signals to linear wideband signals is presented in  FIG. 3 , which illustrates, in form of a schematic block diagram, a second embodiment of the narrowband-to-wideband converter  26  of  FIG. 1 . 
   The second embodiment of the narrowband-to-wideband converter  26  comprises a first processing branch with means  33 ,  34  for up-sampling and lowpass filtering received signals. A second processing branch includes means for a narrowband analysis  35  and means for an upper band signal generation  36 . The output of both processing branches is connected to summing means  37 , which form the output of the narrowband-to-wideband converter  26  of  FIG. 1 . 
   In the first processing branch of the converter  26 , the linear narrowband signal is first up-sampled and then low-pass filtered by the corresponding means  33 ,  34  in order to obtain a distortion free linear wideband signal of a lower frequency band. 
   In the second processing branch of the converter  26 , the upper frequency band of the wideband signal is statistically recovered by using the spectral characteristics of the frequency components of the received narrowband signal. 
   The means for a narrowband analysis  35  in the second processing branch first perform a spectrum analysis of the received narrowband signal. The means for upper band signal generation  36  have access to a stored codebook of narrowband speech spectral parameters and to a corresponding stored codebook of upper band wideband speech spectral parameters. The means for upper band signal generation  36  are therefore able to perform a mapping between narrowband and wideband codebook values, wherein the required narrowband codebook values are calculated from the spectrum analysis of narrowband signal. The mapping is thus suited for predicting the upper band spectrum of wideband speech based on the received narrowband signal. The upper band signal is generated more specifically by shaping upper band excitation signal with a spectrum-shaping filter which is defined by the determined wideband codebook values. The upper band excitation signal can be a locally generated noise or pulse excitation, like in a linear prediction coding (LPC) based speech codec. Alternatively, the upper band excitation signal could be a mirror image of the narrow band signal, like in the first embodiment, or a frequency shaped mirror image. 
   In order to obtain finally the complete wideband signal, the artificially generated upper band signal output by the second processing branch is added by the means for summing  37  to the lower band signal output by the first processing branch. 
   The wideband-to-narrowband conversion in the wideband-to-narrowband converter  16  of  FIG. 1  can be realized for example by lowpass filtering the linear wideband signal and by then down-sampling the low-pass filtered wideband signal. The result is an alias distortion free narrowband signal. 
   In both directions, the transcoding can equally be achieved with other suitable methods, as long as the transcoding of a narrowband signal to a wideband signal results in an at least partially artificial wideband signal of a broader frequency range than the original narrowband signal. 
   With reference to  FIGS. 4 ,  5  and  6 , three different situations will now be presented, in which the invention can be employed advantageously. Corresponding elements are referred to in these figures by the same reference signs. 
     FIG. 4  is a schematic block diagram of a communications system and illustrates a first situation in which audio signals are to be transmitted between a wideband terminal and a narrowband terminal. The system basically corresponds to the system of  FIG. 1 , only the network part being depicted in more detail. In  FIG. 4 , a wideband terminal  0 . 40  has access to a first UTRAN-RAN (radio access network) or a GSM-BSS (base station system)  41 . The first RAN or BSS  41  is connected via a first core network with a first network element  42 , via an interconnect network  43  and via a second core network with a second network element  44  to a second RAN or BSS  45 . A narrowband terminal  46  has access to the second RAN or BSS  45 . The depicted network elements  42 ,  44  are both either a 3G media gateway MGW or a 2G transcoder TC, which are employed in core networks for performing the transcoding between different speech coding schemes. The first network element  42  comprises means  47  for artificial bandwidth expansion of received narrowband audio signals, and means  48  for bandwidth reduction of received wideband audio signals. Transmissions between the wideband terminal  40  and the narrowband terminal  46  are indicated in the figure by arrows. 
   A wideband audio signal transmitted by the wideband terminal  40  and addressed to the narrowband terminal  46  of  FIG. 4  is received by the first RAN or BSS  41  and forwarded to the depicted network element  42  of the connected core network. In the network element  42 , the wideband audio signal is subject to a bandwidth reduction in order to obtain a narrowband audio signal. The bandwidth reduction is achieved by the means  48  for bandwidth reduction, for example as mentioned above by employing a lowpass filtering. The narrowband audio signal is then transmitted via the interconnect network  43 , the second network element  44  of the second core network and the second RAN or BSS  45  to the narrowband terminal  46 . The narrowband terminal  46  is able to present the received narrowband audio signal to a user. 
   In the opposite direction, a narrowband audio signal transmitted by the narrowband terminal  46  and addressed to the wideband terminal  40  of  FIG. 4  is received by the second RAN or BSS  45  and forwarded to the depicted network element  44  of the connected core network. The narrowband audio signal is then further transmitted via the interconnect network  43 , to the network element  42  of the first core network. In this network element  42 , an artificial wideband audio signal is generated by the means for bandwidth extension  47  out of the received narrowband audio signal, e.g. according to one of the methods described with reference to  FIGS. 1 to 3 . The generated wideband audio signal is then forwarded via the first RAN or BSS  41  to the wideband terminal  40 . The wideband terminal  40  is able to present the received audio signal to a user without subjective reduction of speech quality compared to received original wideband speech. 
     FIG. 5  illustrates a second situation in which audio signals are to be transmitted between terminals and a speech-based network service equipment. The figure shows a communications system which corresponds to the system of  FIG. 4 , except that both terminals  40 ,  46  have in addition access to a speech-based network service equipment  50  via the respective RAN or BSS  41 ,  45 , the respective core network with network element  42  or  44  and the interconnect network  43 . The speech-based network service equipment  50  only stores narrowband speech samples and is only designed for handling narrowband audio signals. 
   Transmissions between the speech-based network service equipment  50  and the narrowband terminal  46  are carried out in either direction via the interconnect network  43 , the second network element  44  of the second core network and the second RAN or BSS  45  without a transcoding according to the invention. 
   Transmissions between the speech-based network service equipment  50  and the wideband terminal  40  are indicated in the figure by arrows. They are carried out via the interconnect network  43 , the first network element  42  of the first core network and the first RAN or BSS  41 , or in reversed order respectively. In this case, however, wideband audio signals originating from the wideband terminal  40  are reduced in bandwidth by the corresponding means  48  of the first network element  42  to form a narrowband audio signal, and narrowband audio signals originating from the speech-based network service equipment  50  are expanded to artificial wideband audio signals by the corresponding means  47  of the first network element. The processing is thus analogous to the processing in the situation of  FIG. 4 . 
   It depends on the respective application whether a bandwidth reduction, an artificial bandwidth expansion or both are required in the network element  42  of the first core network. 
   In the situation of  FIG. 5 , instead of a speech-based network service equipment  50 , also a PSTN narrowband terminal could be connected to the interconnect network  43 . In order to enable a communication according to the invention between such a PSTN narrowband terminal and the depicted wideband terminal  40 , a processing corresponding to the processing described with reference to  FIG. 5  for the communication involving a speech-based network service equipment  50  can be employed. 
   In a last presented situation, signals are to be transmitted between two wideband terminals.  FIG. 6  therefore shows a communications system which corresponds again largely to the system of  FIG. 4 , except that in this communications system, both terminals  40 ,  60  are wideband terminals. The first wideband terminal  40  is connected to the first RAN or BSS  41  as in  FIG. 4 , and the second wideband terminal  60  is connected to the second RAN or BSS  45  as the narrowband terminal in  FIG. 4 . The interconnect network  43  of the system of  FIG. 6  is moreover supposed to be a trunking network which was exclusively designed for transmitting narrowband signals. In the system of  FIG. 6 , also the second network element  44  comprises means  61  for artificial bandwidth expansion of received narrowband audio signals, and means  62  for bandwidth reduction of received wideband audio signals. 
   In the system of  FIG. 6 , it is not different kinds of units employed for transmitting and receiving audio signals which make a transcoding necessary, since both involved units  40 ,  60  are wideband terminals. Rather, a transcoding is employed for enabling a low capacity transmission via the trunking network  43 . 
   Thus, whenever the first wideband terminal  40  transmits a wideband audio signal addressed to the second wideband terminal  60 , the audio signal is forwarded via the first RAN or BSS  41  to the first network element  42 , where it is processed a first time. More specifically, the means  48  for bandwidth reduction of the first network element  42  are employed for generating based on the received wideband audio signal a narrowband audio signal, which can be transmitted by the employed interconnect network  43 . The interconnect network  43  forwards the narrowband audio signal to the second network element  44  of the second core network. The means  61  for artificial bandwidth expansion of the second network element  44  convert the received narrowband audio signal artificially into a wideband audio signal again. The wideband audio signal is forwarded via the second RAN or BSS  45  to the second wideband terminal  60 . For bandwidth reduction and expansion, for example the methods described with reference to  FIGS. 1 to 3  can be employed, just as for the situations illustrated in  FIGS. 4 and 5 . By this proceeding, the second wideband terminal  60  can be supplied with wideband speech via the trunking network  43  without a significant reduction of subjective speech quality. 
   In the opposite direction, the processing is exactly the same, a bandwidth reduction of a transmitted wideband audio signal being carried out by the corresponding means  62  of the second network element  44 , and a generation of an artificial wideband audio signal by the corresponding means  47  of the first network element  42 . 
   The methods for artificial bandwidth generation employed in the above described embodiments of the invention are to be understood as exemplary methods. Any other suited method can be utilized instead.